Refactored, added comments

src/Ubc/Parse/Syntax/Import.hs

@@ -11,7 +11,7 @@ import Control.Monad (filterM)
 import Control.Monad.Catch ( MonadThrow, )
 import Control.Monad.Trans (lift, MonadIO (liftIO))
 import Control.Monad.Trans.Except ()
-import Control.Monad.Trans.Reader (asks)
+import Control.Monad.Trans.Reader (asks, ReaderT)
 
 import Data.Functor ((<&>))
 
@@ -22,18 +22,20 @@ import qualified Data.List.NonEmpty as NonEmpty
 
 import qualified System.Directory as IO
 
-import Text.Parsec (sepBy1, ParsecT, anyChar, many, char, choice, notFollowedBy, oneOf)
+import Text.Parsec (sepBy1, ParsecT, anyChar, many, char, choice, notFollowedBy, oneOf, option)
 import qualified Text.Parsec as Parsec
 
 import Path as Path ( Path, File, parseRelFile, Rel, parseRelDir, Dir, Abs, (</>), fromAbsFile, fromRelFile )
 
 import Ubc.Parse.Syntax (Transformer)
+import Ubc.Parse.Syntax.Config (Config)
 import qualified Ubc.Parse.Syntax.Config as Config
 import qualified Ubc.Parse.Syntax.File as File
 import qualified Ubc.Parse.Syntax.Language as UbcLanguage
 
 data Import = Import
         { file :: File.File
+        , alias :: String
         }
 
 importPath :: Monad m => ParsecT String u m (NonEmpty String)
@@ -50,19 +52,27 @@ parse :: ParsecT String u Transformer Import
 parse = do
         UbcLanguage.reserved "import"
         fragments <- importPath
+        importAs <- option (NonEmpty.last fragments) importAlias
 
-        case parseRelPath fragments of
+        -- concatenate directory and file parts, also utf-decoding
+        relFile <- case parseRelPath fragments of
                         Left err -> fail $ "Failed to decode path:\n" <> show err
-                Right relFile -> do
-                        possiblePaths <- lift $ constructPaths relFile
-                        existingFiles <- liftIO $ filterInexistent possiblePaths
-                        tryImport relFile possiblePaths existingFiles
+                        Right relFile -> pure relFile
 
-tryImport :: Path Rel File -> [Path Abs File] -> [Path Abs File] -> ParsecT String u Transformer Import
-tryImport targetFile searchedPaths = \case
-        [] -> fail $ notFoundMessage targetFile searchedPaths
-        [path] -> parseFile path
-        fs@_ -> fail $ multipleFoundMessage targetFile fs
+        -- appended to all search directories
+        possiblePaths <- lift $ constructPaths relFile
+        -- only existing files
+        existingFiles <- liftIO $ filterInexistent possiblePaths
+
+        -- fail if there are multiple files
+        case existingFiles of
+                [] -> fail $ notFoundMessage relFile possiblePaths
+                [path] -> parseFile path importAs
+                fs@_ -> fail $ multipleFoundMessage relFile fs
+
+importAlias :: Monad m => ParsecT String u m String
+importAlias = UbcLanguage.reserved "as" 
+        *> UbcLanguage.identifier
 
 multipleFoundMessage :: Path Rel File -> [Path Abs File] -> String
 multipleFoundMessage relFile foundFiles = "Could not identify imported file " <> Path.fromRelFile relFile <> "\n"
@@ -74,8 +84,8 @@ notFoundMessage relFile searchedLocations = "Could not locate import file path"
                 <> "Searched locations were:"
                 <> (unlines . List.map (('\t':) . Path.fromAbsFile) $ searchedLocations)
 
-parseFile :: MonadIO m => Path Abs File -> ParsecT String u m Import
-parseFile path = do
+parseFile :: MonadIO m => Path Abs File -> String -> ParsecT String u m Import
+parseFile path importAs = do
         let stringPath = Path.fromAbsFile path
         contents <- liftIO . readFile $ stringPath
 
@@ -98,7 +108,7 @@ parseFile path = do
         Parsec.setInput oldInput
         Parsec.setPosition oldPosition
 
-        pure $ Import importedFile
+        pure $ Import importedFile importAs
 
 parseRelPath :: MonadThrow m => NonEmpty FilePath -> m (Path Rel File)
 parseRelPath fragments = do
@@ -112,7 +122,7 @@ constructRelative = foldr (</>)
 filterInexistent :: [Path Abs File] -> IO [Path Abs File]
 filterInexistent paths = filterM (IO.doesFileExist . Path.fromAbsFile) paths
 
-constructPaths :: Path Rel File -> Transformer [Path Abs File]
+constructPaths :: Monad m => Path Rel t -> ReaderT Config m [Path Abs t]
 constructPaths suffix = do
         paths <- asks Config.includePaths
         Set.toList