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

1 Aug 2017

      #14070: Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               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 Iceland_jack):

 That sounds excellent, I'm thrilled about this feature.

 Incidentally is it possible to coerce `Lens s a = forall f. Functor f =>
 (a -> f a)` types?

 {{{#!hs
 class X f where
   x :: Lens (f a) a

 newtype WrappedX f a = WrapX (f a)

 instance X f => X (WrappedX f) where
   x :: forall a. Lens (WrappedX f a) a
   x = unsafeCoerce x' where

     x' :: Lens (t a) a
     x' = x @t @a
 }}}

 fails with

 {{{
     • Could not deduce (Functor f0) arising from a use of ‘x'’
       from the context: X t
         bound by the instance declaration at /tmp/uuu.hs:21:10-25
       or from: Functor f
         bound by the type signature for:
                    x :: Functor f => (a -> f a) -> Foo t a -> f (Foo t a)
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14070#comment:6
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