Somewhat working lambdas

Architectural changes to lambda evaluation and parsing to allow
for correct expression evaluation. Contains several failing AI-generated
tests and we're still failing tests for erroring incomplete definitions

src/Eval.hs

@@ -2,21 +2,27 @@ module Eval where
 
 import Parser
 import Research
-import Data.Set (Set)
-import qualified Data.Set as Set
-import Data.List (foldl')
-import qualified Data.Map as Map
-import Data.Map (Map)
 
-evalSingle :: Map.Map String T -> SaplingAST -> Map.Map String T
+import Data.Map (Map)
+import qualified Data.Map as Map
+import Data.List (foldl')
+import qualified Data.Set as Set
+
+evalSingle :: Map String T -> SaplingAST -> Map String T
 evalSingle env term = case term of
   SFunc name [] body ->
-        let result = evalAST env body
+    let
+        lineNoLambda = eliminateLambda body
+        result = evalAST env lineNoLambda
     in Map.insert name result env
+  SLambda _ body ->
+    let result = evalAST env body
+    in Map.insert "__result" result env
   SApp func arg ->
     let result = apply (evalAST env func) (evalAST env arg)
     in Map.insert "__result" result env
-    SVar name -> case Map.lookup name env of
+  SVar name ->
+    case Map.lookup name env of
       Just value -> Map.insert "__result" value env
       Nothing -> error $ "Variable " ++ name ++ " not defined"
   _ ->
@@ -26,97 +32,63 @@ evalSingle env term = case term of
 evalSapling :: Map String T -> [SaplingAST] -> Map String T
 evalSapling env [] = env
 evalSapling env [lastLine] =
-    let
+    let lastLineNoLambda = eliminateLambda lastLine
-        lastLineNoLambda = eliminateLambda lastLine
         updatedEnv = evalSingle env lastLineNoLambda
     in Map.insert "__result" (result updatedEnv) updatedEnv
 evalSapling env (line:rest) =
-    let
+    let lineNoLambda = eliminateLambda line
-        lineNoLambda = eliminateLambda line
         updatedEnv = evalSingle env lineNoLambda
     in evalSapling updatedEnv rest
 
 evalAST :: Map String T -> SaplingAST -> T
 evalAST env term = case term of
-    SVar name ->
+    SVar name -> case Map.lookup name env of
-        case Map.lookup name env of
         Just value -> value
         Nothing -> error $ "Variable " ++ name ++ " not defined"
     TLeaf -> Leaf
-    TStem t ->
+    TStem t -> Stem (evalAST env t)
-        Stem (evalAST env t)
+    TFork t1 t2 -> Fork (evalAST env t1) (evalAST env t2)
-    TFork t1 t2 ->
+    SApp t1 t2 -> apply (evalAST env t1) (evalAST env t2)
-        Fork (evalAST env t1) (evalAST env t2)
-    SApp t1 t2 ->
-        apply (evalAST env t1) (evalAST env t2)
     SStr str -> toString str
     SInt num -> toNumber num
     SList elems -> toList (map (evalAST Map.empty) elems)
     SFunc name args body ->
         error $ "Unexpected function definition " ++ name
         ++ " in evalAST; define via evalSingle."
-    SLambda {} ->
+    SLambda {} -> error "Internal error: SLambda found in evalAST after elimination."
-        error "Internal error: SLambda found in evalAST after elimination."
-
-result :: Map String T -> T
-result r = case Map.lookup "__result" r of
-    Just a -> a
-    Nothing -> error "No __result field found in provided environment"
-
 
