Pattern synonyms and record fields for Cont, Writer, Reader, State, ...

[Baldur Blöndal]

This will look like the original definitions [1]

> newtype State s a = State { runState :: s -> (a, s) }

before they became type synonyms

> type State s = StateT s Identity

by defining pattern synonyms

> pattern State :: (s -> (a, s)) -> State s a
> pattern State {runState} <- S.runState -> runState
>   where State a           = state a

Trac ticket #12767 for further information. [2]

[1] http://book.realworldhaskell.org/read/monads.html#id643643
[2] https://ghc.haskell.org/trac/ghc/ticket/12767
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