46 changed files with 1061 additions and 473 deletions
--- a/.gitea/workflows/test-and-build.yml
+++ b/.gitea/workflows/test-and-build.yml
@ -62,4 +62,4 @@ jobs:
            ./tricu
          token: '${{ secrets.RELEASE_TOKEN }}'
          body: '${{ gitea.event.head_commit.message }}'
-          pre_release: true
+          prerelease: true
--- a/.gitignore
+++ b/.gitignore
@ -9,3 +9,4 @@
 WD
 bin/
 dist*
+.tricu_history
--- a/README.md
+++ b/README.md
@ -2,34 +2,36 @@

 ## Introduction

-tricu (pronounced "tree-shoe") is a purely functional interpreted language implemented in Haskell. It is fundamentally based on the application of [Tree Calculus](https://github.com/barry-jay-personal/typed_tree_calculus/blob/main/typed_program_analysis.pdf) terms, but minimal syntax sugar is included to provide a useful programming tool. tricu is under active development and you can expect breaking changes with nearly every commit.
+tricu (pronounced "tree-shoe") is a purely functional interpreted language implemented in Haskell. It is fundamentally based on the application of [Tree Calculus](https://github.com/barry-jay-personal/typed_tree_calculus/blob/main/typed_program_analysis.pdf) terms, but minimal syntax sugar is included to provide a useful programming tool. 
+
+*tricu is under active development and you should expect breaking changes with every commit.*

 tricu is the word for "tree" in Lojban: `(x1) is a tree of species/cultivar (x2)`.

 ## Features

 - Tree Calculus operator: `t`
- Assignments: `x = t t`
- Lambda abstraction syntax: `id = (\a : a)`
+- Immutable definitions: `x = t t`
+- Lambda abstraction: `id = (a : a)`
 - List, Number, and String literals: `[(2) ("Hello")]` 
 - Function application: `not (not false)`
- Higher order/first-class functions: `map (\a : lconcat a "!") [("Hello")]`
+- Higher order/first-class functions: `map (a : append a "!") [("Hello")]`
 - Intensionality blurs the distinction between functions and data (see REPL examples)
- Immutability
+- Simple module system for code organization

 ## REPL examples

 ```
 tricu < -- Anything after `--` on a single line is a comment
-tricu < id = (\a : a) -- Lambda abstraction is eliminated to tree calculus terms
-tricu < head (map (\i : lconcat i " world!") [("Hello, ")])
+tricu < id = (a : a) -- Lambda abstraction is eliminated to tree calculus terms
+tricu < head (map (i : append i " world!") [("Hello, ")])
 tricu > "Hello,  world!"
-tricu < id (head (map (\i : lconcat i " world!") [("Hello, ")]))
+tricu < id (head (map (i : append i " world!") [("Hello, ")]))
 tricu > "Hello,  world!"

 tricu < -- Intensionality! We can inspect the structure of a function or data.
-tricu < triage = (\a b c : t (t a b) c)
-tricu < test = triage "Leaf" (\z : "Stem") (\a b : "Fork")
+tricu < triage = (a b c : t (t a b) c)
+tricu < test = triage "Leaf" (z : "Stem") (a b : "Fork")
 tricu < test (t t)
 tricu > "Stem"
 tricu < -- We can even convert a term back to source code (/demos/toSource.tri)
@ -38,13 +40,21 @@ tricu > "(t (t (t t) (t t t)) (t t (t t t)))"
 tricu < -- or calculate its size (/demos/size.tri)
 tricu < size not?
 tricu > 12
+
+tricu < -- REPL Commands:
+tricu < !definitions  -- Lists all available definitions
+tricu < !output       -- Change output format (Tree, FSL, AST, etc.)
+tricu < !import       -- Import definitions from a file
+tricu < !exit         -- Exit the REPL
+tricu < !clear        -- ANSI screen clear
+tricu < !save         -- Save all REPL definitions to a file that you can !import
+tricu < !reset        -- Clear all REPL definitions
+tricu < !version      -- Print tricu version
 ```

 ## Installation and Use

-[Releases are available for Linux.](https://git.eversole.co/James/tricu/releases)
-
-Or you can easily build and/or run this project using [Nix](https://nixos.org/download/).
+You can easily build and run this project using [Nix](https://nixos.org/download/).

 - Quick Start (REPL): 
  - `nix run git+https://git.eversole.co/James/tricu`
@ -81,6 +91,12 @@ tricu decode [OPTIONS]
                    Defaults to stdin.
 ```

+## Collaborating
+
+I am happy to accept issue reports, pull requests, or questions about tricu [via email](mailto:james@eversole.co).
+
+If you want to collaborate but don't want to email back-and-forth, please reach out via email once to let me know and I will provision a git.eversole.co account for you.
+
 ## Acknowledgements 

 Tree Calculus was discovered by [Barry Jay](https://github.com/barry-jay-personal/blog). 
--- a/demos/equality.tri
+++ b/demos/equality.tri
@ -1,3 +1,6 @@
+!import "../lib/base.tri" !Local
+!import "../lib/list.tri" !Local
+
 main = lambdaEqualsTC

 -- We represent `false` with a Leaf and `true` with a Stem Leaf
@ -8,20 +11,17 @@ demo_true  = t t
 not_TC?     = t (t (t t) (t t t)) (t t (t t t))

 -- /demos/toSource.tri contains an explanation of `triage`
-demo_triage = \a b c : t (t a b) c
-demo_matchBool = (\ot of : demo_triage
-  of
-  (\_ : ot)
-  (\_ _ : ot)
-)
+demo_triage = a b c : t (t a b) c
+demo_matchBool = a b : demo_triage b (_ : a) (_ _ : a)
+
 -- Lambda representation of the Boolean `not` function
 not_Lambda? = demo_matchBool demo_false demo_true

-- Since tricu eliminates Lambda terms to SKI combinators, the tree form of many
+-- As tricu eliminates Lambda terms to SKI combinators, the tree form of many
 -- functions defined via Lambda terms are larger than the most efficient TC
-- representation. Between different languages that evaluate to tree calculus 
-- terms, the exact implementation of Lambda elimination may differ and lead 
-- to different tree representations even if they share extensional behavior.
+-- representation possible. Between different languages that evaluate to tree
+-- calculus terms, the exact implementation of Lambda elimination may differ
+-- and lead to different trees even if they share extensional behavior.

 -- Let's see if these are the same:
 lambdaEqualsTC = equal? not_TC? not_Lambda?
--- a/demos/levelOrderTraversal.tri
+++ b/demos/levelOrderTraversal.tri
@ -1,3 +1,6 @@
+!import "../lib/base.tri" Lib
+!import "../lib/list.tri" !Local
+
 main = exampleTwo
 -- Level Order Traversal of a labelled binary tree
 -- Objective: Print each "level" of the tree on a separate line
@ -15,47 +18,47 @@ main = exampleTwo
 --    /   /  \
 --   4   5    6

-label = \node : head node
+label = node : head node

-left = (\node : if (emptyList? node)
-  [] 
-  (if (emptyList? (tail node)) 
-    [] 
+left = node : (if (emptyList? node)
+  []
+  (if (emptyList? (tail node))
+    []
    (head (tail node))))

-right = (\node : if (emptyList? node) 
-  [] 
-  (if (emptyList? (tail node)) 
-    [] 
-    (if (emptyList? (tail (tail node))) 
-      [] 
+right = node : (if (emptyList? node)
+  []
+  (if (emptyList? (tail node))
+    []
+    (if (emptyList? (tail (tail node)))
+      []
      (head (tail (tail node))))))

-processLevel = y (\self queue : if (emptyList? queue) 
-  [] 
-  (pair (map label queue) (self (filter 
-    (\node : not? (emptyList? node)) 
-      (lconcat (map left queue) (map right queue))))))
+processLevel = y (self queue : if (emptyList? queue)
+  []
+  (pair (map label queue) (self (filter
+    (node : not? (emptyList? node))
+      (append (map left queue) (map right queue))))))

-levelOrderTraversal_ = \a : processLevel (t a t)
+levelOrderTraversal_ = a : processLevel (t a t)

-toLineString = y (\self levels : if (emptyList? levels) 
-  "" 
-  (lconcat 
-    (lconcat (map (\x : lconcat x " ") (head levels)) "") 
-    (if (emptyList? (tail levels)) "" (lconcat (t (t 10 t) t) (self (tail levels))))))
+toLineString = y (self levels : if (emptyList? levels)
+  ""
+  (append
+    (append (map (x : append x " ") (head levels)) "")
+    (if (emptyList? (tail levels)) "" (append (t (t 10 t) t) (self (tail levels))))))

-levelOrderToString = \s : toLineString (levelOrderTraversal_ s)
+levelOrderToString = s : toLineString (levelOrderTraversal_ s)

-flatten = foldl (\acc x : lconcat acc x) ""
+flatten = foldl (acc x : append acc x) ""

-levelOrderTraversal = \s : lconcat (t 10 t) (flatten (levelOrderToString s))
+levelOrderTraversal = s : append (t 10 t) (flatten (levelOrderToString s))

-exampleOne = levelOrderTraversal [("1") 
-                                 [("2") [("4") t t] t] 
+exampleOne = levelOrderTraversal [("1")
+                                 [("2") [("4") t t] t]
                                 [("3") [("5") t t] [("6") t t]]]

-exampleTwo = levelOrderTraversal [("1") 
-                                 [("2") [("4") [("8") t t] [("9") t t]] 
-                                        [("6") [("10") t t] [("12") t t]]] 
+exampleTwo = levelOrderTraversal [("1")
+                                 [("2") [("4") [("8") t t] [("9") t t]]
+                                        [("6") [("10") t t] [("12") t t]]]
                                 [("3") [("5") [("11") t t] t] [("7") t t]]]
--- a/demos/patternMatching.tri
+++ b/demos/patternMatching.tri
@ -0,0 +1,37 @@
+!import "../lib/patterns.tri" !Local
+
+-- We can do conditional pattern matching by providing a list of lists, where
+-- each sublist contains a boolean expression and a function to return if said
+-- boolean expression evaluates to true.
+
+value = 42
+main  = match value [[(equal? "Hello") (_ : ", world!")] [(equal? 42) (_ : "The answer.")]]
+
+-- < main
+-- > "The answer."
+
+matchExample = (x : match x
+  [[(equal? 1)  (_ : "one")]
+   [(equal? 2)  (_ : "two")]
+   [(equal? 3)  (_ : "three")]
+   [(equal? 4)  (_ : "four")]
+   [(equal? 5)  (_ : "five")]
+   [(equal? 6)  (_ : "six")]
+   [(equal? 7)  (_ : "seven")]
+   [(equal? 8)  (_ : "eight")]
+   [(equal? 9)  (_ : "nine")]
+   [(equal? 10) (_ : "ten")]
+   [ otherwise  (_ : "I ran out of fingers!")]])
+
+-- < matchExample 3
+-- > "three"
+-- < matchExample 5
+-- > "five"
+-- < matchExample 9
+-- > "nine"
+-- < matchExample 11
+-- > "I ran out of fingers!"
+-- < matchExample "three"
+-- > "I ran out of fingers!"
+-- < matchExample [("hello") ("world")]
+-- > "I ran out of fingers!"