 eliminateLambda :: SaplingAST -> SaplingAST
 eliminateLambda (SLambda (v:vs) body)
   | null vs = lambdaToT v (eliminateLambda body)
-    | otherwise =
+  | otherwise = eliminateLambda (SLambda [v] (SLambda vs body))
-        eliminateLambda (SLambda [v] (SLambda vs body))
+eliminateLambda (SApp f arg) = SApp (eliminateLambda f) (eliminateLambda arg)
-eliminateLambda (SApp f arg) =
+eliminateLambda (TStem t) = TStem (eliminateLambda t)
-    SApp (eliminateLambda f) (eliminateLambda arg)
+eliminateLambda (TFork l r) = TFork (eliminateLambda l) (eliminateLambda r)
-eliminateLambda (TStem t) =
+eliminateLambda (SList xs) = SList (map eliminateLambda xs)
-    TStem (eliminateLambda t)
-eliminateLambda (TFork l r) =
-    TFork (eliminateLambda l) (eliminateLambda r)
-eliminateLambda (SList xs) =
-    SList (map eliminateLambda xs)
-eliminateLambda (SFunc n vs b) =
-    SFunc n vs (eliminateLambda b)
 eliminateLambda other = other
 
 lambdaToT :: String -> SaplingAST -> SaplingAST
 lambdaToT x (SVar y)
   | x == y = tI
 lambdaToT x (SVar y)
-    | x /= y =
+  | x /= y = SApp tK (SVar y)
-        SApp tK (SVar y)
 lambdaToT x t
-    | not (isFree x t) =
+  | not (isFree x t) = SApp tK t
-        SApp tK t
 lambdaToT x (SApp n u)
-    | not (isFree x (SApp n u)) =
+  | not (isFree x (SApp n u)) = SApp tK (SApp (eliminateLambda n) (eliminateLambda u))
-        SApp tK (SApp (eliminateLambda n) (eliminateLambda u))
+lambdaToT x (SApp n u) = SApp (SApp tS (lambdaToT x (eliminateLambda n))) (lambdaToT x (eliminateLambda u))
-lambdaToT x (SApp n u) =
-    SApp
-    (SApp tS (lambdaToT x (eliminateLambda n)))
-    (lambdaToT x (eliminateLambda u))
-lambdaToT x (SApp f args) = lambdaToT x f
 lambdaToT x body
-    | not (isFree x body) =
+  | not (isFree x body) = SApp tK body
-        SApp tK body
+  | otherwise = SApp (SApp tS (lambdaToT x body)) TLeaf
-    | otherwise =
-        SApp
-        (SApp tS (lambdaToT x body))
-        tLeaf
 
-tLeaf :: SaplingAST
+freeVars :: SaplingAST -> Set.Set String
-tLeaf = TLeaf
-
-freeVars :: SaplingAST -> Set String
 freeVars (SVar v) = Set.singleton v
 freeVars (SInt _) = Set.empty
 freeVars (SStr _) = Set.empty
 freeVars (SList xs) = foldMap freeVars xs
-freeVars (SFunc _ _ b) = freeVars b
 freeVars (SApp f arg) = freeVars f <> freeVars arg
 freeVars TLeaf = Set.empty
+freeVars (SFunc _ _ b) = freeVars b
 freeVars (TStem t) = freeVars t
 freeVars (TFork l r) = freeVars l <> freeVars r
 freeVars (SLambda vs b) = foldr Set.delete (freeVars b) vs
@@ -130,11 +102,15 @@ toAST (Stem a) = TStem (toAST a)
 toAST (Fork a b) = TFork (toAST a) (toAST b)
 
 tI :: SaplingAST
-tI = toAST _I
+tI = SApp (SApp TLeaf (SApp TLeaf (SApp TLeaf TLeaf))) TLeaf
 
 tK :: SaplingAST
-tK = toAST _K
+tK = SApp TLeaf TLeaf
 
 tS :: SaplingAST
-tS = toAST _S
+tS = SApp (SApp TLeaf (SApp TLeaf (SApp (SApp TLeaf TLeaf) TLeaf))) TLeaf
 
