Re: [GHC] #13585: ala from Control.Lens.Wrapped panics

18 Apr 2017

      #13585: ala from Control.Lens.Wrapped panics
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        Reporter:  fumieval          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by pacak):

 Lens.hs
 {{{

 {-# LANGUAGE KindSignatures, RankNTypes, TypeFamilies,
 MultiParamTypeClasses, FlexibleInstances,UndecidableInstances #-}

 module Lens where

 import Data.Monoid (First(..))
 import Data.Functor.Identity

 class Profunctor p where
   dimap :: (a -> b) -> (c -> d) -> p b c -> p a d
   dimap f g = lmap f . rmap g
   {-# INLINE dimap #-}

   lmap :: (a -> b) -> p b c -> p a c
   lmap f = dimap f id
   {-# INLINE lmap #-}

   rmap :: (b -> c) -> p a b -> p a c
   rmap = dimap id
   {-# INLINE rmap #-}

 data Exchange a b s t = Exchange (s -> a) (b -> t)

 instance Functor (Exchange a b s) where
   fmap f (Exchange sa bt) = Exchange sa (f . bt)
   {-# INLINE fmap #-}

 instance Profunctor (Exchange a b) where
   dimap f g (Exchange sa bt) = Exchange (sa . f) (g . bt)
   {-# INLINE dimap #-}
   lmap f (Exchange sa bt) = Exchange (sa . f) bt
   {-# INLINE lmap #-}
   rmap f (Exchange sa bt) = Exchange sa (f . bt)
   {-# INLINE rmap #-}

 withIso :: AnIso s t a b -> ((s -> a) -> (b -> t) -> r) -> r
 withIso ai k = case ai (Exchange id Identity) of
   Exchange sa bt -> k sa (runIdentity undefined bt)
 {-# INLINE withIso #-}

 type Iso s t a b = forall p f. (Profunctor p, Functor f) => p a (f b) -> p
 s (f t)
 type Iso' s a = Iso s s a a
 type AnIso s t a b = Exchange a b a (Identity b) -> Exchange a b s
 (Identity t)

 class    (Rewrapped s t, Rewrapped t s) => Rewrapping s t
 instance (Rewrapped s t, Rewrapped t s) => Rewrapping s t

 instance (t ~ First b) => Rewrapped (First a) t
 instance Wrapped (First a) where
     type Unwrapped (First a) = Maybe a
     _Wrapped' = iso getFirst First
     {-# INLINE _Wrapped' #-}

 class Wrapped s => Rewrapped (s :: *) (t :: *)

 class Wrapped s where
     type Unwrapped s :: *
     _Wrapped' :: Iso' s (Unwrapped s)

 _Wrapping :: Rewrapping s t => (Unwrapped s -> s) -> Iso s t (Unwrapped s)
 (Unwrapped t)
 _Wrapping _ = _Wrapped
 {-# INLINE _Wrapping #-}

 iso :: (s -> a) -> (b -> t) -> Iso s t a b
 iso sa bt = dimap sa (fmap bt)
 {-# INLINE iso #-}

 _Wrapped :: Rewrapping s t => Iso s t (Unwrapped s) (Unwrapped t)
 _Wrapped = withIso _Wrapped' $ \ sa _ -> withIso _Wrapped' $ \ _ bt -> iso
 sa bt
 {-# INLINE _Wrapped #-}

 au :: Functor f => AnIso s t a b -> ((b -> t) -> f s) -> f a
 au k = withIso k $ \ sa bt f -> fmap sa (f bt)
 {-# INLINE au #-}

 ala :: (Functor f, Rewrapping s t) => (Unwrapped s -> s) -> ((Unwrapped t
 -> t) -> f s) -> f (Unwrapped s)
 ala = au . _Wrapping
 {-# INLINE ala #-}

 }}}

 Panic.hs

 {{{
 module Panic where

 import Lens
 import Data.Monoid

 extractZonedTime :: Maybe ()
 extractZonedTime = ala First foldMap [Nothing]

 }}}

 Main.hs

 {{{
 module Main where
 import Panic (extractZonedTime)

 main :: IO ()
 main = print extractZonedTime

 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13585#comment:4
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