--- a/demos/size.tri
+++ b/demos/size.tri
@ -1,21 +1,11 @@
+!import "../lib/base.tri" !Local
+!import "../lib/list.tri" !Local
+
 main = size size

-compose = \f g x : f (g x)
-
-succ = y (\self :
-  triage
-    1
-    t
-    (triage
-      (t (t t))
-      (\_ tail : t t (self tail))
-      t))
-
-size = (\x :
-  (y (\self x :
-    compose succ
-      (triage
-        (\x : x)
-        self
-        (\x y : compose (self x) (self y))
-        x)) x 0))
+size = x : y (self x : compose succ (triage
+  id
+  self
+  (x y : compose (self x) (self y))
+  x)
+) x 0
--- a/demos/toSource.tri
+++ b/demos/toSource.tri
@ -1,3 +1,6 @@
+!import "../lib/base.tri" !Local
+!import "../lib/list.tri" !Local
+
 main = toSource not?
 -- Thanks to intensionality, we can inspect the structure of a given value
 -- even if it's a function. This includes lambdas which are eliminated to
@ -15,25 +18,25 @@ main = toSource not?
 sourceLeaf = t (head "t")

 -- Stem case
-sourceStem = (\convert : (\a rest :
+sourceStem = convert : (a rest :
  t (head "(")                       -- Start with a left parenthesis "(".
    (t (head "t")                    -- Add a "t"
      (t (head " ")                  -- Add a space.
        (convert a                   -- Recursively convert the argument.
-          (t (head ")") rest))))))   -- Close with ")" and append the rest.
+          (t (head ")") rest)))))   -- Close with ")" and append the rest.

 -- Fork case
-sourceFork = (\convert : (\a b rest :
+sourceFork = convert : (a b rest :
  t (head "(")                           -- Start with a left parenthesis "(".
    (t (head "t")                        -- Add a "t"
      (t (head " ")                      -- Add a space.
        (convert a                       -- Recursively convert the first arg.
          (t (head " ")                  -- Add another space.
            (convert b                   -- Recursively convert the second arg.
-              (t (head ")") rest)))))))) -- Close with ")" and append the rest.
+              (t (head ")") rest))))))) -- Close with ")" and append the rest.

 -- Wrapper around triage 
-toSource_ = y (\self arg :
+toSource_ = y (self arg :
  triage
    sourceLeaf        -- `triage` "a" case, Leaf
    (sourceStem self) -- `triage` "b" case, Stem
@ -41,7 +44,7 @@ toSource_ = y (\self arg :
    arg)              -- The term to be inspected

 -- toSource takes a single TC term and returns a String
-toSource = \v : toSource_ v ""
+toSource = v : toSource_ v ""

 exampleOne = toSource true -- OUT: "(t t)"
 exampleTwo = toSource not? -- OUT: "(t (t (t t) (t t t)) (t t (t t t)))"
--- a/flake.nix
+++ b/flake.nix
@ -34,6 +34,7 @@
        devShells.default = pkgs.mkShell {
          buildInputs = with pkgs; [
            haskellPackages.cabal-install
+            haskellPackages.ghc-events
            haskellPackages.ghcid
            customGHC
            upx
--- a/lib/base.tri
+++ b/lib/base.tri
@ -1,83 +1,74 @@
 false = t
 _     = t
 true  = t t
-k     = t t
-i     = t (t k) t
-s     = t (t (k t)) t
-m     = s i i
-b     = s (k s) k
-c     = s (s (k s) (s (k k) s)) (k k)
-id    = \a : a
+id    = a : a
+const = a b : a
 pair  = t
-if    = \cond then else : t (t else (t t then)) t cond
+if    = cond then else : t (t else (t t then)) t cond

-y = ((\mut wait fun : wait mut (\x : fun (wait mut x)))
-     (\x : x x)
-     (\a0 a1 a2 : t (t a0) (t t a2) a1))
+y = ((mut wait fun : wait mut (x : fun (wait mut x)))
+     (x : x x)
+     (a0 a1 a2 : t (t a0) (t t a2) a1))

-triage = \leaf stem fork : t (t leaf stem) fork
-test   = triage "Leaf" (\_ : "Stem") (\_ _ : "Fork")
+compose = f g x : f (g x)

-matchBool = (\ot of : triage
+triage = leaf stem fork : t (t leaf stem) fork
+test   = triage "Leaf" (_ : "Stem") (_ _ : "Fork")
+
+matchBool = (ot of : triage
  of
-  (\_ : ot)
-  (\_ _ : ot)
+  (_ : ot)
+  (_ _ : ot)
 )

-matchList = \a b : triage a _ b
-
-matchPair = \a   : triage _ _ a
-
-not? = matchBool false true
-and? = matchBool id (\_ : false)
-emptyList? = matchList true (\_ _ : false)
-
-head = matchList t (\head _ : head)
-tail = matchList t (\_ tail : tail)
-
-lconcat = y (\self : matchList
-  (\k : k)
-  (\h r k : pair h (self r k)))
-
 lAnd = (triage
-  (\_     : false)
-  (\_ x   : x)
-  (\_ _ x : x))
+  (_     : false)
+  (_ x   : x)
+  (_ _ x : x))

 lOr = (triage
-  (\x     : x)
-  (\_ _   : true)
-  (\_ _ _ : true))
+  (x     : x)
+  (_ _   : true)
+  (_ _ _ : true))

-map_ = y (\self :
-  matchList
-    (\_ : t)
-    (\head tail f : pair (f head) (self tail f)))
-map = \f l : map_ l f
+matchPair = a : triage _ _ a

-equal? = y (\self : triage
+not? = matchBool false true
+and? = matchBool id (_ : false)
+
+or? = (x z : 
+  matchBool 
+    (matchBool true true z)
+    (matchBool true false z)
+    x)
+
+xor? = (x z : 
+  matchBool 
+    (matchBool false true z)
+    (matchBool true false z)
+    x)
+
+equal? = y (self : triage
  (triage
    true
-    (\_   : false)
-    (\_ _ : false))
-  (\ax :
+    (_   : false)
+    (_ _ : false))
+  (ax :
    triage
      false
      (self ax)
-      (\_ _ : false))
-  (\ax ay :
+      (_ _ : false))
+  (ax ay :
    triage
      false
-      (\_ : false)
-      (\bx by : lAnd (self ax bx) (self ay by))))
+      (_ : false)
+      (bx by : lAnd (self ax bx) (self ay by))))

-filter_ = y (\self : matchList
-  (\_ : t)
-  (\head tail f : matchBool (t head) i (f head) (self tail f)))
-filter  = \f l : filter_ l f
-
-foldl_ = y (\self f l x : matchList (\acc : acc) (\head tail acc : self f tail (f acc head)) l x)
-foldl  = \f x l : foldl_ f l x
-
-foldr_ = y (\self x f l : matchList x (\head tail : f (self x f tail) head) l)
-foldr  = \f x l : foldr_ x f l
+succ = y (self :
+  triage
+    1
+    t
+    (triage
+      (t (t t))
+      (_ tail : t t (self tail))
+      t))
--- a/lib/list.tri
+++ b/lib/list.tri
@ -0,0 +1,70 @@
+!import "base.tri" !Local
+
+_ = t
+
+matchList = a b : triage a _ b
+
+emptyList? = matchList true (_ _ : false)
+head = matchList t (head _ : head)
+tail = matchList t (_ tail : tail)
+
+append = y (self : matchList
+  (k : k)
+  (h r k : pair h (self r k)))
+
+lExist? = y (self x : matchList
+  false
+  (h z : or? (equal? x h) (self x z)))
+
+map_ = y (self :
+  matchList
+    (_ : t)
+    (head tail f : pair (f head) (self tail f)))
+map = f l : map_ l f
+
+filter_ = y (self : matchList
+  (_ : t)
+  (head tail f : matchBool (t head) id (f head) (self tail f)))
+filter  = f l : filter_ l f
+
+foldl_ = y (self f l x : matchList (acc : acc) (head tail acc : self f tail (f acc head)) l x)
+foldl  = f x l : foldl_ f l x
+
+foldr_ = y (self x f l : matchList x (head tail : f (self x f tail) head) l)
+foldr  = f x l : foldr_ x f l
+
+length = y (self : matchList
+  0
+  (_ tail : succ (self tail)))
+
+reverse = y (self : matchList
+  t
+  (head tail : append (self tail) (pair head t)))
+
+snoc = y (self x : matchList
+  (pair x t)
+  (h z : pair h (self x z)))
+
+count = y (self x : matchList
+  0
+  (h z : matchBool
+    (succ (self x z))
+    (self x z)
+    (equal? x h)))
+
+last = y (self : matchList
+  t
+  (hd tl : matchBool
+    hd
+    (self tl)
+    (emptyList? tl)))
+
+all? = y (self pred : matchList
+  true
+  (h z : and? (pred h) (self pred z)))
+
+any? = y (self pred : matchList
+  false
+  (h z : or? (pred h) (self pred z)))
+
+intersect = xs ys : filter (x : lExist? x ys) xs
--- a/lib/patterns.tri
+++ b/lib/patterns.tri
@ -0,0 +1,24 @@
+!import "base.tri" !Local
+!import "list.tri" List
+
+match_ = y (self value patterns :
+  triage
+    t
+    (_ : t)
+    (pattern rest :
+      triage
+        t
+        (_ : t)
+        (test result :
+          if (test value)
+             (result value)
+             (self value rest))
+        pattern)
+    patterns)
+
+match = (value patterns :
+  match_ value (List.map (sublist :
+    pair (List.head sublist) (List.head (List.tail sublist)))
+    patterns))
+
+otherwise = const (t t)
--- a/src/Eval.hs
+++ b/src/Eval.hs
@ -3,33 +3,33 @@ module Eval where
 import Parser
 import Research

-import Data.List (partition)
+import Data.List (partition, (\\))
 import Data.Map  (Map)
 import qualified Data.Map as Map
 import qualified Data.Set as Set

 evalSingle :: Env -> TricuAST -> Env
 evalSingle env term
-  | SDef name [] body <- term =
-      if
-        | Map.member name env ->
-            errorWithoutStackTrace $
-              "Error: Identifier '" ++ name ++ "' is already defined."
-        | otherwise ->
-            let res = evalAST env body
-            in Map.insert "__result" res (Map.insert name res env)
-  | SApp func arg <- term =
-      let res = apply (evalAST env func) (evalAST env arg)
-      in Map.insert "__result" res env
-  | SVar name <- term =
-      case Map.lookup name env of
-        Just v  ->
-          Map.insert "__result" v env
+  | SDef name [] body <- term
+  = case Map.lookup name env of
+      Just existingValue
+        | existingValue == evalAST env body -> env
+        | otherwise -> errorWithoutStackTrace $
+            "Unable to rebind immutable identifier: " ++ name
+      Nothing ->
+        let res = evalAST env body
+        in Map.insert "!result" res (Map.insert name res env)
+  | SApp func arg <- term
+  = let res = apply (evalAST env func) (evalAST env arg)
+      in Map.insert "!result" res env
+  | SVar name <- term
+  = case Map.lookup name env of
+        Just v  -> Map.insert "!result" v env
        Nothing ->
          errorWithoutStackTrace $ "Variable `" ++ name ++ "` not defined\n\
          \This error should never occur here. Please report this as an issue."
