Error on import cycles

src/FileEval.hs

@@ -10,6 +10,7 @@ import Control.Monad   (foldM)
 import System.IO
 
 import qualified Data.Map as Map
+import qualified Data.Set as Set
 
 evaluateFileResult :: FilePath -> IO T
 evaluateFileResult filePath = do
@@ -61,18 +62,24 @@ mainAlias moduleName env =
     Nothing    -> env
 
 preprocessFile :: FilePath -> IO [TricuAST]
-preprocessFile filePath = do
-  contents <- readFile filePath
-  let tokens = lexTricu contents
-  case parseProgram tokens of
-    Left err -> errorWithoutStackTrace (handleParseError err)
-    Right asts -> do
-      let (moduleName, restAST) = extractModule asts
-      let (imports, nonImports) = partition isImport restAST
-      importedASTs <- concat <$> mapM (processImport moduleName) imports
-      let namespacedAST = namespaceDefinitions moduleName nonImports
-      let fullyNamespacedImports = map (namespaceBody moduleName) importedASTs
-      pure $ fullyNamespacedImports ++ namespacedAST
+preprocessFile filePath = preprocessFile' Set.empty filePath
+
+preprocessFile' :: Set.Set FilePath -> FilePath -> IO [TricuAST]
+preprocessFile' inProgress filePath
+  | filePath `Set.member` inProgress = 
+      errorWithoutStackTrace $ "Encountered cyclic import: " ++ filePath
+  | otherwise = do
+      contents <- readFile filePath
+      let tokens = lexTricu contents
+      case parseProgram tokens of
+        Left err -> errorWithoutStackTrace (handleParseError err)
+        Right asts -> do
+          let (moduleName, restAST) = extractModule asts
+          let (imports, nonImports) = partition isImport restAST
+          let newInProgress = Set.insert filePath inProgress
+          importedASTs <- concat <$> mapM (processImport newInProgress) imports
+          let namespacedAST = namespaceDefinitions moduleName nonImports
+          pure $ importedASTs ++ namespacedAST
   where
     extractModule :: [TricuAST] -> (String, [TricuAST])
     extractModule ((SModule name) : xs) = (name, xs)
@@ -82,11 +89,11 @@ preprocessFile filePath = do
     isImport (SImport _ _) = True
     isImport _             = False
 
-    processImport :: String -> TricuAST -> IO [TricuAST]
-    processImport _ (SImport filePath moduleName) = do
-      importedAST <- preprocessFile filePath
+    processImport :: Set.Set FilePath -> TricuAST -> IO [TricuAST]
+    processImport inProgress (SImport filePath moduleName) = do
+      importedAST <- preprocessFile' inProgress filePath
       pure $ namespaceDefinitions moduleName importedAST
-    processImport _ _ = error "Unexpected non-import AST node in processImport"
+    processImport _ _ = error "Unexpected non-import in processImport"
 
 namespaceDefinitions :: String -> [TricuAST] -> [TricuAST]
 namespaceDefinitions moduleName = map (namespaceDefinition moduleName)

test/cycle-1.tri

@@ -0,0 +1,5 @@
+!module Cycle
+
+!import "test/cycle-2.tri" Cycle2
+
+cycle1 = t Cycle2.cycle2

test/cycle-2.tri

@@ -0,0 +1,5 @@
+!module Cycle2
+
+!import "test/cycle-1.tri" Cycle1
+
+cycle2 = t Cycle1.cycle1