[GHC] #8761: Make pattern synonyms work with Template Haskell

[GHC]

#8761: Make pattern synonyms work with Template Haskell
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Template Haskell  |              Version:
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by sboo):

 one motivation is for fixed points
 https://www.reddit.com/r/haskell/comments/3qrkzr/one_weird_trick_for_nicer_functor_fixpoints/cwhsy6i

 for example:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell, PatternSynonyms, DeriveFunctor #-}

 data TreeF a r
  = LitF a
  | VarF String
  | BinF r Operation r
  deriving Functor

 data Operation = Add | ...

 makeFix ''TreeF
 }}}

 would generate:

 {{{#!hs
 type Tree a = Fix (TreeF a)

 pattern Lit :: a -> Tree a
 pattern Lit a = Fix (LitF a)

 pattern Var :: String -> Tree a
 pattern Var s = Fix (VarF s)

 pattern Bin :: Tree a -> Operation -> Tree a -> Tree a
 pattern Bin l op r = Fix (BinF l op r)
 }}}

 which would make `TreeF` as easy to use as `Tree` when using pattern
 matching:

 {{{#!hs
 optimize :: Tree a -> Tree a
 optimize (Bin (Lit x) Add (Lit y)) = Lit (x + y)
 optimize ... = ...
 }}}

 and still as easy when using recursion schemes:

 {{{#!hs
 evalTreeF :: Map String Integer -> TreeF Integer (Maybe Integer) -> Maybe
 Integer
 evalTreeF environment (BinF (Just l) Add (Just r)) = Just (l + r)
 evalTreeF ...

 evalTree :: Map String Integer -> Tree Integer -> Maybe Integer
 evalTree environment = cata (evalTreeF environment)
 }}}

 (sorry if here's the wrong place, this is my first post)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8761#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler