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, 3 insertions, 2 deletions

diff --git a/utils/ghctags/Main.hs b/utils/ghctags/Main.hs
index 0f9886fc14..9fffd52464 100644
--- a/utils/ghctags/Main.hs
+++ b/utils/ghctags/Main.hs
@@ -257,7 +257,7 @@ boundValues mod group =
   let vals = case hs_valds group of
                ValBindsOut nest _sigs ->
                    [ x | (_rec, binds) <- nest
-                       , bind <- bagToList binds
+                       , (_, bind) <- bagToList binds
                        , x <- boundThings mod bind ]
                _other -> error "boundValues"
       tys = [ n | ns <- map hsLTyClDeclBinders (tyClGroupConcat (hs_tyclds group))
@@ -284,6 +284,7 @@ boundThings modname lbinding =
     PatBind { pat_lhs = lhs } -> patThings lhs []
     VarBind { var_id = id } -> [FoundThing modname (getOccString id) (startOfLocated lbinding)]
     AbsBinds { } -> [] -- nothing interesting in a type abstraction
+    PatSynBind { patsyn_id = id } -> [thing id]
   where thing = foundOfLName modname
         patThings lpat tl =
           let loc = startOfLocated lpat
@@ -299,7 +300,7 @@ boundThings modname lbinding =
                TuplePat ps _ _ -> foldr patThings tl ps
                PArrPat ps _ -> foldr patThings tl ps
                ConPatIn _ conargs -> conArgs conargs tl
-               ConPatOut _ _ _ _ conargs _ -> conArgs conargs tl
+               ConPatOut{ pat_args = conargs } -> conArgs conargs tl
                LitPat _ -> tl
                NPat _ _ _ -> tl -- form of literal pattern?
                NPlusKPat id _ _ _ -> thing id : tl