Fix parsing of //\\ (#666)

The issue was that the parser was attempting to parse // first, which
will succeed on the prefix of //\\, then the parser will get an error
because it expects a sub expression but the input is \\.

dhall/dhall.cabal

@@ -28,6 +28,7 @@ Extra-Source-Files:
     Prelude/Monoid
     tests/format/*.dhall
     tests/normalization/tutorial/combineTypes/*.dhall
+    tests/normalization/tutorial/prefer/*.dhall
     tests/normalization/tutorial/projection/*.dhall
     tests/normalization/tutorial/access/*.dhall
     tests/normalization/*.dhall

dhall/src/Dhall/Parser/Expression.hs

@@ -158,8 +158,8 @@ completeExpression embedded = completeExpression_
             <|> Combine     <$ _combine
 
     precedence2Operator =
-                Prefer       <$ _prefer
-            <|> CombineTypes <$ _combineTypes
+                CombineTypes <$ _combineTypes
+            <|> Prefer       <$ _prefer
             <|> NaturalTimes <$ _times
             <|> BoolNE       <$ _notEqual
 

dhall/tests/Normalization.hs

@@ -40,6 +40,8 @@ tutorialExamples :: TestTree
 tutorialExamples =
     testGroup "Tutorial examples"
         [ shouldNormalize "⩓" "./tutorial/combineTypes/0"
+        , shouldNormalize "//\\\\" "./tutorial/combineTypes/1"
+        , shouldNormalize "//" "./tutorial/prefer/0"
         , shouldNormalize "projection" "./tutorial/projection/0"
         , shouldNormalize "access record" "./tutorial/access/0"
         , shouldNormalize "access union" "./tutorial/access/1"

dhall/tests/normalization/tutorial/combineTypes/1A.dhall

@@ -0,0 +1 @@
+{ foo : { bar : Text } } //\\ { foo : { baz : Bool }, qux : Integer }

dhall/tests/normalization/tutorial/combineTypes/1B.dhall

@@ -0,0 +1 @@
+{ foo : { bar : Text, baz : Bool }, qux : Integer }

dhall/tests/normalization/tutorial/prefer/0A.dhall

@@ -0,0 +1 @@
+{ foo = 1, bar = "ABC" } // { baz = True }

dhall/tests/normalization/tutorial/prefer/0B.dhall

@@ -0,0 +1 @@
+{ bar = "ABC", baz = True, foo = 1 }