Do not try flatparse scanner for .md files (#2934)

- In https://github.com/anoma/juvix/pull/2925, the flatparse scanner
always fails for .md files and silently falls back to megaparsec.
- In https://github.com/anoma/juvix/pull/2929, a warning is introduced
that informs the user when the fallback parser is being used. This is
causing a warning every time we scan a markdown file.
- This pr fixes the problem by changing the default strategy to directly
use megaparsec when a .md file is given.

src/Juvix/Compiler/Concrete/Translation/ImportScanner.hs

@@ -41,7 +41,7 @@ scanBSImports ::
   ByteString ->
   Sem r ScanResult
 scanBSImports fp inputBS = do
-  strat <- ask
+  strat <- adaptStrategy <$> ask
   case strat of
     ImportScanStrategyFallback ->
       case FlatParse.scanBSImports fp inputBS of
@@ -59,6 +59,12 @@ scanBSImports fp inputBS = do
       Just r -> return r
     ImportScanStrategyMegaparsec -> Megaparsec.scanBSImports fp inputBS
   where
+    adaptStrategy :: ImportScanStrategy -> ImportScanStrategy
+    adaptStrategy = \case
+      ImportScanStrategyFallback
+        | not (isJuvixFile fp) -> ImportScanStrategyMegaparsec
+      s -> s
+
     fileLoc :: Interval
     fileLoc =
       Interval
@@ -66,6 +72,7 @@ scanBSImports fp inputBS = do
           _intervalStart = tmpFileLoc,
           _intervalEnd = tmpFileLoc
         }
+
     tmpFileLoc :: FileLoc
     tmpFileLoc =
       FileLoc

src/Juvix/Compiler/Concrete/Translation/ImportScanner/FlatParse.hs

@@ -12,10 +12,7 @@ import Juvix.Prelude.FlatParse qualified as FP
 import Juvix.Prelude.FlatParse.Lexer qualified as L
 
 scanBSImports :: Path Abs File -> ByteString -> Maybe ScanResult
-scanBSImports fp
+scanBSImports fp = fromResult . scanner fp
-  -- FlatParse only supports .juvix files
-  | isJuvixFile fp = fromResult . scanner fp
-  | otherwise = const Nothing
   where
     fromResult :: Result () ok -> Maybe ok
     fromResult = \case