• Herbert Valerio Riedel's avatar
    Provide default implementation of `Monad(return)` · a741e69a
    Herbert Valerio Riedel authored
    This was dropped last-minute from d94de872
    (re #4834) together with the default implementation for `(>>)`
    (see 65f887e1 for explanation).
    
    However, the risk of accidental mutually recursive definitions of
    `return`/`pure` is rather low as unlike with the `(>>) = (*>)` default,
    any cyclic definitions would necessarily wind up being new ones, rather
    than changing the semantics for old operations and introducing bottoms.
    
    On the other hand, not having the default method implementation in place
    in GHC 7.10 would complicate/delay any future attempt to clean-up the
    `Monad`-class.
    
    This finally allows (for `base >= 4.8`) to define a F/A/M instance-chain
    with the following minimal definitions (while ignoring that `return` is
    still a class-method in `Monad`)
    
      instance Functor M where
        fmap  = ...
    
      instance Applicative M where
        pure  = ...
        (<*>) = ...
    
      instance Monad M where
        (>>=) = ...
    
    Reviewed By: ekmett, austin
    
    Differential Revision: https://phabricator.haskell.org/D647
    a741e69a
Base.hs 40.3 KB