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

26 Aug 2017

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

Comment (by Iceland_jack):

 I'll keep it, I want to add another example I that could be derived

 {{{#!hs
 newtype Foo a = Foo (forall xx. Show a => IO xx)
   deriving newtype
     Alternative

 instance Functor Foo where

 instance Applicative Foo where
 }}}

 currently we can write this mess

 {{{#!hs
 instance Alternative Foo where
   empty :: Foo a
   empty = Foo (coerce (empty @IO @xx) :: forall xx. IO xx)

   (<|>) :: Foo a -> Foo a -> Foo a
   Foo a <|> Foo b = Foo
     (coerce ((<|>) @IO @xx a b) :: forall xx. IO xx)
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14070#comment:18
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler