report multiple start sections in text representation as parsing error

src/Language/Wasm/Parser.y

@@ -1501,6 +1501,7 @@ constInstructionToValue _ = error "Only const instructions supported as argument
 desugarize :: [ModuleField] -> Either String S.Module
 desugarize fields = do
     checkImportsOrder fields
+    checkStartCount fields
     let mod = Module {
         types = reverse $ foldl' extractTypeDef (reverse $ explicitTypeDefs fields) fields,
         functions = extract extractFunction fields,
@@ -1563,6 +1564,15 @@ desugarize fields = do
                 checkDef _ (MFTable _) = return True
                 checkDef nonImportOccured _ = return nonImportOccured
         
+        checkStartCount :: [ModuleField] -> Either String ()
+        checkStartCount fields = foldM checkDef False fields >> return ()
+            where
+                checkDef startOccured (MFStart _) =
+                    if startOccured
+                    then Left "Multiple start sections"
+                    else Right True
+                checkDef startOccured _ = return startOccured
+
         extractTypeDef :: [TypeDef] -> ModuleField -> [TypeDef]
         extractTypeDef defs (MFType _) = defs -- should be extracted before implicit defs
         extractTypeDef defs (MFImport Import { desc = ImportFunc _ typeUse }) =

tests/Test.hs

@@ -17,7 +17,7 @@ import qualified Data.List as List
 main :: IO ()
 main = do
   files <- filter (List.isSuffixOf ".wast") <$> Directory.listDirectory "tests/spec"
-  -- let files = ["linking.wast"]
+  -- let files = ["start.wast"]
   scriptTestCases <- (`mapM` files) $ \file -> do
     test <- LBS.readFile ("tests/spec/" ++ file)
     return $ testCase file $ do