+result :: Map String T -> T
+result r = case Map.lookup "__result" r of
+    Just a -> a
+    Nothing -> error "No __result field found in provided environment"

src/Lexer.hs

@@ -7,6 +7,7 @@ import Data.Void
 import qualified Data.Set as Set
 
 type Lexer = Parsec Void String
+
 data LToken
   = LKeywordT
   | LIdentifier String
@@ -44,7 +45,7 @@ stringLiteral = do
   if null content
     then fail "Empty string literals are not allowed"
     else do
-      char '"' -- "
+      char '"'
       return (LStringLiteral content)
 
 assign :: Lexer LToken

src/Main.hs

@@ -11,3 +11,6 @@ import Text.Megaparsec (runParser)
 
 main :: IO ()
 main = repl Map.empty --(Map.fromList [("__result", Leaf)])
+
+runSapling :: String -> String
+runSapling s = show $ result (evalSapling Map.empty $ parseSapling s)

src/Parser.hs

@@ -1,10 +1,8 @@
 module Parser where
 
 import Debug.Trace
-
 import Lexer
 import Research hiding (toList)
-
 import Control.Exception (throw)
 import Data.List.NonEmpty (toList)
 import qualified Data.Set as Set
@@ -14,6 +12,7 @@ import Text.Megaparsec.Char
 import Text.Megaparsec.Error (errorBundlePretty, ParseErrorBundle)
 
 type Parser = Parsec Void [LToken]
+
 data SaplingAST
   = SVar String
   | SInt Int
@@ -45,6 +44,7 @@ parseExpression :: Parser SaplingAST
 parseExpression = choice
   [ try parseFunction
   , try parseLambda
+  , try parseLambdaExpression
   , try parseListLiteral
   , try parseApplication
   , try parseTreeTerm
@@ -59,6 +59,19 @@ parseFunction = do
   body <- parseExpression
   return (SFunc name (map getIdentifier args) body)
 
+parseAtomicBase :: Parser SaplingAST
+parseAtomicBase = choice
+    [ try parseVarWithoutAssignment
+    , parseTreeLeaf
+    , parseGrouped
+    ]
+parseVarWithoutAssignment :: Parser SaplingAST
+parseVarWithoutAssignment = do
+    LIdentifier name <- satisfy isIdentifier
+    if (name == "t" || name == "__result")
+    then fail $ "Reserved keyword: " ++ name ++ " cannot be assigned."
+    else notFollowedBy (satisfy (== LAssign)) *> return (SVar name)
+
 parseLambda :: Parser SaplingAST
 parseLambda = between (satisfy (== LOpenParen)) (satisfy (== LCloseParen)) $ do
   satisfy (== LBackslash)
@@ -81,6 +94,7 @@ parseAtomicLambda = choice
   , parseTreeLeaf
   , parseLiteral
   , parseListLiteral
+  , try parseLambda
   , between (satisfy (== LOpenParen)) (satisfy (== LCloseParen)) parseLambdaExpression
   ]
 
@@ -102,13 +116,6 @@ isTreeTerm (TStem _)   = True
 isTreeTerm (TFork _ _) = True
 isTreeTerm _ = False
 
-parseAtomicBase :: Parser SaplingAST
-parseAtomicBase = choice
-  [ parseVar
-  , parseTreeLeaf
-  , parseGrouped
-  ]
-
 parseTreeLeaf :: Parser SaplingAST
 parseTreeLeaf = satisfy isKeywordT *> notFollowedBy (satisfy (== LAssign)) *> pure TLeaf
 
@@ -147,7 +154,6 @@ parseAtomic = choice
   , parseLiteral
   ]
 
-
 parseGrouped :: Parser SaplingAST
 parseGrouped = between (satisfy (== LOpenParen)) (satisfy (== LCloseParen)) parseExpression
 
@@ -218,21 +224,16 @@ parseStrLiteral = do
 -- Boolean Helpers
 isKeywordT (LKeywordT) = True
 isKeywordT _ = False
-
 isIdentifier (LIdentifier _) = True
 isIdentifier _ = False
-
 isIntegerLiteral (LIntegerLiteral _) = True
 isIntegerLiteral _ = False
-
 isStringLiteral (LStringLiteral _) = True
 isStringLiteral _ = False
-
 isLiteral (LIntegerLiteral _) = True
 isLiteral (LStringLiteral _) = True
 isLiteral _ = False
-
+isNewline (LNewline) = True
-esNewline        (LNewline)          = True
 isNewline _ = False
 
 -- Error Handling
@@ -252,3 +253,4 @@ showError (FancyError offset fancy) =
 showError (TrivialError offset Nothing expected) =
   "Parse error at offset " ++ show offset ++ ": expected one of "
   ++ show (Set.toList expected)
+

test/Spec.hs

@@ -5,15 +5,20 @@ import Lexer
 import Parser
 import Research
 import Control.Exception (evaluate, try, SomeException)
-import qualified Data.Map as Map
 import Test.Tasty
 import Test.Tasty.HUnit
 import Test.Tasty.QuickCheck
 import Text.Megaparsec (runParser)
 
+import qualified Data.Map as Map
+import qualified Data.Set as Set
+
 main :: IO ()
 main = defaultMain tests
 
+runSapling :: String -> String
+runSapling s = show $ result (evalSapling Map.empty $ parseSapling s)
+
 tests :: TestTree
 tests = testGroup "Sapling Tests"
   [ lexerTests
@@ -21,6 +26,7 @@ tests = testGroup "Sapling Tests"
   , integrationTests
   , evaluationTests
   , propertyTests
+  , lambdaEvalTests
   ]
 
 lexerTests :: TestTree
@@ -70,75 +76,70 @@ parserTests = testGroup "Parser Tests"
       case (runParser parseExpression "" input) of
         Left _ -> return ()
         Right _ -> assertFailure "Expected failure when trying to assign the value of T"
-    , testCase "Error when parsing bodyless definitions with arguments" $ do
-        let input = lexSapling "x a b = "
-        case (runParser parseExpression "" input) of
-            Left _ -> return ()
-            Right _ -> assertFailure "Expected failure on invalid input"
   , testCase "Parse function definitions" $ do
-        let input = "x a b c = a"
+      let input = "x = (\\a b c : a)"
-        let expect = SFunc "x" ["a","b","c"] (SVar "a")
+          expect = SFunc "x" [] (SLambda ["a"] (SLambda ["b"] (SLambda ["c"] (SVar "a"))))
       parseSingle input @?= expect
   , testCase "Parse nested Tree Calculus terms" $ do
       let input = "t (t t) t"
-        let expect = SApp (SApp TLeaf (SApp TLeaf TLeaf)) TLeaf
+          expect = SApp (SApp TLeaf (SApp TLeaf TLeaf)) TLeaf
       parseSingle input @?= expect
   , testCase "Parse sequential Tree Calculus terms" $ do
       let input = "t t t"
-        let expect = SApp (SApp TLeaf TLeaf) TLeaf
+          expect = SApp (SApp TLeaf TLeaf) TLeaf
       parseSingle input @?= expect
   , testCase "Parse mixed list literals" $ do
       let input = "[t (\"hello\") t]"
-        let expect = SList [TLeaf, SStr "hello", TLeaf]
+          expect = SList [TLeaf, SStr "hello", TLeaf]
       parseSingle input @?= expect
   , testCase "Parse function with applications" $ do
-        let input = "f x = t x"
+      let input  = "f = (\\x : t x)"
-        let expect = SFunc "f" ["x"] (SApp TLeaf (SVar "x"))
+          expect = SFunc "f" [] (SLambda ["x"] (SApp TLeaf (SVar "x")))
       parseSingle input @?= expect
   , testCase "Parse nested lists" $ do
       let input  = "[t [(t t)]]"
-        let expect = SList [TLeaf,SList [SApp TLeaf TLeaf]]
+          expect = SList [TLeaf,SList [SApp TLeaf TLeaf]]
       parseSingle input @?= expect
   , testCase "Parse complex parentheses" $ do
       let input  = "t (t t (t t))"
-        let expect = SApp TLeaf (SApp (SApp TLeaf TLeaf) (SApp TLeaf TLeaf))
+          expect = SApp TLeaf (SApp (SApp TLeaf TLeaf) (SApp TLeaf TLeaf))
       parseSingle input @?= expect
   , testCase "Parse empty list" $ do
       let input  = "[]"
-        let expect = SList []
+          expect = SList []
       parseSingle input @?= expect
   , testCase "Parse multiple nested lists" $ do
       let input  = "[[t t] [t (t t)]]"
-        let expect = SList [SList [TLeaf,TLeaf],SList [TLeaf,SApp TLeaf TLeaf]]
+          expect = SList [SList [TLeaf,TLeaf],SList [TLeaf,SApp TLeaf TLeaf]]
       parseSingle input @?= expect
   , testCase "Parse whitespace variance" $ do
       let input1 = "[t t]"
       let input2 = "[ t t ]"
-        let expect = SList [TLeaf, TLeaf]
+          expect = SList [TLeaf, TLeaf]
       parseSingle input1 @?= expect
       parseSingle input2 @?= expect
   , testCase "Parse string in list" $ do
       let input  = "[(\"hello\")]"
-        let expect = SList [SStr "hello"]
+          expect = SList [SStr "hello"]
       parseSingle input @?= expect
   , testCase "Parse parentheses inside list" $ do
       let input  = "[t (t t)]"
-        let expect = SList [TLeaf,SApp TLeaf TLeaf]
+          expect = SList [TLeaf,SApp TLeaf TLeaf]
       parseSingle input @?= expect
   , testCase "Parse nested parentheses in function body" $ do
-        let input = "f = t (t (t t))"
+      let input  = "f = (\\x : t (t (t t)))"
-        let expect = SFunc "f" [] (SApp TLeaf (SApp TLeaf (SApp TLeaf TLeaf)))
+          expect = SFunc "f" [] (SLambda ["x"] (SApp TLeaf (SApp TLeaf (SApp TLeaf TLeaf))))
       parseSingle input @?= expect
   , testCase "Parse lambda abstractions" $ do
       let input  = "(\\a : a)"
-        let expect = (SLambda ["a"] (SVar "a"))
+          expect = (SLambda ["a"] (SVar "a"))
       parseSingle input @?= expect
   , testCase "Parse multiple arguments to lambda abstractions" $ do
       let input  = "x = (\\a b : a)"
-        let expect = SFunc "x" [] (SLambda ["a"] (SLambda ["b"] (SVar "a")))
+          expect = SFunc "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)\n" <> "x (t)"
+      let input  = "x = (\\a : a)\nx (t)"
           expect = [SFunc "x" [] (SLambda ["a"] (SVar "a")),SApp (SVar "x") TLeaf]
       parseSapling input @?= expect
   ]
@@ -147,11 +148,11 @@ integrationTests :: TestTree
 integrationTests = testGroup "Integration Tests"
   [ testCase "Combine lexer and parser" $ do
       let input = "x = t t t"
-        let expect = SFunc "x" [] (SApp (SApp TLeaf TLeaf) TLeaf)
+          expect = SApp (SVar "x") (SApp (SApp TLeaf TLeaf) TLeaf)
       parseSingle input @?= expect
   , testCase "Complex Tree Calculus expression" $ do
       let input = "t (t t t) t"
-        let expect = SApp (SApp TLeaf (SApp (SApp TLeaf TLeaf) TLeaf)) TLeaf
+          expect = SApp (SApp TLeaf (SApp (SApp TLeaf TLeaf) TLeaf)) TLeaf
       parseSingle input @?= expect
   ]
 
@@ -180,16 +181,16 @@ evaluationTests = testGroup "Evaluation Tests"
         Fork (Fork (Stem Leaf) (Fork Leaf Leaf)) Leaf
   , testCase "Environment updates with definitions" $ do
       let input = "x = t\ny = x"
-        let env = evalSapling Map.empty (parseSapling input)
+          env = evalSapling Map.empty (parseSapling input)
       Map.lookup "x" env @?= Just Leaf
       Map.lookup "y" env @?= Just Leaf
   , testCase "Variable substitution" $ do
       let input = "x = t t\ny = t x\ny"
-        let env = evalSapling Map.empty (parseSapling input)
+          env = evalSapling Map.empty (parseSapling input)
       (result env) @?= Stem (Stem Leaf)
   , testCase "Multiline input evaluation" $ do
       let input = "x = t\ny = t t\nx"
-        let env = evalSapling Map.empty (parseSapling input)
+          env = evalSapling Map.empty (parseSapling input)
       (result env) @?= Leaf
   , testCase "Evaluate string literal" $ do
       let input = "\"hello\""
@@ -209,26 +210,55 @@ evaluationTests = testGroup "Evaluation Tests"
                   \ z = y\n \
                   \ variablewithamuchlongername = z\n \
                   \ variablewithamuchlongername"
-        let env = evalSapling Map.empty (parseSapling input)
+          env = evalSapling Map.empty (parseSapling input)
       (result env) @?= (Stem (Stem Leaf))
   , testCase "Evaluate variable shadowing" $ do
       let input = "x = t t\nx = t\nx"
-        let env = evalSapling Map.empty (parseSapling input)
-        (result env) @?= Leaf
-    , testCase "Lambda identity" $ do
-        let input = "(\\a : a)"
           env = evalSapling Map.empty (parseSapling input)
-        result env @?= Fork (Stem (Stem Leaf)) (Stem Leaf)
+      (result env) @?= Leaf
     , testCase "Apply identity to Boolean Not" $ do
       let not = "(t (t (t t) (t t t)) t)"
-            input = "x = (\\a : a)\nx " ++ not
+      let input = "x = (\\a : a)\nx " ++ not
           env = evalSapling Map.empty (parseSapling input)
       result env @?= Fork (Fork (Stem Leaf) (Fork Leaf Leaf)) Leaf
     , testCase "Constant function matches" $ do
       let input = "k = (\\a b : a)\nk (t t) t"
           env = evalSapling Map.empty (parseSapling input)
       result env @?= Stem Leaf
+   , testCase "Boolean AND_ TF" $ do
+      let input = "and (t t) (t)"
+          env = evalSapling boolEnv (parseSapling input)
+      result env @?= Leaf
+   , testCase "Boolean AND_ FT" $ do
+      let input = "and (t) (t t)"
+          env = evalSapling boolEnv (parseSapling input)
+      result env @?= Leaf
+   , testCase "Boolean AND_ FF" $ do
+      let input = "and (t) (t)"
+          env = evalSapling boolEnv (parseSapling input)
+      result env @?= Leaf
+    , testCase "Boolean AND_ TT" $ do
+      let input = "and (t t) (t t)"
+          env = evalSapling boolEnv (parseSapling input)
+      result env @?= Stem Leaf
+    , testCase "Verifying Equality" $ do
+      let input = "equal (t t t) (t t t)"
+          env = evalSapling boolEnv (parseSapling input)
+      result env @?= Stem Leaf
   ]
+  where
+    boolEnv = evalSapling Map.empty $ parseSapling 
+      "false = t\n \ 
+     \ true = t t\n \
+     \ falseL = (\\z : false)\n \
+     \ id = (\\a : a)\n \
+     \ triage = (\\a b c : (t (t a b) c))\n \
+     \ match_bool = (\\ot of : triage of (\\z : ot) t)\n \
+     \ and = match_bool id falseL\n \
+     \ fix = (\\m wait f : wait m (\\x : f (wait m x))) (\\x : x x) (\\a b c : (t (t a) (t t c) b))\n \
+     \ equal = fix ((\\self : triage (triage true (\\z : false) (\\z x : false)) (\\ax : triage false (self ax) (\\z x : false)) (\\ax ay : triage false (\\z : false) (\\bx by : and (self ax bx) (self ay by)))))\
+     \ "
+
 
 propertyTests :: TestTree
 propertyTests = testGroup "Property Tests"
@@ -239,3 +269,75 @@ propertyTests = testGroup "Property Tests"
           Left _ -> property True
           Right ast -> parseSingle input === ast
   ]
+
+lambdaEvalTests :: TestTree
+lambdaEvalTests = testGroup "Lambda Evaluation Tests"
+  [ testCase "Lambda Identity Function" $ do
+      let input = "id = (\\x : x)\nid t"
+      runSapling input @?= "Leaf"
+
+  , testCase "Lambda Constant Function (K combinator)" $ do
+      let input = "k = (\\x y : x)\nk t (t t)"
+      runSapling input @?= "Leaf"
+
+  , testCase "Lambda Application with Variable" $ do
+      let input = "id = (\\x : x)\nval = t t\nid val"
+      runSapling 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)"
+      runSapling 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"
+      runSapling input @?= "Leaf"
+
+  , testCase "Lambda with a complex body" $ do
+      let input = "f = (\\x : t (t x))\nf t"
+      runSapling input @?= "Stem (Stem Leaf)"
+
+  , testCase "Lambda returning a function" $ do
+      let input = "f = (\\x : (\\y : x))\ng = f t\ng (t t)"
+      runSapling input @?= "Leaf"
+
+  , testCase "Lambda with Shadowing" $ do
+      let input = "f = (\\x : (\\x : x))\nf t (t t)"
+      runSapling input @?= "Stem Leaf"
+
+   , testCase "Lambda returning another lambda" $ do
+      let input = "k = (\\x : (\\y : x))\nk_app = k t\nk_app (t t)"
+      runSapling input @?= "Leaf"
+
+   , testCase "Lambda with free variables" $ do
+      let input = "y = t t\nf = (\\x : y)\nf t"
+      runSapling 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)"
+      runSapling input @?= "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)"
+      runSapling input @?= "Fork (Fork Leaf Leaf) Leaf"
+
+   , testCase "Lambda with nested application in the body" $ do
+      let input = "f = (\\x : t (t (t x)))\nf t"
+      runSapling 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)"
+        runSapling input @?= "Fork Leaf (Stem Leaf)"
+    , testCase "Lambda applying a variable" $ do
+        let input = "id = (\\x : x)\na = t t\nid a"
+        runSapling input @?= "Stem Leaf"
+    , testCase "Multiple lambda abstractions in the same expression" $ do
+        let input = "f = (\\x : (\\y : x y))\ng = (\\z : z)\nf g t"
+        runSapling input @?= "Stem Leaf"
+  , testCase "Lambda with a string literal" $ do
+        let input = "f = (\\x : x)\nf \"hello\""
+        runSapling input @?= "Fork (Fork (Fork (Stem Leaf) (Fork Leaf Leaf)) (Fork (Stem Leaf) (Fork Leaf Leaf))) (Fork (Fork (Stem Leaf) (Fork Leaf Leaf)) (Fork (Stem Leaf) (Fork Leaf Leaf)))"
+   , testCase "Lambda with an integer literal" $ do
+        let input = "f = (\\x : x)\nf 42"
+        runSapling input @?= "Fork (Leaf) (Fork (Stem Leaf) (Fork Leaf Leaf))"
+   , testCase "Lambda with a list literal" $ do
+        let input = "f = (\\x : x)\nf [t (t t)]"
+        runSapling input @?= "Fork Leaf (Fork (Stem Leaf) Leaf)"
+  ]