[GHC] #12857: associate pattern synonyms with a type synonym

20 Nov 2016


      #12857: associate pattern synonyms with a type synonym
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           Reporter:  int-e          |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I believe it would be useful to bundle pattern synonyms with type
 synonyms, which is currently not supported. For example, the `State` type
 synonym from monad transformers could profit from such bundling, as it
 would allow users to use `State (..)` in an import list:
 {{{#!hs
 {-# LANGUAGE ViewPatterns #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module State (
     State (State, runState)
 ) where

 import Control.Monad

 newtype Identity a = Identity { runIdentity :: a }

 newtype StateT s m a = StateT { runStateT :: s -> m (s, a) }

 type State s a = StateT s Identity a

 pattern State { runState } <- ((runIdentity .) . runStateT -> runState)
     where State runState = StateT (Identity . runState)
 }}}
 (I would have a use for this in `haskell-src-exts-simple` package, which,
 similar to the above example, uses type synonyms to instantiate a type
 parameter a few datatypes to a fixed type.)

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12857
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