Add support for Unix shebangs (#2175)

… as standardized in dhall-lang/dhall-lang#1158

Author: Gabriella439

dhall/dhall-lang

@@ -10,7 +10,6 @@ module Dhall.Parser.Expression where
 import Control.Applicative     (Alternative (..), liftA2, optional)
 import Data.ByteArray.Encoding (Base (..))
 import Data.Foldable           (foldl')
-import Data.Functor            (void)
 import Data.List.NonEmpty      (NonEmpty (..))
 import Data.Text               (Text)
 import Dhall.Src               (Src (..))
@@ -113,7 +112,17 @@ data Parsers a = Parsers
 parsers :: forall a. Parser a -> Parsers a
 parsers embedded = Parsers {..}
   where
-    completeExpression_ = whitespace *> expression <* whitespace
+    completeExpression_ =
+        optional shebang *> whitespace *> expression <* whitespace
+
+    shebang = do
+        _ <- text "#!"
+
+        let predicate c = ('\x20' <= c && c <= '\x10FFFF') || c == '\t'
+
+        _ <- Dhall.Parser.Combinators.takeWhile predicate
+
+        endOfLine
 
     expression =
         noted
@@ -718,7 +727,7 @@ parsers embedded = Parsers {..}
                 , unescapedCharacterFast
                 , unescapedCharacterSlow
                 , tab
-                , endOfLine
+                , endOfLine_
                 ]
           where
                 escapeSingleQuotes = do
@@ -757,7 +766,7 @@ parsers embedded = Parsers {..}
                   where
                     predicate c = c == '$' || c == '\''
 
-                endOfLine = do
+                endOfLine_ = do
                     a <- "\n" <|> "\r\n"
                     b <- singleQuoteContinue
                     return (Chunks [] a <> b)
@@ -769,12 +778,12 @@ parsers embedded = Parsers {..}
 
     singleQuoteLiteral = do
             _ <- text "''"
+
             _ <- endOfLine
+
             a <- singleQuoteContinue
 
             return (Dhall.Syntax.toDoubleQuoted a)
-          where
-            endOfLine = (void (char '\n') <|> void (text "\r\n")) <?> "newline"
 
     textLiteral = (do
             literal <- doubleQuotedLiteral <|> singleQuoteLiteral

dhall/src/Dhall/Parser/Expression.hs

@@ -4,6 +4,7 @@
 -- | Parse Dhall tokens. Even though we don't have a tokenizer per-se this
 ---  module is useful for keeping some small parsing utilities.
 module Dhall.Parser.Token (
+    endOfLine,
     validCodepoint,
     whitespace,
     lineComment,
@@ -135,6 +136,12 @@ import qualified Text.Parser.Token
 
 import Numeric.Natural (Natural)
 
+-- | Match an end-of-line character sequence
+endOfLine :: Parser Text
+endOfLine =
+    (   Text.Parser.Char.text "\n"  
+    <|> Text.Parser.Char.text "\r\n"
+    ) <?> "newline"
 
 -- | Returns `True` if the given `Int` is a valid Unicode codepoint
 validCodepoint :: Int -> Bool
@@ -360,14 +367,9 @@ lineComment = do
 
     commentText <- Dhall.Parser.Combinators.takeWhile predicate
 
-    endOfLine
+    _ <- endOfLine
 
     return ("--" <> commentText)
-  where
-    endOfLine =
-        (   void (Text.Parser.Char.char '\n'  )
-        <|> void (Text.Parser.Char.text "\r\n")
-        ) <?> "newline"
 
 -- | Parsed text doesn't include opening braces
 blockComment :: Parser Text
@@ -396,8 +398,6 @@ blockCommentChunk =
       where
         predicate c = '\x20' <= c && c <= '\x10FFFF' || c == '\n' || c == '\t'
 
-    endOfLine = (Text.Parser.Char.text "\r\n" <?> "newline")
-
 blockCommentContinue :: Parser Text
 blockCommentContinue = endOfComment <|> continue
   where