-  | otherwise =
-      Map.insert "__result" (evalAST env term) env
+  | otherwise
+  = Map.insert "!result" (evalAST env term) env

 evalTricu :: Env -> [TricuAST] -> Env
 evalTricu env x = go env (reorderDefs env x)
@ -37,7 +37,7 @@ evalTricu env x = go env (reorderDefs env x)
    go env []     = env
    go env [x]    =
      let updatedEnv = evalSingle env x
-      in Map.insert "__result" (result updatedEnv) updatedEnv
+      in Map.insert "!result" (result updatedEnv) updatedEnv
    go env (x:xs) =
      evalTricu (evalSingle env x) xs

@ -70,13 +70,14 @@ elimLambda = go
      | body == triageBody         = _TRIAGE
      where
        triageBody =
-          (SApp (SApp TLeaf (SApp (SApp TLeaf (SVar a)) (SVar b))) (SVar c))
+          SApp (SApp TLeaf (SApp (SApp TLeaf (SVar a)) (SVar b))) (SVar c)
    -- Composition optimization
    go (SLambda [f] (SLambda [g] (SLambda [x] body)))
-      | body == composeBody        = _COMPOSE
-      where
-        composeBody = SApp (SVar f) (SApp (SVar g) (SVar x))
+      | body == SApp (SVar f) (SApp (SVar g) (SVar x)) = _B
    -- General elimination
+    go (SLambda [v] (SList xs))
+      = elimLambda (SLambda [v] (foldr wrapTLeaf TLeaf xs))
+        where wrapTLeaf m r = SApp (SApp TLeaf m) r
    go (SLambda (v:vs) body)
      | null vs                    = toSKI v (elimLambda body)
      | otherwise                  = elimLambda (SLambda [v] (SLambda vs body))
@ -96,8 +97,8 @@ elimLambda = go
    _S       = parseSingle "t (t (t t t)) t"
    _K       = parseSingle "t t"
    _I       = parseSingle "t (t (t t)) t"
+    _B       = parseSingle "t (t (t t (t (t (t t t)) t))) (t t)"
    _TRIAGE  = parseSingle "t (t (t t (t (t (t t t))))) t"
-    _COMPOSE = parseSingle "t (t (t t (t (t (t t t)) t))) (t t)"

 isFree :: String -> TricuAST -> Bool
 isFree x = Set.member x . freeVars
@ -108,11 +109,12 @@ freeVars (SInt    _    ) = Set.empty
 freeVars (SStr    _    ) = Set.empty
 freeVars (SList   s    ) = foldMap freeVars s
 freeVars (SApp    f a  ) = freeVars f <> freeVars a
-freeVars (TLeaf        ) = Set.empty
-freeVars (SDef   _ _ b) = freeVars b
+freeVars  TLeaf          = Set.empty
+freeVars (SDef   _ _ b)  = freeVars b
 freeVars (TStem   t    ) = freeVars t
 freeVars (TFork   l r  ) = freeVars l <> freeVars r
 freeVars (SLambda v b  ) = foldr Set.delete (freeVars b) v
+freeVars  _              = Set.empty

 reorderDefs :: Env -> [TricuAST] -> [TricuAST]
 reorderDefs env defs
@ -122,51 +124,60 @@ reorderDefs env defs
  | otherwise = orderedDefs ++ others
  where
    (defsOnly, others) = partition isDef defs
-    graph              = buildDepGraph defsOnly
-    sortedDefs         = sortDeps graph
-    defMap             = Map.fromList [(name, def) | def@(SDef name _ _) <- defsOnly]
-    orderedDefs        = map (\name -> defMap Map.! name) sortedDefs
-    topDefNames        = Set.fromList (Map.keys defMap)
-    envNames           = Set.fromList (Map.keys env)
-    freeVarsDefs       = foldMap (\(SDef _ _ body) -> freeVars body) defsOnly
-    freeVarsOthers     = foldMap freeVars others
-    allFreeVars        = freeVarsDefs <> freeVarsOthers
-    validNames         = topDefNames `Set.union` envNames
-    missingDeps        = Set.toList (allFreeVars `Set.difference` validNames)
+    defNames = [ name | SDef name _ _ <- defsOnly ]
+
+    defsWithFreeVars = [(def, freeVars body) | def@(SDef _ _ body) <- defsOnly]
+
+    graph = buildDepGraph defsOnly
+    sortedDefs = sortDeps graph
+    defMap = Map.fromList [(name, def) | def@(SDef name _ _) <- defsOnly]
+    orderedDefs = map (\name -> defMap Map.! name) sortedDefs
+
+    freeVarsDefs = foldMap snd defsWithFreeVars
+    freeVarsOthers = foldMap freeVars others
+    allFreeVars = freeVarsDefs <> freeVarsOthers
+    validNames = Set.fromList defNames `Set.union` Set.fromList (Map.keys env)
+    missingDeps = Set.toList (allFreeVars `Set.difference` validNames)

    isDef (SDef _ _ _) = True
    isDef _            = False

 buildDepGraph :: [TricuAST] -> Map.Map String (Set.Set String)
 buildDepGraph topDefs
-  | not (null duplicateNames) =
+  | not (null conflictingDefs) =
      errorWithoutStackTrace $
-        "Duplicate definitions detected: " ++ show duplicateNames
+        "Conflicting definitions detected: " ++ show conflictingDefs
  | otherwise =
      Map.fromList
        [ (name, depends topDefs (SDef name [] body))
        | SDef name _ body <- topDefs]
  where
-    names = [name | SDef name _ _ <- topDefs]
-    duplicateNames =
-      [ name | (name, count) <- Map.toList (countOccurrences names) , count > 1]
-    countOccurrences = foldr (\x -> Map.insertWith (+) x 1) Map.empty
+    defsMap = Map.fromListWith (++)
+      [(name, [(name, body)]) | SDef name _ body <- topDefs]
+
+    conflictingDefs =
+      [ name
+      | (name, defs) <- Map.toList defsMap
+      , let bodies = map snd defs
+      , not $ all (== head bodies) (tail bodies)
+      ]

 sortDeps :: Map.Map String (Set.Set String) -> [String]
-sortDeps graph = go [] (Map.keys graph)
+sortDeps graph = go [] Set.empty (Map.keys graph)
  where
-    go sorted [] = sorted
-    go sorted remaining
-      | null ready =
-          errorWithoutStackTrace
-            "ERROR: Top-level cyclic dependency detected and prohibited\n\
-           \RESOLVE: Use nested lambdas"
-      | otherwise = go (sorted ++ ready) notReady
-      where
-        ready    = [ name | name <- remaining
-                    , all (`elem` sorted) (Set.toList (graph Map.! name))]
-        notReady =
-          [ name | name <- remaining , name `notElem` ready]
+    go sorted sortedSet [] = sorted
+    go sorted sortedSet remaining =
+      let ready = [ name | name <- remaining
+                        , let deps = Map.findWithDefault Set.empty name graph
+                        , Set.isSubsetOf deps sortedSet ]
+          notReady = remaining \\ ready
+      in if null ready
+         then errorWithoutStackTrace
+          "ERROR: Cyclic dependency detected and prohibited.\n\
+          \RESOLVE: Use nested lambdas."
+         else go (sorted ++ ready)
+                 (Set.union sortedSet (Set.fromList ready))
+                 notReady

 depends :: [TricuAST] -> TricuAST -> Set.Set String
 depends topDefs (SDef _ _ body) =
@ -176,9 +187,9 @@ depends topDefs (SDef _ _ body) =
 depends _ _ = Set.empty

 result :: Env -> T
-result r = case Map.lookup "__result" r of
+result r = case Map.lookup "!result" r of
  Just a -> a
-  Nothing -> errorWithoutStackTrace "No __result field found in provided env"
+  Nothing -> errorWithoutStackTrace "No !result field found in provided env"

 mainResult :: Env -> T
 mainResult r = case Map.lookup "main" r of
--- a/src/FileEval.hs
+++ b/src/FileEval.hs
@ -1,30 +1,158 @@
 module FileEval where

 import Eval
+import Lexer
 import Parser
 import Research

+import Data.List       (partition)
+import Data.Maybe      (mapMaybe)
+import Control.Monad   (foldM)
 import System.IO
+import System.FilePath (takeDirectory, normalise, (</>))

 import qualified Data.Map as Map
+import qualified Data.Set as Set
+
+extractMain :: Env -> Either String T
+extractMain env =
+  case Map.lookup "main" env of
+    Just result -> Right result
+    Nothing -> Left "No `main` function detected"
+
+processImports :: Set.Set FilePath -> FilePath -> FilePath -> [TricuAST]
+                 -> Either String ([TricuAST], [(FilePath, String, FilePath)])
+processImports seen base currentPath asts =
+  let (imports, nonImports) = partition isImp asts
+      importPaths = mapMaybe getImportInfo imports
+  in if currentPath `Set.member` seen
+     then Left $ "Encountered cyclic import: " ++ currentPath
+     else Right (nonImports, importPaths)
+  where
+    isImp (SImport _ _) = True
+    isImp _ = False
+    getImportInfo (SImport p n) = Just (p, n, makeRelativeTo currentPath p)
+    getImportInfo _ = Nothing

 evaluateFileResult :: FilePath -> IO T
 evaluateFileResult filePath = do
  contents <- readFile filePath
-  let asts = parseTricu contents
-  let finalEnv = evalTricu Map.empty asts
-  case Map.lookup "main" finalEnv of
-    Just finalResult -> return finalResult
-    Nothing -> errorWithoutStackTrace "No `main` function detected"
+  let tokens = lexTricu contents
+  case parseProgram tokens of
+    Left err -> errorWithoutStackTrace (handleParseError err)
+    Right ast -> do
+      processedAst <- preprocessFile filePath
+      let finalEnv = evalTricu Map.empty processedAst
+      case extractMain finalEnv of
+        Right result -> return result
+        Left err -> errorWithoutStackTrace err

 evaluateFile :: FilePath -> IO Env
 evaluateFile filePath = do
  contents <- readFile filePath
-  let asts = parseTricu contents
-  pure $ evalTricu Map.empty asts
+  let tokens = lexTricu contents
+  case parseProgram tokens of
+    Left err -> errorWithoutStackTrace (handleParseError err)
+    Right ast -> do
+      ast <- preprocessFile filePath
+      pure $ evalTricu Map.empty ast

 evaluateFileWithContext :: Env -> FilePath -> IO Env
 evaluateFileWithContext env filePath = do
  contents <- readFile filePath
-  let asts = parseTricu contents
-  pure $ evalTricu env asts
+  let tokens = lexTricu contents
+  case parseProgram tokens of
+    Left err -> errorWithoutStackTrace (handleParseError err)
+    Right ast -> do
+      ast <- preprocessFile filePath
+      pure $ evalTricu env ast
+
+preprocessFile :: FilePath -> IO [TricuAST]
+preprocessFile p = preprocessFile' Set.empty p p
+
+preprocessFile' :: Set.Set FilePath -> FilePath -> FilePath -> IO [TricuAST]
+preprocessFile' seen base currentPath = do
+  contents <- readFile currentPath
+  let tokens = lexTricu contents
+  case parseProgram tokens of
+    Left err -> errorWithoutStackTrace (handleParseError err)
+    Right ast ->
+      case processImports seen base currentPath ast of
+        Left err -> errorWithoutStackTrace err
+        Right (nonImports, importPaths) -> do
+          let seen' = Set.insert currentPath seen
+          imported <- concat <$> mapM (processImportPath seen' base) importPaths
+          pure $ imported ++ nonImports
+  where
+    processImportPath seen base (path, name, importPath) = do
+      ast <- preprocessFile' seen base importPath
+      pure $ map (nsDefinition (if name == "!Local" then "" else name))
