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associate pattern synonyms with a type synonym

    • Open Issue created by Bertram Felgenhauer @int-e

    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:

    {-# 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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    Trac field Value
    Version 8.0.1
    Type FeatureRequest
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
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