Re: [GHC] #14070: Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’

24 Feb 2018

      #14070: Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  duplicate         |             Keywords:
                                     |  QuantifiedConstraints, deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #2893             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Sure. Here's a test adapted from the original post:

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE ImpredicativeTypes #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}

 import Data.Coerce
 import Data.Kind

 class Monad m => MonadJoin m where
   join_ :: m (m a) -> m a

 newtype T m a = T (m a)
   deriving (Functor, Applicative, Monad)

 type Representational1 f = (forall a b. Coercible a b => Coercible (f a)
 (f b) :: Constraint)

 instance (MonadJoin m, Representational1 m) => MonadJoin (T m) where
   join_ = coerce @(forall a.   m (  m a) ->   m a)
                  @(forall a. T m (T m a) -> T m a)
                  join_
 }}}

 As this is essentially the code that this proposed deriving strategy would
 have generated (except with `unsafeCoerce` replaced with `coerce`).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14070#comment:23
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