+           $ filter (not . isImp) ast
+    isImp (SImport _ _) = True
+    isImp _ = False
+
+makeRelativeTo :: FilePath -> FilePath -> FilePath
+makeRelativeTo f i =
+    let d = takeDirectory f
+    in normalise $ d </> i
+
+nsDefinitions :: String -> [TricuAST] -> [TricuAST]
+nsDefinitions moduleName = map (nsDefinition moduleName)
+
+nsDefinition :: String -> TricuAST -> TricuAST
+nsDefinition "" def = def
+nsDefinition moduleName (SDef name args body)
+  | isPrefixed name = SDef name args (nsBody moduleName body)
+  | otherwise = SDef (nsVariable moduleName name)
+                        args (nsBody moduleName body)
+nsDefinition moduleName other =
+  nsBody moduleName other
+
+nsBody :: String -> TricuAST -> TricuAST
+nsBody moduleName (SVar name)
+  | isPrefixed name = SVar name
+  | otherwise = SVar (nsVariable moduleName name)
+nsBody moduleName (SApp func arg) =
+  SApp (nsBody moduleName func) (nsBody moduleName arg)
+nsBody moduleName (SLambda args body) =
+  SLambda args (nsBodyScoped moduleName args body)
+nsBody moduleName (SList items) =
+  SList (map (nsBody moduleName) items)
+nsBody moduleName (TFork left right) =
+  TFork (nsBody moduleName left) (nsBody moduleName right)
+nsBody moduleName (TStem subtree) =
+  TStem (nsBody moduleName subtree)
+nsBody moduleName (SDef name args body)
+  | isPrefixed name = SDef name args (nsBody moduleName body)
+  | otherwise = SDef (nsVariable moduleName name)
+                        args (nsBody moduleName body)
+nsBody _ other = other
+
+nsBodyScoped :: String -> [String] -> TricuAST -> TricuAST
+nsBodyScoped moduleName args body = case body of
+  SVar name ->
+    if name `elem` args
+      then SVar name
+      else nsBody moduleName (SVar name)
+  SApp func arg ->
+    SApp (nsBodyScoped moduleName args func) (nsBodyScoped moduleName args arg)
+  SLambda innerArgs innerBody ->
+    SLambda innerArgs (nsBodyScoped moduleName (args ++ innerArgs) innerBody)
+  SList items ->
+    SList (map (nsBodyScoped moduleName args) items)
+  TFork left right ->
+    TFork (nsBodyScoped moduleName args left)
+          (nsBodyScoped moduleName args right)
+  TStem subtree ->
+    TStem (nsBodyScoped moduleName args subtree)
+  SDef name innerArgs innerBody ->
+    SDef (nsVariable moduleName name) innerArgs
+         (nsBodyScoped moduleName (args ++ innerArgs) innerBody)
+  other -> other
+
+isPrefixed :: String -> Bool
+isPrefixed name = '.' `elem` name
+
+nsVariable :: String -> String -> String
+nsVariable "" name = name
+nsVariable moduleName name = moduleName ++ "." ++ name
--- a/src/Lexer.hs
+++ b/src/Lexer.hs
@ -3,6 +3,7 @@ module Lexer where
 import Research

 import Control.Monad               (void)
+import Data.Functor                (($>))
 import Data.Void
 import Text.Megaparsec
 import Text.Megaparsec.Char hiding (space)
@ -12,21 +13,107 @@ import qualified Data.Set as Set

 type Lexer = Parsec Void String

+tricuLexer :: Lexer [LToken]
+tricuLexer = do
+  sc
+  header <- many $ do
+    tok <- choice
+      [ try lImport
+      , lnewline
+      ]
+    sc
+    pure tok
+  tokens <- many $ do
+    tok <- choice tricuLexer'
+    sc
+    pure tok
+  sc
+  eof
+  pure (header ++ tokens)
+  where
+    tricuLexer' =
+      [ try lnewline
+      , try namespace
+      , try dot
+      , try identifier
+      , try keywordT
+      , try integerLiteral
+      , try stringLiteral
+      , assign
+      , colon
+      , openParen
+      , closeParen
+      , openBracket
+      , closeBracket
+      ]
+
+lexTricu :: String -> [LToken]
+lexTricu input = case runParser tricuLexer "" input of
+  Left err -> errorWithoutStackTrace $ "Lexical error:\n" ++ errorBundlePretty err
+  Right tokens -> tokens
+
+
 keywordT :: Lexer LToken
-keywordT = string "t" *> notFollowedBy alphaNumChar *> pure LKeywordT
+keywordT = string "t" *> notFollowedBy alphaNumChar $> LKeywordT

 identifier :: Lexer LToken
 identifier = do
-  first <- letterChar <|> char '_'
-  rest  <- many $ letterChar 
-              <|> digitChar 
-              <|> char '_' <|> char '-' <|> char '?' <|> char '!'
-              <|> char '$' <|> char '#' <|> char '@' <|> char '%'
+  first <- lowerChar <|> char '_'
+  rest  <- many $ letterChar
+              <|> digitChar <|> char '_' <|> char '-' <|> char '?'
+              <|> char '$'  <|> char '#' <|> char '@' <|> char '%'
  let name = first : rest
-  if (name == "t" || name == "__result")
-    then fail "Keywords (`t`, `__result`) cannot be used as an identifier"
+  if name == "t" || name == "!result"
+    then fail "Keywords (`t`, `!result`) cannot be used as an identifier"
    else return (LIdentifier name)

+namespace :: Lexer LToken
+namespace = do
+  name <- try (string "!Local") <|> do
+    first <- upperChar
+    rest <- many (letterChar <|> digitChar)
+    return (first:rest)
+  return (LNamespace name)
+
+dot :: Lexer LToken
+dot = char '.' $> LDot
+
+lImport :: Lexer LToken
+lImport = do
+  _ <- string "!import"
+  space1
+  LStringLiteral path <- stringLiteral
+  space1
+  LNamespace name <- namespace
+  return (LImport path name)
+
+assign :: Lexer LToken
+assign = char '=' $> LAssign
+
+colon :: Lexer LToken
+colon = char ':' $> LColon
+
+openParen :: Lexer LToken
+openParen = char '(' $> LOpenParen
+
+closeParen :: Lexer LToken
+closeParen = char ')' $> LCloseParen
+
+openBracket :: Lexer LToken
+openBracket = char '[' $> LOpenBracket
+
+closeBracket :: Lexer LToken
+closeBracket = char ']' $> LCloseBracket
+
+lnewline :: Lexer LToken
+lnewline = char '\n' $> LNewline
+
+sc :: Lexer ()
+sc = space
+  (void $ takeWhile1P (Just "space") (\c -> c == ' ' || c == '\t'))
+  (skipLineComment "--")
+  (skipBlockComment "|-" "-|")
+
 integerLiteral :: Lexer LToken
 integerLiteral = do
  num <- some digitChar
@ -35,67 +122,22 @@ integerLiteral = do
 stringLiteral :: Lexer LToken
 stringLiteral = do
  char '"'
-  content <- many (noneOf ['"'])
-  char '"' --"
+  content <- manyTill Lexer.charLiteral (char '"')
  return (LStringLiteral content)

-assign :: Lexer LToken
-assign = char '=' *> pure LAssign
-
-colon :: Lexer LToken
-colon = char ':' *> pure LColon
-
-backslash :: Lexer LToken
-backslash = char '\\' *> pure LBackslash
-
-openParen :: Lexer LToken
-openParen = char '(' *> pure LOpenParen
-
-closeParen :: Lexer LToken
-closeParen = char ')' *> pure LCloseParen
-
-openBracket :: Lexer LToken
-openBracket = char '[' *> pure LOpenBracket
-
-closeBracket :: Lexer LToken
-closeBracket = char ']' *> pure LCloseBracket
-
-lnewline :: Lexer LToken
-lnewline = char '\n' *> pure LNewline
-
-sc :: Lexer ()
-sc = space
-  (void $ takeWhile1P (Just "space") (\c -> c == ' ' || c == '\t'))
-  (skipLineComment "--")
-  (skipBlockComment "|-" "-|")
-
-tricuLexer :: Lexer [LToken]
-tricuLexer = do
-  sc
-  tokens <- many $ do
-    tok <- choice tricuLexer'
-    sc
-    pure tok
-  sc
-  eof
-  pure tokens
-    where
-      tricuLexer' = 
-        [ try lnewline
-        , try identifier
-        , try keywordT
-        , try integerLiteral
-        , try stringLiteral
-        , assign
-        , colon
-        , backslash
-        , openParen
-        , closeParen
-        , openBracket
-        , closeBracket
-        ]
-
-lexTricu :: String -> [LToken]
-lexTricu input = case runParser tricuLexer "" input of
-  Left err -> errorWithoutStackTrace $ "Lexical error:\n" ++ errorBundlePretty err
-  Right tokens -> tokens
+charLiteral :: Lexer Char
+charLiteral = escapedChar <|> normalChar
+  where
+    normalChar = noneOf ['"', '\\']
+    escapedChar = do
+      void $ char '\\'
+      c <- oneOf ['n', 't', 'r', 'f', 'b', '\\', '"', '\'']
+      return $ case c of
+        'n'  -> '\n'
+        't'  -> '\t'
+        'r'  -> '\r'
+        'f'  -> '\f'
+        'b'  -> '\b'
+        '\\' -> '\\'
+        '"'  -> '"'
+        '\'' -> '\''
--- a/src/Main.hs
+++ b/src/Main.hs
@ -8,7 +8,9 @@ import Research

 import Control.Monad          (foldM)
 import Control.Monad.IO.Class (liftIO)
+import Data.Version           (showVersion)
 import Text.Megaparsec        (runParser)
+import Paths_tricu            (version)
 import System.Console.CmdArgs

 import qualified Data.Map as Map
@ -52,16 +54,17 @@ decodeMode = TDecode

 main :: IO ()
 main = do
+  let versionStr = "tricu Evaluator and REPL " ++ showVersion version
  args <- cmdArgs $ modes [replMode, evaluateMode, decodeMode]
    &= help "tricu: Exploring Tree Calculus"
    &= program "tricu"
-    &= summary "tricu Evaluator and REPL"
+    &= summary versionStr
+    &= versionArg [explicit, name "version", summary versionStr]
  case args of
    Repl -> do
      putStrLn "Welcome to the tricu REPL"
-      putStrLn "You can exit with `CTRL+D` or the `:_exit` command.`"
-      library <- liftIO $ evaluateFile "./lib/base.tri"
-      repl $ Map.delete "__result" library
+      putStrLn "You can exit with `CTRL+D` or the `!exit` command.`"
+      repl Map.empty
    Evaluate { file = filePaths, form = form } -> do
      result <- case filePaths of
        [] -> do
@ -77,8 +80,7 @@ main = do
      value <- case filePaths of
        [] -> getContents
        (filePath:_) -> readFile filePath
-      library <- liftIO $ evaluateFile "./lib/base.tri"
-      putStrLn $ decodeResult $ result $ evalTricu library $ parseTricu value
+      putStrLn $ decodeResult $ result $ evalTricu Map.empty $ parseTricu value

 runTricu :: String -> T
 runTricu input =
--- a/src/Parser.hs
+++ b/src/Parser.hs
@ -3,12 +3,12 @@ module Parser where
 import Lexer
 import Research

-import Control.Monad (void)
+import Control.Monad             (void)
 import Control.Monad.State
-import Data.List.NonEmpty (toList)
-import Data.Void (Void)
+import Data.List.NonEmpty        (toList)
+import Data.Void                 (Void)
 import Text.Megaparsec
-import Text.Megaparsec.Error (ParseErrorBundle, errorBundlePretty)
+import Text.Megaparsec.Error     (ParseErrorBundle, errorBundlePretty)
 import qualified Data.Set as Set

 data PState = PState
@ -74,9 +74,22 @@ parseSingle input =
 parseProgramM :: ParserM [TricuAST]
 parseProgramM = do
  skipMany topLevelNewline
+  importNodes <- many (do
+    node <- parseImportM
+    skipMany topLevelNewline
+    return node)
+  skipMany topLevelNewline
  exprs <- sepEndBy parseOneExpression (some topLevelNewline)
  skipMany topLevelNewline
-  return exprs
