Use : in syntax for union types

src/Dhall/Core.hs

@@ -607,7 +607,7 @@ buildTagTypes   [a]  = buildTagType a
 buildTagTypes (a:bs) = buildTagType a <> " | " <> buildTagTypes bs
 
 buildTagType :: Buildable a => (Text, Expr a) -> Builder
-buildTagType (a, b) = build a <> " " <> buildExpr0 b
+buildTagType (a, b) = build a <> " : " <> buildExpr0 b
 
 buildUnionLit :: Buildable a => Text -> Expr a -> Map Text (Expr a) -> Builder
 buildUnionLit a b c

src/Dhall/Parser.y

@@ -339,8 +339,8 @@ UnionLit
         { UnionLit $2 $4 (Data.Map.fromList $6) }
 
 TagType
-    : label Expr0
-        { ($1, $2) } 
+    : label ':' Expr0
+        { ($1, $3) } 
 
 Import
     : file