Implement pattern synonyms

This patch implements Pattern Synonyms (enabled by -XPatternSynonyms),
allowing y ou to assign names to a pattern and abstract over it.

The rundown is this:

  * Named patterns are introduced by the new 'pattern' keyword, and can
    be either *unidirectional* or *bidirectional*. A unidirectional
    pattern is, in the simplest sense, simply an 'alias' for a pattern,
    where the LHS may mention variables to occur in the RHS. A
    bidirectional pattern synonym occurs when a pattern may also be used
    in expression context.

  * Unidirectional patterns are declared like thus:

        pattern P x <- x:_

    The synonym 'P' may only occur in a pattern context:

        foo :: [Int] -> Maybe Int
        foo (P x) = Just x
        foo _     = Nothing

  * Bidirectional patterns are declared like thus:

        pattern P x y = [x, y]

    Here, P may not only occur as a pattern, but also as an expression
    when given values for 'x' and 'y', i.e.

        bar :: Int -> [Int]
        bar x = P x 10

  * Patterns can't yet have their own type signatures; signatures are inferred.

  * Pattern synonyms may not be recursive, c.f. type synonyms.

  * Pattern synonyms are also exported/imported using the 'pattern'
    keyword in an import/export decl, i.e.

        module Foo (pattern Bar) where ...

    Note that pattern synonyms share the namespace of constructors, so
    this disambiguation is required as a there may also be a 'Bar'
    type in scope as well as the 'Bar' pattern.

  * The semantics of a pattern synonym differ slightly from a typical
    pattern: when using a synonym, the pattern itself is matched,
    followed by all the arguments. This means that the strictness
    differs slightly:

        pattern P x y <- [x, y]

        f (P True True) = True
        f _             = False

        g [True, True] = True
        g _            = False

    In the example, while `g (False:undefined)` evaluates to False,
    `f (False:undefined)` results in undefined as both `x` and `y`
    arguments are matched to `True`.

For more information, see the wiki:

    https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms
    https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms/Implementation

Reviewed-by: Simon Peyton Jones <simonpj@microsoft.com>
Signed-off-by: Austin Seipp <austin@well-typed.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/ghc/GhciTags.hs b/ghc/GhciTags.hs
index 2815a74dcb..b250637b07 100644
--- a/ghc/GhciTags.hs
+++ b/ghc/GhciTags.hs
@@ -22,6 +22,7 @@ import Outputable
 -- into the GHC API instead
 import Name (nameOccName)
 import OccName (pprOccName)
+import ConLike
 import MonadUtils
 
 import Data.Function
@@ -103,10 +104,11 @@ listModuleTags m = do
                      ]
 
   where
-    tyThing2TagKind (AnId _)     = 'v'
-    tyThing2TagKind (ADataCon _) = 'd'
-    tyThing2TagKind (ATyCon _)   = 't'
-    tyThing2TagKind (ACoAxiom _) = 'x'
+    tyThing2TagKind (AnId _)                 = 'v'
+    tyThing2TagKind (AConLike RealDataCon{}) = 'd'
+    tyThing2TagKind (AConLike PatSynCon{})   = 'p'
+    tyThing2TagKind (ATyCon _)               = 't'
+    tyThing2TagKind (ACoAxiom _)             = 'x'
 
 
 data TagInfo = TagInfo