Re: [GHC] #13403: Derive instances (Applicative, Monad, ...) for structures lifted over functors

9 Mar 2017

      #13403: Derive instances (Applicative, Monad, ...) for structures lifted over
functors
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Moreover, it wouldn't take much code at all to set up the machinery needed
 to do this:

 {{{#!hs
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleContexts #-}
 module DeriveApplicativeMonad where

 import GHC.Generics

 -- Applicative
 genericPure :: (Generic1 f, Applicative (Rep1 f))
             => a -> f a
 genericPure = to1 . pure

 genericAp :: (Generic1 f, Applicative (Rep1 f))
           => f (a -> b) -> f a -> f b
 genericAp f x = to1 $ from1 f <*> from1 x

 -- Monad
 genericBind :: (Generic1 m, Monad (Rep1 m))
             => m a -> (a -> m b) -> m b
 genericBind m f = to1 $ from1 m >>= from1 . f

 -- Example
 data Product f g h a = Product (f (g (f a))) (h (f (g a)))
   deriving (Functor, Generic1)
 instance (Applicative f, Applicative g, Applicative h)
     => Applicative (Product f g h) where
   pure  = genericPure
   (<*>) = genericAp
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13403#comment:5
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