+  return (importNodes ++ exprs)
+
+parseImportM :: ParserM TricuAST
+parseImportM = do
+  LImport filePath moduleName <- satisfyM isImport
+  pure (SImport filePath moduleName)
+  where
+    isImport (LImport _ _) = True
+    isImport _             = False

 parseOneExpression :: ParserM TricuAST
 parseOneExpression = scnParserM *> parseExpressionM
@ -117,7 +130,6 @@ parseFunctionM = do
 parseLambdaM :: ParserM TricuAST
 parseLambdaM = do
  let ident = (\case LIdentifier _ -> True; _ -> False)
-  _      <- satisfyM (== LBackslash)
  params <- some (satisfyM ident)
  _      <- satisfyM (== LColon)
  scnParserM
@ -132,11 +144,11 @@ parseLambdaExpressionM = choice

 parseAtomicLambdaM :: ParserM TricuAST
 parseAtomicLambdaM = choice
-  [ parseVarM
+  [ try parseLambdaM
+  , parseVarM
  , parseTreeLeafM
  , parseLiteralM
  , parseListLiteralM
-  , try parseLambdaM
  , between (satisfyM (== LOpenParen)) (satisfyM (== LCloseParen)) parseLambdaExpressionM
  ]

@ -192,7 +204,8 @@ parseTreeLeafOrParenthesizedM = choice

 parseAtomicM :: ParserM TricuAST
 parseAtomicM = choice
-  [ parseVarM
+  [ try parseLambdaM
+  , parseVarM
  , parseTreeLeafM
  , parseListLiteralM
  , parseGroupedM
@ -242,12 +255,19 @@ parseSingleItemM = do

 parseVarM :: ParserM TricuAST
 parseVarM = do
-  satisfyM (\case LIdentifier _ -> True; _ -> False) >>= \case
+  token <- satisfyM (\case
+    LNamespace _ -> True
+    LIdentifier _ -> True
+    _ -> False)
+  case token of
+    LNamespace ns -> do
+      _ <- satisfyM (== LDot)
+      LIdentifier name <- satisfyM (\case LIdentifier _ -> True; _ -> False)
+      pure $ SVar (ns ++ "." ++ name)
    LIdentifier name
-      | name == "t" || name == "__result" ->
+      | name == "t" || name == "!result" ->
        fail ("Reserved keyword: " ++ name ++ " cannot be assigned.")
-      | otherwise                         ->
-         pure (SVar name)
+      | otherwise -> pure (SVar name)
    _ -> fail "Unexpected token while parsing variable"

 parseIntLiteralM :: ParserM TricuAST
--- a/src/REPL.hs
+++ b/src/REPL.hs
@ -6,56 +6,185 @@ import Lexer
 import Parser
 import Research

-import Control.Exception         (SomeException, catch)
+import Control.Exception         (IOException, SomeException, catch
+                                 , displayException)
+import Control.Monad             (forM_)
+import Control.Monad.Catch       (handle, MonadCatch)
 import Control.Monad.IO.Class    (liftIO)
-import Data.Char                 (isSpace)
-import Data.List                 (dropWhile, dropWhileEnd, intercalate)
+import Control.Monad.Trans.Class (lift)
+import Control.Monad.Trans.Maybe (MaybeT(..), runMaybeT)
+import Data.Char                 (isSpace, isUpper)
+import Data.List                 (dropWhile, dropWhileEnd, isPrefixOf)
+import Data.Version (showVersion)
+import Paths_tricu (version)
 import System.Console.Haskeline

 import qualified Data.Map as Map
+import qualified Data.Text as T
+import qualified Data.Text.IO as T

 repl :: Env -> IO ()
-repl env = runInputT defaultSettings (loop env)
+repl env = runInputT settings (withInterrupt (loop env Decode))
  where
-    loop :: Env -> InputT IO ()
-    loop env = do
-      minput <- getInputLine "tricu < "
-      if
-        | Nothing <- minput                     -> outputStrLn "Exiting tricu"
-        | Just s  <- minput, strip s == "!exit" -> outputStrLn "Exiting tricu"
-        | Just s  <- minput, strip s == ""      -> do
-          outputStrLn ""
-          loop env
-        | Just s  <- minput, strip s == "!load" -> do
-          path <- getInputLine "File path to load < "
-          if
-            | Nothing <- path -> do
-              outputStrLn "No input received; stopping import."
-              loop env
-            | Just p  <- path -> do
-              loadedEnv <- liftIO $ evaluateFileWithContext env (strip p) `catch` \e -> errorHandler env e
-              loop $ Map.delete "__result" (Map.union loadedEnv env)
-        | Just s <- minput -> do
-          if
-            | take 2 s == "--" -> loop env
-            | otherwise -> do
-              newEnv <- liftIO $ processInput env s `catch` errorHandler env
-              loop newEnv
+    settings :: Settings IO
+    settings = Settings
+      { complete = completeWord Nothing " \t" completeCommands
+      , historyFile = Just "~/.local/state/tricu/history"
+      , autoAddHistory = True
+      }

-    processInput :: Env -> String -> IO Env
-    processInput env input = do
+    completeCommands :: String -> IO [Completion]
+    completeCommands str = return $ map simpleCompletion $
+      filter (str `isPrefixOf`) commands
+      where
+        commands = [ "!exit"
+                   , "!output"
+                   , "!definitions"
+                   , "!import"
+                   , "!clear"
+                   , "!save"
+                   , "!reset"
+                   , "!version"
+                   ]
+
+    loop :: Env -> EvaluatedForm -> InputT IO ()
+    loop env form = handle (interruptHandler env form) $ do
+      minput <- getInputLine "tricu < "
+      case minput of
+        Nothing -> outputStrLn "Exiting tricu"
+        Just s
+          | strip s == "" -> loop env form
+          | strip s == "!exit" -> outputStrLn "Exiting tricu"
+          | strip s == "!clear" -> do
+              liftIO $ putStr "\ESC[2J\ESC[H"
+              loop env form
+          | strip s == "!reset" -> do
+              outputStrLn "Environment reset to initial state"
+              loop Map.empty form
+          | strip s == "!version" -> do
+              outputStrLn $ "tricu version " ++ showVersion version
+              loop env form
+          | "!save" `isPrefixOf` strip s -> handleSave env form
+          | strip s == "!output" -> handleOutput env form
+          | strip s == "!definitions" -> do
+              let defs = Map.keys $ Map.delete "!result" env
+              if null defs
+                then outputStrLn "No definitions discovered."
+                else do
+                  outputStrLn "Available definitions:"
+                  mapM_ outputStrLn defs
+              loop env form
+          | "!import" `isPrefixOf` strip s -> handleImport env form
+          | take 2 s == "--" -> loop env form
+          | otherwise -> do
+              newEnv <- liftIO $ processInput env s form `catch` errorHandler env
+              loop newEnv form
+
+    handleOutput :: Env -> EvaluatedForm -> InputT IO ()
+    handleOutput env currentForm = do
+      let formats = [Decode, TreeCalculus, FSL, AST, Ternary, Ascii]
+      outputStrLn "Available output formats:"
+      mapM_ (\(i, f) -> outputStrLn $ show i ++ ". " ++ show f)
+        (zip [1..] formats)
+
+      result <- runMaybeT $ do
+        input <- MaybeT $ getInputLine "Select output format (1-6) < "
+        case reads input of
+          [(n, "")] | n >= 1 && n <= 6 ->
+            return $ formats !! (n-1)
+          _ -> MaybeT $ return Nothing
+
+      case result of
+        Nothing -> do
+          outputStrLn "Invalid selection. Keeping current output format."
+          loop env currentForm
+        Just newForm -> do
+          outputStrLn $ "Output format changed to: " ++ show newForm
+          loop env newForm
+
+    handleImport :: Env -> EvaluatedForm -> InputT IO ()
+    handleImport env form = do
+      res <- runMaybeT $ do
+        let fset = setComplete completeFilename defaultSettings
+        path <- MaybeT $ runInputT fset $
+          getInputLineWithInitial "File path to load < " ("", "")
+
+        text <- MaybeT $ liftIO $ handle (\e -> do
+            putStrLn $ "Error reading file: " ++ displayException (e :: IOException)
+            return Nothing
+          ) $ Just <$> readFile (strip path)
+
+        case parseProgram (lexTricu text) of
+          Left err -> do
+            lift $ outputStrLn $ "Parse error: " ++ handleParseError err
+            MaybeT $ return Nothing
+          Right ast -> do
+            ns <- MaybeT $ runInputT defaultSettings $
+              getInputLineWithInitial "Namespace (or !Local for no namespace) < " ("", "")
+
+            let name = strip ns
+            if (name /= "!Local" && (null name || not (isUpper (head name)))) then do
+                lift $ outputStrLn "Namespace must start with an uppercase letter"
+                MaybeT $ return Nothing
+            else do
+                prog <- liftIO $ preprocessFile (strip path)
+                let code = case name of
+                      "!Local" -> prog
+                      _ -> nsDefinitions name prog
+                    env' = evalTricu env code
+                return env'
+      case res of
+        Nothing -> do
+          outputStrLn "Import cancelled"
+          loop env form
+        Just env' ->
+          loop (Map.delete "!result" env') form
+
+    interruptHandler :: Env -> EvaluatedForm -> Interrupt -> InputT IO ()
+    interruptHandler env form _ = do
+      outputStrLn "Interrupted with CTRL+C\n\
+                  \You can use the !exit command or CTRL+D to exit"
+      loop env form
+
+    processInput :: Env -> String -> EvaluatedForm -> IO Env
+    processInput env input form = do
      let asts   = parseTricu input
          newEnv = evalTricu env asts
-      if
-        | Just r <- Map.lookup "__result" newEnv -> do
-          putStrLn $ "tricu > " ++ decodeResult r
-        | otherwise -> return ()
+      case Map.lookup "!result" newEnv of
+        Just r -> do
+          putStrLn $ "tricu > " ++ formatResult form r
+        Nothing -> pure ()
      return newEnv
-    
+
    errorHandler :: Env -> SomeException -> IO (Env)
    errorHandler env e = do
      putStrLn $ "Error: " ++ show e
      return env
-    
+
    strip :: String -> String
    strip = dropWhileEnd isSpace . dropWhile isSpace
+
+    handleSave :: Env -> EvaluatedForm -> InputT IO ()
+    handleSave env form = do
+      let fset = setComplete completeFilename defaultSettings
+      path <- runInputT fset $
+        getInputLineWithInitial "File to save < " ("", "")
+
+      case path of
+        Nothing -> do
+          outputStrLn "Save cancelled"
+          loop env form
+        Just p -> do
+          let definitions = Map.toList $ Map.delete "!result" env
+              filepath = strip p
+
+          outputStrLn "Starting save..."
+          liftIO $ writeFile filepath ""
+          outputStrLn "File created..."
+          forM_ definitions $ \(name, value) -> do
+            let content = name ++ " = " ++ formatResult TreeCalculus value ++ "\n"
+            outputStrLn $ "Writing definition: " ++ name ++ " with length " ++ show (length content)
+            liftIO $ appendFile filepath content
+          outputStrLn $ "Saved " ++ show (length definitions) ++ " definitions to " ++ p
+
+          loop env form
--- a/src/Research.hs
+++ b/src/Research.hs
@ -1,6 +1,5 @@
 module Research where

-import Control.Monad.State
 import Data.List                 (intercalate)
 import Data.Map                  (Map)
 import Data.Text                 (Text, replace)
@ -26,22 +25,25 @@ data TricuAST
  | TFork TricuAST TricuAST
  | SLambda [String] TricuAST
  | SEmpty
+  | SImport String String
  deriving (Show, Eq, Ord)

 -- Lexer Tokens
 data LToken
  = LKeywordT
  | LIdentifier String
+  | LNamespace String
  | LIntegerLiteral Int
  | LStringLiteral String
  | LAssign
  | LColon
-  | LBackslash
+  | LDot
  | LOpenParen
  | LCloseParen
  | LOpenBracket
  | LCloseBracket
  | LNewline
+  | LImport String String
  deriving (Show, Eq, Ord)

 -- Output formats
@ -49,17 +51,26 @@ data EvaluatedForm = TreeCalculus | FSL | AST | Ternary | Ascii | Decode
  deriving (Show, Data, Typeable)

 -- Environment containing previously evaluated TC terms
-type Env = Map.Map String T 
+type Env = Map.Map String T

-- Tree Calculus Reduction
+-- Tree Calculus Reduction Rules
+{- 
+  The t operator is left associative.
+  1.  t  t      a b       -> a
+  2.  t (t a)   b c       -> a c (b c)
+  3a. t (t a b) c t       -> a
+  3b. t (t a b) c (t u)   -> b u
+  3c. t (t a b) c (t u v) -> c u v
+-}
 apply :: T -> T -> T
-apply Leaf b                            = Stem b
-apply (Stem a) b                        = Fork a b
-apply (Fork Leaf a) _                   = a
-apply (Fork (Stem a1) a2) b             = apply (apply a1 b) (apply a2 b)
-apply (Fork (Fork a1 a2) a3) Leaf       = a1
-apply (Fork (Fork a1 a2) a3) (Stem u)   = apply a2 u
-apply (Fork (Fork a1 a2) a3) (Fork u v) = apply (apply a3 u) v
+apply (Fork Leaf a)         _         = a
+apply (Fork (Stem a) b) c             = apply (apply a c) (apply b c)
+apply (Fork (Fork a  b) c)  Leaf      = a
+apply (Fork (Fork a  b) c) (Stem u)   = apply b u
+apply (Fork (Fork a  b) c) (Fork u v) = apply (apply c u) v
+-- Left associative `t`
+apply  Leaf b                         = Stem b
+apply (Stem a) b                      = Fork a b

 -- Booleans
 _false :: T
@ -73,7 +84,7 @@ _not = Fork (Fork _true (Fork Leaf _false)) Leaf

 -- Marshalling
 ofString :: String -> T
-ofString str = ofList (map ofNumber (map fromEnum str))
+ofString str = ofList $ map (ofNumber . fromEnum) str

 ofNumber :: Int -> T
 ofNumber 0 = Leaf
@ -83,8 +94,7 @@ ofNumber n =
    (ofNumber (n `div` 2))

 ofList :: [T] -> T
-ofList [] = Leaf
-ofList (x:xs) = Fork x (ofList xs)
+ofList = foldr Fork Leaf

 toNumber :: T -> Either String Int
 toNumber Leaf = Right 0
@ -118,11 +128,11 @@ formatResult Ascii        = toAscii
 formatResult Decode       = decodeResult

 toSimpleT :: String -> String
-toSimpleT s = T.unpack 
+toSimpleT s = T.unpack
  $ replace "Fork" "t"
  $ replace "Stem" "t"
  $ replace "Leaf" "t"
-  $ (T.pack s)
+  $ T.pack s

 toTernaryString :: T -> String
 toTernaryString Leaf = "0"
@ -149,8 +159,18 @@ toAscii tree = go tree "" True
        ++ go right (prefix ++ (if isLast then "    " else "|   ")) True

 decodeResult :: T -> String
-decodeResult tc
-  | Right num  <- toNumber tc = show num
-  | Right str  <- toString tc = "\"" ++ str ++ "\""
-  | Right list <- toList tc   = "[" ++ intercalate ", " (map decodeResult list) ++ "]"
-  | otherwise                 = formatResult TreeCalculus tc
+decodeResult Leaf = "t"
+decodeResult tc =
+  case (toString tc, toList tc, toNumber tc) of
+    (Right s, _, _) | all isCommonChar s -> "\"" ++ s ++ "\""
+    (_, _, Right n) -> show n
+    (_, Right xs@(_:_), _) -> "[" ++ intercalate ", " (map decodeResult xs) ++ "]"
+    (_, Right [], _) -> "[]"
+    _ -> formatResult TreeCalculus tc
+  where
+    isCommonChar c =
+      let n = fromEnum c
+      in (n >= 32 && n <= 126)
+         || n == 9
+         || n == 10
+         || n == 13
--- a/test/Spec.hs
+++ b/test/Spec.hs
@ -7,12 +7,13 @@ import Parser
 import REPL
 import Research

-import Control.Exception (evaluate, try, SomeException)
+import Control.Exception      (evaluate, try, SomeException)
 import Control.Monad.IO.Class (liftIO)
+import Data.List              (isInfixOf)
 import Test.Tasty
 import Test.Tasty.HUnit
 import Test.Tasty.QuickCheck
-import Text.Megaparsec (runParser)
+import Text.Megaparsec        (runParser)

 import qualified Data.Map as Map
 import qualified Data.Set as Set
@ -29,9 +30,11 @@ tests = testGroup "Tricu Tests"
  , parser
  , simpleEvaluation
  , lambdas
-  , baseLibrary
+  , providedLibraries
  , fileEval
+  , modules
  , demos
+  , decoding
  ]

 lexer :: TestTree
@ -48,7 +51,22 @@ lexer = testGroup "Lexer Tests"

  , testCase "Lex escaped characters in strings" $ do
      let input = "\"hello\\nworld\""
-          expect = Right [LStringLiteral "hello\\nworld"]
+          expect = Right [LStringLiteral "hello\nworld"]
+      runParser tricuLexer "" input @?= expect
+
+  , testCase "Lex multiple escaped characters in strings" $ do
+      let input = "\"tab:\\t newline:\\n quote:\\\" backslash:\\\\\""
+          expect = Right [LStringLiteral "tab:\t newline:\n quote:\" backslash:\\"]
+      runParser tricuLexer "" input @?= expect
+
+  , testCase "Lex escaped characters in string literals" $ do
+      let input = "x = \"line1\\nline2\\tindented\""
+          expect = Right [LIdentifier "x", LAssign, LStringLiteral "line1\nline2\tindented"]
+      runParser tricuLexer "" input @?= expect
+
+  , testCase "Lex empty string with escape sequence" $ do
+      let input = "\"\\\"\""
+          expect = Right [LStringLiteral "\""]
      runParser tricuLexer "" input @?= expect

  , testCase "Lex mixed literals" $ do
@ -70,9 +88,9 @@ lexer = testGroup "Lexer Tests"
        Right i -> i @?= expect

  , testCase "Error when using invalid characters in identifiers" $ do
-        case (runParser tricuLexer "" "__result = 5") of
+        case (runParser tricuLexer "" "!result = 5") of
          Left _ -> return ()
-          Right _ -> assertFailure "Expected failure when trying to assign the value of __result"
+          Right _ -> assertFailure "Expected failure when trying to assign the value of !result"
  ]

 parser :: TestTree
@ -84,7 +102,7 @@ parser = testGroup "Parser Tests"
        Right _ -> assertFailure "Expected failure when trying to assign the value of T"

  , testCase "Parse function definitions" $ do
-      let input = "x = (\\a b c : a)"
+      let input = "x = (a b c : a)"
          expect = SDef "x" [] (SLambda ["a"] (SLambda ["b"] (SLambda ["c"] (SVar "a"))))
      parseSingle input @?= expect

@ -104,7 +122,7 @@ parser = testGroup "Parser Tests"
      parseSingle input @?= expect

  , testCase "Parse function with applications" $ do
-      let input  = "f = (\\x : t x)"
+      let input  = "f = (x : t x)"
          expect = SDef "f" [] (SLambda ["x"] (SApp TLeaf (SVar "x")))
      parseSingle input @?= expect

@ -146,22 +164,22 @@ parser = testGroup "Parser Tests"
      parseSingle input @?= expect

  , testCase "Parse nested parentheses in function body" $ do
-      let input  = "f = (\\x : t (t (t t)))"
+      let input  = "f = (x : t (t (t t)))"
          expect = SDef "f" [] (SLambda ["x"] (SApp TLeaf (SApp TLeaf (SApp TLeaf TLeaf))))
      parseSingle input @?= expect

  , testCase "Parse lambda abstractions" $ do
-      let input  = "(\\a : a)"
+      let input  = "(a : a)"
          expect = (SLambda ["a"] (SVar "a"))
      parseSingle input @?= expect

  , testCase "Parse multiple arguments to lambda abstractions" $ do
-      let input  = "x = (\\a b : a)"
+      let input  = "x = (a b : a)"
          expect = SDef "x" [] (SLambda ["a"] (SLambda ["b"] (SVar "a")))
      parseSingle input @?= expect

  , testCase "Grouping T terms with parentheses in function application" $ do
-      let input  = "x = (\\a : a)\nx (t)"
+      let input  = "x = (a : a)\nx (t)"
          expect = [SDef "x" [] (SLambda ["a"] (SVar "a")),SApp (SVar "x") TLeaf]
      parseTricu input @?= expect

@ -256,7 +274,7 @@ simpleEvaluation = testGroup "Evaluation Tests"

  , testCase "Apply identity to Boolean Not" $ do
      let not = "(t (t (t t) (t t t)) t)"
-      let input = "x = (\\a : a)\nx " ++ not
+      let input = "x = (a : a)\nx " ++ not
          env = evalTricu Map.empty (parseTricu input)
      result env @?= Fork (Fork (Stem Leaf) (Fork Leaf Leaf)) Leaf
  ]
@ -264,214 +282,182 @@ simpleEvaluation = testGroup "Evaluation Tests"
 lambdas :: TestTree
 lambdas = testGroup "Lambda Evaluation Tests"
  [ testCase "Lambda Identity Function" $ do
-      let input = "id = (\\x : x)\nid t"
+      let input = "id = (x : x)\nid t"
      runTricu input @?= "Leaf"

  , testCase "Lambda Constant Function (K combinator)" $ do
-      let input = "k = (\\x y : x)\nk t (t t)"
+      let input = "k = (x y : x)\nk t (t t)"
      runTricu input @?= "Leaf"

  , testCase "Lambda Application with Variable" $ do
-      let input = "id = (\\x : x)\nval = t t\nid val"
+      let input = "id = (x : x)\nval = t t\nid val"
      runTricu input @?= "Stem Leaf"

  , testCase "Lambda Application with Multiple Arguments" $ do
-      let input = "apply = (\\f x y : f x y)\nk = (\\a b : a)\napply k t (t t)"
+      let input = "apply = (f x y : f x y)\nk = (a b : a)\napply k t (t t)"
      runTricu input @?= "Leaf"

   , testCase "Nested Lambda Application" $ do
-      let input = "apply = (\\f x y : f x y)\nid = (\\x : x)\napply (\\f x : f x) id t"
+      let input = "apply = (f x y : f x y)\nid = (x : x)\napply (f x : f x) id t"
      runTricu input @?= "Leaf"

  , testCase "Lambda with a complex body" $ do
-      let input = "f = (\\x : t (t x))\nf t"
+      let input = "f = (x : t (t x))\nf t"
      runTricu input @?= "Stem (Stem Leaf)"

  , testCase "Lambda returning a function" $ do
-      let input = "f = (\\x : (\\y : x))\ng = f t\ng (t t)"
+      let input = "f = (x : (y : x))\ng = f t\ng (t t)"
      runTricu input @?= "Leaf"

  , testCase "Lambda with Shadowing" $ do
-      let input = "f = (\\x : (\\x : x))\nf t (t t)"
+      let input = "f = (x : (x : x))\nf t (t t)"
      runTricu input @?= "Stem Leaf"

   , testCase "Lambda returning another lambda" $ do
-      let input = "k = (\\x : (\\y : x))\nk_app = k t\nk_app (t t)"
+      let input = "k = (x : (y : x))\nk_app = k t\nk_app (t t)"
      runTricu input @?= "Leaf"

   , testCase "Lambda with free variables" $ do
-      let input = "y = t t\nf = (\\x : y)\nf t"
+      let input = "y = t t\nf = (x : y)\nf t"
      runTricu input @?= "Stem Leaf"

   , testCase "SKI Composition" $ do
-      let input = "s = (\\x y z : x z (y z))\nk = (\\x y : x)\ni = (\\x : x)\ncomp = s k i\ncomp t (t t)"
+      let input = "s = (x y z : x z (y z))\nk = (x y : x)\ni = (x : x)\ncomp = s k i\ncomp t (t t)"
      runTricu input @?= "Stem (Stem Leaf)"

   , testCase "Lambda with multiple parameters and application" $ do
-      let input = "f = (\\a b c : t a b c)\nf t (t t) (t t t)"
+      let input = "f = (a b c : t a b c)\nf t (t t) (t t t)"
      runTricu input @?= "Stem Leaf"

   , testCase "Lambda with nested application in the body" $ do
-      let input = "f = (\\x : t (t (t x)))\nf t"
+      let input = "f = (x : t (t (t x)))\nf t"
      runTricu input @?= "Stem (Stem (Stem Leaf))"

    , testCase "Lambda returning a function and applying it" $ do
-        let input = "f = (\\x : (\\y : t x y))\ng = f t\ng (t t)"
+        let input = "f = (x : (y : t x y))\ng = f t\ng (t t)"
        runTricu input @?= "Fork Leaf (Stem Leaf)"

    , testCase "Lambda applying a variable" $ do
-        let input = "id = (\\x : x)\na = t t\nid a"
+        let input = "id = (x : x)\na = t t\nid a"
        runTricu input @?= "Stem Leaf"

    , testCase "Nested lambda abstractions in the same expression" $ do
-        let input = "f = (\\x : (\\y : x y))\ng = (\\z : z)\nf g t"
+        let input = "f = (x : (y : x y))\ng = (z : z)\nf g t"
        runTricu input @?= "Leaf"

-  , testCase "Lambda with a string literal" $ do
-        let input = "f = (\\x : x)\nf \"hello\""
+  , testCase "Lambda applied to string literal" $ do
+        let input = "f = (x : x)\nf \"hello\""
        runTricu input @?= "Fork (Fork Leaf (Fork Leaf (Fork Leaf (Fork (Stem Leaf) (Fork Leaf (Fork (Stem Leaf) (Fork (Stem Leaf) Leaf))))))) (Fork (Fork (Stem Leaf) (Fork Leaf (Fork (Stem Leaf) (Fork Leaf (Fork Leaf (Fork (Stem Leaf) (Fork (Stem Leaf) Leaf))))))) (Fork (Fork Leaf (Fork Leaf (Fork (Stem Leaf) (Fork (Stem Leaf) (Fork Leaf (Fork (Stem Leaf) (Fork (Stem Leaf) Leaf))))))) (Fork (Fork Leaf (Fork Leaf (Fork (Stem Leaf) (Fork (Stem Leaf) (Fork Leaf (Fork (Stem Leaf) (Fork (Stem Leaf) Leaf))))))) (Fork (Fork (Stem Leaf) (Fork (Stem Leaf) (Fork (Stem Leaf) (Fork (Stem Leaf) (Fork Leaf (Fork (Stem Leaf) (Fork (Stem Leaf) Leaf))))))) Leaf))))"


-   , testCase "Lambda with an integer literal" $ do
-        let input = "f = (\\x : x)\nf 42"
+   , testCase "Lambda applied to integer literal" $ do
+        let input = "f = (x : x)\nf 42"
        runTricu input @?= "Fork Leaf (Fork (Stem Leaf) (Fork Leaf (Fork (Stem Leaf) (Fork Leaf (Fork (Stem Leaf) Leaf)))))"

-   , testCase "Lambda with a list literal" $ do
-        let input = "f = (\\x : x)\nf [t (t t)]"
+   , testCase "Lambda applied to list literal" $ do
+        let input = "f = (x : x)\nf [t (t t)]"
        runTricu input @?= "Fork Leaf (Fork (Stem Leaf) Leaf)"
+
+  , testCase "Lambda containing list literal" $ do
+      let input = "(a : [(a)]) 1"
+      runTricu input @?= "Fork (Fork (Stem Leaf) Leaf) Leaf"
  ]

-baseLibrary :: TestTree
-baseLibrary = testGroup "Library Tests"
-  [ testCase "K combinator 1" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "k (t) (t t)"
-          env = evalTricu library (parseTricu input)
-      result env @?= Leaf
-
-  , testCase "K combinator 2" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "k (t t) (t)"
-          env = evalTricu library (parseTricu input)
-      result env @?= Stem Leaf
-
-  , testCase "K combinator 3" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "k (t t t) (t)"
-          env = evalTricu library (parseTricu input)
-      result env @?= Fork Leaf Leaf
-
-  , testCase "S combinator" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "s (t) (t) (t)"
-          env = evalTricu library (parseTricu input)
-      result env @?= Fork Leaf (Stem Leaf)
-
-  , testCase "SKK == I (fully expanded)" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "s k k"
-          env = evalTricu library (parseTricu input)
-      result env @?= Fork (Stem (Stem Leaf)) (Stem Leaf)
-
-  , testCase "I combinator" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "i not?"
-          env = evalTricu library (parseTricu input)
-      result env @?= Fork (Fork (Stem Leaf) (Fork Leaf Leaf)) (Fork Leaf (Fork Leaf Leaf))
-
-  , testCase "Triage test Leaf" $ do
-      library <- evaluateFile "./lib/base.tri"
+providedLibraries :: TestTree
+providedLibraries = testGroup "Library Tests"
+  [ testCase "Triage test Leaf" $ do
+      library <- evaluateFile "./lib/list.tri"
      let input = "test t"
          env = decodeResult $ result $ evalTricu library (parseTricu input)
      env @?= "\"Leaf\""

  , testCase "Triage test (Stem Leaf)" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "test (t t)"
          env = decodeResult $ result $ evalTricu library (parseTricu input)
      env @?= "\"Stem\""

  , testCase "Triage test (Fork Leaf Leaf)" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "test (t t t)"
          env = decodeResult $ result $ evalTricu library (parseTricu input)
      env @?= "\"Fork\""

  , testCase "Boolean NOT: true" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "not? true"
          env = result $ evalTricu library (parseTricu input)
      env @?= Leaf

  , testCase "Boolean NOT: false" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "not? false"
          env = result $ evalTricu library (parseTricu input)
      env @?= Stem Leaf


  , testCase "Boolean AND TF" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "and? (t t) (t)"
          env = evalTricu library (parseTricu input)
      result env @?= Leaf

  , testCase "Boolean AND FT" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "and? (t) (t t)"
          env = evalTricu library (parseTricu input)
      result env @?= Leaf

  , testCase "Boolean AND FF" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "and? (t) (t)"
          env = evalTricu library (parseTricu input)
      result env @?= Leaf

  , testCase "Boolean AND TT" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "and? (t t) (t t)"
          env = evalTricu library (parseTricu input)
      result env @?= Stem Leaf

  , testCase "List head" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "head [(t) (t t) (t t t)]"
          env = evalTricu library (parseTricu input)
      result env @?= Leaf

  , testCase "List tail" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "head (tail (tail [(t) (t t) (t t t)]))"
          env = evalTricu library (parseTricu input)
      result env @?= Fork Leaf Leaf

  , testCase "List map" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "head (tail (map (\\a : (t t t)) [(t) (t) (t)]))"
+      library <- evaluateFile "./lib/list.tri"
+      let input = "head (tail (map (a : (t t t)) [(t) (t) (t)]))"
          env = evalTricu library (parseTricu input)
      result env @?= Fork Leaf Leaf

  , testCase "Empty list check" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "emptyList? []"
          env = evalTricu library (parseTricu input)
      result env @?= Stem Leaf

  , testCase "Non-empty list check" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "not? (emptyList? [(1) (2) (3)])"
          env = evalTricu library (parseTricu input)
      result env @?= Stem Leaf

  , testCase "Concatenate strings" $ do
-      library <- evaluateFile "./lib/base.tri"
-      let input = "lconcat \"Hello, \" \"world!\""
+      library <- evaluateFile "./lib/list.tri"
+      let input = "append \"Hello, \" \"world!\""
          env = decodeResult $ result $ evalTricu library (parseTricu input)
      env @?= "\"Hello, world!\""

  , testCase "Verifying Equality" $ do
-      library <- evaluateFile "./lib/base.tri"
+      library <- evaluateFile "./lib/list.tri"
      let input = "equal? (t t t) (t t t)"
