Re: [GHC] #13264: GHC panic with (->) generalization branch while compiling lens

10 Feb 2017

      #13264: GHC panic with (->) generalization branch while compiling lens
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      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 bgamari):

 Here is a standalone reproducer,
 {{{#!hs
 {-# LANGUAGE Rank2Types #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module Control.Lens.Traversal where

 import Control.Category
 import Prelude hiding ((.),id)

 type Over p f s t a b = p a (f b) -> s -> f t

 newtype Bazaar p a b t = Bazaar { runBazaar :: forall f. Applicative f =>
 p a (f b) -> f t }

 newtype Pretext p a b t = Pretext { runPretext :: forall f. Functor f => p
 a (f b) -> f t }

 newtype Tagged s b = Tagged { unTagged :: b }

 class Conjoined p where

 holesOf :: forall p s t a. Conjoined p => Over p (Bazaar p a a) s t a a ->
 s -> [Pretext p a a t]
 holesOf l s = unTagged
   ( conjoined
      (Tagged $ let
         f [] _ = []
         f (x:xs) g = Pretext (\xfy -> g . (:xs) <$> xfy x) : f xs (g .
 (x:))
       in f (ins b) (unsafeOuts b))
      (Tagged $ let
         f [] _ = []
         f (wx:xs) g = Pretext (\wxfy -> g . (:Prelude.map extract xs) <$>
 cosieve wxfy wx) : f xs (g . (extract wx:))
       in f (pins b) (unsafeOuts b))
     :: Tagged (p a b) [Pretext p a a t]
   ) where b = l sell s
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13264#comment:1
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