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

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