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>

2 files changed, 14 insertions, 0 deletions

diff --git a/compiler/utils/UniqFM.lhs b/compiler/utils/UniqFM.lhs
index 7fde82a3c7..52cd3dd791 100644
--- a/compiler/utils/UniqFM.lhs
+++ b/compiler/utils/UniqFM.lhs
@@ -73,6 +73,7 @@ import qualified Data.Foldable as Foldable
 import qualified Data.Traversable as Traversable
 import Data.Typeable
 import Data.Data
+import Data.Monoid
 \end{code}
 
 %************************************************************************
@@ -185,6 +186,18 @@ ufmToList       :: UniqFM elt -> [(Unique, elt)]
 
 %************************************************************************
 %*                                                                      *
+\subsection{Monoid interface}
+%*                                                                      *
+%************************************************************************
+
+\begin{code}
+instance Monoid (UniqFM a) where
+    mempty = emptyUFM
+    mappend = plusUFM
+\end{code}
+
+%************************************************************************
+%*                                                                      *
 \subsection{Implementation using ``Data.IntMap''}
 %*                                                                      *
 %************************************************************************
diff --git a/compiler/utils/UniqSet.lhs b/compiler/utils/UniqSet.lhs
index 1653f2dc43..fae5ddabb6 100644
--- a/compiler/utils/UniqSet.lhs
+++ b/compiler/utils/UniqSet.lhs
@@ -75,6 +75,7 @@ isEmptyUniqSet :: UniqSet a -> Bool
 lookupUniqSet :: Uniquable a => UniqSet a -> a -> Maybe a
 uniqSetToList :: UniqSet a -> [a]
 \end{code}
+
 %************************************************************************
 %*                                                                      *
 \subsection{Implementation using ``UniqFM''}