          env = evalTricu library (parseTricu input)
      result env @?= Stem Leaf
@ -488,32 +474,99 @@ fileEval = testGroup "File evaluation tests"
      res @?= Fork (Stem Leaf) Leaf

  , testCase "Mapping and Equality" $ do
-      library <- liftIO $ evaluateFile "./lib/base.tri"
+      library <- liftIO $ evaluateFile "./lib/list.tri"
      fEnv    <- liftIO $ evaluateFileWithContext library "./test/map.tri"
      (mainResult fEnv) @?= Stem Leaf

  , testCase "Eval and decoding string" $ do
-      library <- liftIO $ evaluateFile "./lib/base.tri"
+      library <- liftIO $ evaluateFile "./lib/list.tri"
      res <- liftIO $ evaluateFileWithContext library "./test/string.tri"
      decodeResult (result res) @?= "\"String test!\""
  ]

+modules :: TestTree
+modules = testGroup "Test modules"
+  [ testCase "Detect cyclic dependencies" $ do
+      result <- try (liftIO $ evaluateFileResult "./test/cycle-1.tri") :: IO (Either SomeException T)
+      case result of
+        Left e -> do
+          let errorMsg = show e
+          if "Encountered cyclic import" `isInfixOf` errorMsg
+            then return ()
+            else assertFailure $ "Unexpected error: " ++ errorMsg
+        Right _ -> assertFailure "Expected cyclic dependencies"
+  , testCase "Module imports and namespacing" $ do
+      res <- liftIO $ evaluateFileResult "./test/namespace-A.tri"
+      res @?= Leaf
+  , testCase "Multiple imports" $ do
+      res <- liftIO $ evaluateFileResult "./test/vars-A.tri"
+      res @?= Leaf
+  , testCase "Error on unresolved variable" $ do
+      result <- try (liftIO $ evaluateFileResult "./test/unresolved-A.tri") :: IO (Either SomeException T)
+      case result of
+        Left e -> do
+          let errorMsg = show e
+          if "undefinedVar" `isInfixOf` errorMsg
+            then return ()
+            else assertFailure $ "Unexpected error: " ++ errorMsg
+        Right _ -> assertFailure "Expected unresolved variable error"
+  , testCase "Multi-level imports" $ do
+      res <- liftIO $ evaluateFileResult "./test/multi-level-A.tri"
+      res @?= Leaf
+  , testCase "Lambda expression namespaces" $ do
+      res <- liftIO $ evaluateFileResult "./test/lambda-A.tri"
+      res @?= Leaf
+  , testCase "Local namespace import chain" $ do
+      res <- liftIO $ evaluateFileResult "./test/local-ns/1.tri"
+      res @?=  Fork (Stem Leaf) (Fork (Stem Leaf) Leaf)
+  ]
+
+
+-- All of our demo tests are also module tests
 demos :: TestTree
 demos = testGroup "Test provided demo functionality"
  [ testCase "Structural equality demo" $ do
-      library <- liftIO $ evaluateFile "./lib/base.tri"
-      res     <- liftIO $ evaluateFileWithContext library "./demos/equality.tri"
-      decodeResult (result res) @?= "t t"
+      res     <- liftIO $ evaluateFileResult "./demos/equality.tri"
+      decodeResult res @?= "t t"
  , testCase "Convert values back to source code demo" $ do
-      library <- liftIO $ evaluateFile "./lib/base.tri"
-      res     <- liftIO $ evaluateFileWithContext library "./demos/toSource.tri"
-      decodeResult (result res) @?= "\"(t (t (t t) (t t t)) (t t (t t t)))\""
+      res     <- liftIO $ evaluateFileResult "./demos/toSource.tri"
+      decodeResult res @?= "\"(t (t (t t) (t t t)) (t t (t t t)))\""
  , testCase "Determining the size of functions" $ do
-      library <- liftIO $ evaluateFile "./lib/base.tri"
-      res     <- liftIO $ evaluateFileWithContext library "./demos/size.tri"
-      decodeResult (result res) @?= "454"
+      res     <- liftIO $ evaluateFileResult "./demos/size.tri"
+      decodeResult res @?= "454"
  , testCase "Level Order Traversal demo" $ do
-      library <- liftIO $ evaluateFile "./lib/base.tri"
-      res     <- liftIO $ evaluateFileWithContext library "./demos/levelOrderTraversal.tri"
-      decodeResult (result res) @?= "\"\n1 \n2 3 \n4 5 6 7 \n8 11 10 9 12 \""
+      res     <- liftIO $ evaluateFileResult "./demos/levelOrderTraversal.tri"
+      decodeResult res @?= "\"\n1 \n2 3 \n4 5 6 7 \n8 11 10 9 12 \""
+  ]
+
+decoding :: TestTree
+decoding = testGroup "Decoding Tests"
+  [ testCase "Decode Leaf" $ do
+      decodeResult Leaf @?= "t"
+
+  , testCase "Decode list of non-ASCII numbers" $ do
+      let input = ofList [ofNumber 1, ofNumber 14, ofNumber 6]
+      decodeResult input @?= "[1, 14, 6]"
+
+  , testCase "Decode list of ASCII numbers as a string" $ do
+      let input = ofList [ofNumber 97, ofNumber 98, ofNumber 99]
+      decodeResult input @?= "\"abc\""
+
+  , testCase "Decode small number" $ do
+      decodeResult (ofNumber 42) @?= "42"
+
+  , testCase "Decode large number" $ do
+      decodeResult (ofNumber 9999) @?= "9999"
+
+  , testCase "Decode string in list" $ do
+      let input = ofList [ofString "hello", ofString "world"]
+      decodeResult input @?= "[\"hello\", \"world\"]"
+
+  , testCase "Decode mixed list with strings" $ do
+      let input = ofList [ofString "hello", ofNumber 42, ofString "world"]
+      decodeResult input @?= "[\"hello\", 42, \"world\"]"
+
+  , testCase "Decode nested lists with strings" $ do
+      let input = ofList [ofList [ofString "nested"], ofString "string"]
+      decodeResult input @?= "[[\"nested\"], \"string\"]"
  ]
--- a/test/ascii.tri
+++ b/test/ascii.tri
@ -1 +0,0 @@
-t (t (t (t (t t) (t t t)) t) t t) t
--- a/test/assignment.tri
+++ b/test/assignment.tri
@ -1 +0,0 @@
-x = t (t t) t
--- a/test/comments-1.tri
+++ b/test/comments-1.tri
@ -1,9 +1,9 @@
 -- This is a tricu comment!
 -- t (t t) (t (t t t))
 -- t (t t t) (t t)
-- x = (\a : a)
+-- x = (a : a)
 main = t (t t) t -- Fork (Stem Leaf) Leaf
 -- t t
 -- x
-- x = (\a : a)
+-- x = (a : a)
 -- t
--- a/test/cycle-1.tri
+++ b/test/cycle-1.tri
@ -0,0 +1,4 @@
+
+!import "cycle-2.tri" Cycle2
+
+cycle1 = t Cycle2.cycle2
--- a/test/cycle-2.tri
+++ b/test/cycle-2.tri
@ -0,0 +1,4 @@
+
+!import "cycle-1.tri" Cycle1
+
+cycle2 = t Cycle1.cycle1
--- a/test/lambda-A.tri
+++ b/test/lambda-A.tri
@ -0,0 +1 @@
+main = (x : x) t
--- a/test/local-ns/1.tri
+++ b/test/local-ns/1.tri
@ -0,0 +1,4 @@
+
+!import "2.tri" Two
+
+main = Two.x
--- a/test/local-ns/2.tri
+++ b/test/local-ns/2.tri
@ -0,0 +1,2 @@
+
+!import "3.tri" !Local
--- a/test/local-ns/3.tri
+++ b/test/local-ns/3.tri
@ -0,0 +1 @@
+x = 3
--- a/test/map.tri
+++ b/test/map.tri
@ -1,2 +1,2 @@
-x = map (\i : lconcat "Successfully concatenated " i) [("two strings!")]
+x = map (i : append "Successfully concatenated " i) [("two strings!")]
 main = equal? x [("Successfully concatenated two strings!")]
--- a/test/multi-level-A.tri
+++ b/test/multi-level-A.tri
@ -0,0 +1,2 @@
+!import "multi-level-B.tri" B
+main = B.main
--- a/test/multi-level-B.tri
+++ b/test/multi-level-B.tri
@ -0,0 +1,2 @@
+!import "multi-level-C.tri" C
+main = C.val
--- a/test/multi-level-C.tri
+++ b/test/multi-level-C.tri
@ -0,0 +1 @@
+val = t
--- a/test/named-imports/1.tri
+++ b/test/named-imports/1.tri
@ -0,0 +1,7 @@
+
+!import "lib/base.tri"
+
+!import "test/named-imports/2.tri"
+!import "test/named-imports/3.tri" ThreeRenamed
+
+main = equal? (equal? Two.x 2) (equal? ThreeRenamed.x 3)
--- a/test/named-imports/2.tri
+++ b/test/named-imports/2.tri
@ -0,0 +1,2 @@
+
+x = 2
--- a/test/named-imports/3.tri
+++ b/test/named-imports/3.tri
@ -0,0 +1,2 @@
+
+x = 3
--- a/test/namespace-A.tri
+++ b/test/namespace-A.tri
@ -0,0 +1,2 @@
+!import "namespace-B.tri" B
+main = B.x
--- a/test/namespace-B.tri
+++ b/test/namespace-B.tri
@ -0,0 +1 @@
+x = t
--- a/test/size.tri
+++ b/test/size.tri
@ -1,21 +1,21 @@
-compose = \f g x : f (g x)
+compose = f g x : f (g x)

-succ = y (\self :
+succ = y (self :
  triage
    1
    t
    (triage
      (t (t t))
-      (\_ tail : t t (self tail))
+      (_ tail : t t (self tail))
      t))

-size = (\x :
-  (y (\self x :
+size = (x :
+  (y (self x :
    compose succ
      (triage
-        (\x : x)
+        (x : x)
        self
-        (\x y : compose (self x) (self y))
+        (x y : compose (self x) (self y))
        x)) x 0))

 size size
--- a/test/string.tri
+++ b/test/string.tri
@ -1 +1 @@
-head (map (\i : lconcat "String " i) [("test!")])
+head (map (i : append "String " i) [("test!")])
--- a/test/undefined.tri
+++ b/test/undefined.tri
@ -1 +0,0 @@
-namedTerm = undefinedForTesting
--- a/test/unresolved-A.tri
+++ b/test/unresolved-A.tri
@ -0,0 +1 @@
+main = undefinedVar
--- a/test/vars-A.tri
+++ b/test/vars-A.tri
@ -0,0 +1,6 @@
+
+!import "vars-B.tri" B
+
+!import "vars-C.tri" C
+
+main = B.y (C.z)
--- a/test/vars-B.tri
+++ b/test/vars-B.tri
@ -0,0 +1 @@
+y = x : x
--- a/test/vars-C.tri
+++ b/test/vars-C.tri
@ -0,0 +1 @@
+z = t
--- a/tricu.cabal
+++ b/tricu.cabal
@ -1,7 +1,7 @@
 cabal-version: 1.12

 name:           tricu
-version:        0.11.0
+version:        0.18.1
 description:    A micro-language for exploring Tree Calculus
 author:         James Eversole
 maintainer:     james@eversole.co
@ -26,10 +26,13 @@ executable tricu
    base >=4.7
    , cmdargs
    , containers
+    , exceptions
+    , filepath
    , haskeline
    , megaparsec
    , mtl
    , text
+    , transformers
  other-modules:
    Eval
    FileEval
@ -52,6 +55,8 @@ test-suite tricu-tests
    base
    , cmdargs
    , containers
+    , exceptions
+    , filepath
    , haskeline
    , megaparsec
    , mtl
@ -59,6 +64,7 @@ test-suite tricu-tests
    , tasty-hunit
    , tasty-quickcheck
    , text
+    , transformers
  default-language:    Haskell2010
  other-modules:
    Eval