Fix parser swallowing whitespace in Src annotations (#1563)

* Fix swallowed whitespace in Src annotations

The parser annotated application expressions incorrectly: whitespace
between function and argument was swallowed when constructing the Src
annotation. This bug resulted in strange behaviour in the hovering
functionality in dhall-lsp-server.

As an example, previously the Dhall expression `0 0` would be parsed as
```
Note (Src {..., srcText = "00"})
     (App (Note (Src {..., srcText = "0"}) (NaturalLit 0))
          (Note (Src {..., srcText = "0"}) (NaturalLit 0)))
```
while we now get
```
Note (Src {..., srcText = "0 0"})
     (App (Note (Src {..., srcText = "0"}) (NaturalLit 0))
          (Note (Src {..., srcText = "0"}) (NaturalLit 0)))
```
as expected.

* Use foldl' over foldl

As suggested by @sjakobi. Note that there are quite a few more
occurences of foldl where we don't actually need laziness, but I leave
that for someone else to fix :)

* Revert "Use foldl' over foldl"

This reverts commit 1bd9a54.

Author: EggBaconAndSpam

dhall/src/Dhall/Parser/Expression.hs

@@ -323,13 +323,18 @@ parsers embedded = Parsers {..}
             f <-    (Some <$ _Some <* nonemptyWhitespace)
                 <|> return id
             a <- noted importExpression_
-            b <- Text.Megaparsec.many (try (nonemptyWhitespace *> noted importExpression_))
-            return (foldl app (f a) b)
+            bs <- Text.Megaparsec.many . try $ do
+                (sep, _) <- Text.Megaparsec.match nonemptyWhitespace
+                b <- importExpression_
+                return (sep, b)
+            return (foldl app (f a) bs)
           where
-            app nL@(Note (Src before _ bytesL) _) nR@(Note (Src _ after bytesR) _) =
-                Note (Src before after (bytesL <> bytesR)) (App nL nR)
-            app nL nR =
-                App nL nR
+            app a (sep, b)
+                | Note (Src left _ bytesL) _ <- a
+                , Note (Src _ right bytesR) _ <- b
+                = Note (Src left right (bytesL <> sep <> bytesR)) (App a b)
+            app a (_, b) =
+                App a b
 
     importExpression_ = noted (choice [ alternative0, alternative1 ])
           where