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, 5 insertions, 5 deletions

diff --git a/testsuite/tests/ghc-api/T6145.hs b/testsuite/tests/ghc-api/T6145.hs
index 0332b05a51..98e8bd0219 100644
--- a/testsuite/tests/ghc-api/T6145.hs
+++ b/testsuite/tests/ghc-api/T6145.hs
@@ -27,17 +27,17 @@ main = do
                         l <- loadModule d
                         let ts=typecheckedSource l
 --                        liftIO (putStr (showSDocDebug (ppr ts)))
-                        let fs=filterBag getDataCon ts
+                        let fs=filterBag (isDataCon . snd) ts
                         return $ not $ isEmptyBag fs
         removeFile "Test.hs"
         print ok
     where 
-      getDataCon (L _ (AbsBinds { abs_binds = bs }))
-        = not (isEmptyBag (filterBag getDataCon bs))
-      getDataCon (L l (f@FunBind {}))
+      isDataCon (L _ (AbsBinds { abs_binds = bs }))
+        = not (isEmptyBag (filterBag (isDataCon . snd) bs))
+      isDataCon (L l (f@FunBind {}))
         | (MG (m:_) _ _) <- fun_matches f,
           (L _ (c@ConPatOut{}):_)<-hsLMatchPats m,
           (L l _)<-pat_con c
         = isGoodSrcSpan l       -- Check that the source location is a good one
-      getDataCon _ 
+      isDataCon _
         = False