Re: [GHC] #14172: GHC hangs during type-checking

31 Aug 2017

      #14172: GHC hangs during type-checking
-------------------------------------+-------------------------------------
        Reporter:  lightandlight     |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 So this is odd. I tried running the example file (after installing the
 `lens` package) on GHC 8.2.1, and while it didn't loop forever, it //did//
 stack overflow:

 {{{
 $ /opt/ghc/8.2.1/bin/ghc Foo.hs
 [1 of 1] Compiling Foo              ( Foo.hs, Foo.o )

 Foo.hs:7:19: error:
     • Reduction stack overflow; size = 201
       When simplifying the following type: g ~ Compose f'0 g'0
       Use -freduction-depth=0 to disable this check
       (any upper bound you could choose might fail unpredictably with
        minor updates to GHC, so disabling the check is recommended if
        you're sure that type checking should terminate)
     • In the expression: _Wrapping Compose . traverse
       In an equation for ‘traverseCompose’:
           traverseCompose = _Wrapping Compose . traverse
   |
 7 | traverseCompose = _Wrapping Compose . traverse
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 Which certainly isn't good. I then tried to remove external dependencies:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Lens where

 import Data.Coerce
 import Data.Functor.Compose
 import Data.Functor.Identity

 class Profunctor p where
   dimap :: (a -> b) -> (c -> d) -> p b c -> p a d
   (#.) :: Coercible c b => (b -> c) -> p a b -> p a c

 instance Profunctor (->) where
   dimap ab cd bc = cd . bc . ab
   {-# INLINE dimap #-}
   (#.) _ = coerce (\x -> x :: b) :: forall a b. Coercible b a => a -> b
   {-# INLINE (#.) #-}

 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

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

 type AnIso s t a b = Exchange a b a (Identity b) -> Exchange a b s
 (Identity t)

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

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

 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 #. bt)
 {-# INLINE withIso #-}

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

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

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

 instance (t ~ Compose f' g' a') => Rewrapped (Compose f g a) t
 instance Wrapped (Compose f g a) where
   type Unwrapped (Compose f g a) = f (g a)
   _Wrapped' = iso getCompose Compose

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

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

 import Data.Functor.Compose
 import Lens

 traverseCompose :: (a -> f b) -> g a -> f (h _)
 traverseCompose = _Wrapping Compose . traverse
 }}}

 However, once I do that, compiling with GHC 8.2.1 no longer stack
 overflows!

 {{{
 $ /opt/ghc/8.2.1/bin/ghc -c -O Lens.hs
 $ /opt/ghc/8.2.1/bin/ghc -c -O Bug.hs

 Bug.hs:6:46: error:
     • Found type wildcard ‘_’ standing for ‘a'’
       Where: ‘a'’ is a rigid type variable bound by
                the inferred type of
                traverseCompose :: (a -> f b) -> g a -> f (h a')
                at Bug.hs:7:1-46
       To use the inferred type, enable PartialTypeSignatures
     • In the type signature:
         traverseCompose :: (a -> f b) -> g a -> f (h _)
   |
 6 | traverseCompose :: (a -> f b) -> g a -> f (h _)
   |                                              ^

 Bug.hs:7:19: error:
     • Occurs check: cannot construct the infinite type: a ~ g'1 a
       Expected type: (f'0 a -> f (f'0 b))
                      -> Compose f'0 g'1 a -> f (h a')
         Actual type: (Unwrapped (Compose f'0 g'1 a)
                       -> f (Unwrapped (h a')))
                      -> Compose f'0 g'1 a -> f (h a')
     • In the first argument of ‘(.)’, namely ‘_Wrapping Compose’
       In the expression: _Wrapping Compose . traverse
       In an equation for ‘traverseCompose’:
           traverseCompose = _Wrapping Compose . traverse
     • Relevant bindings include
         traverseCompose :: (a -> f b) -> g a -> f (h a')
           (bound at Bug.hs:7:1)
   |
 7 | traverseCompose = _Wrapping Compose . traverse
   |                   ^^^^^^^^^^^^^^^^^

 Bug.hs:7:19: error:
     • Couldn't match type ‘g’ with ‘Compose f'0 g'1’
       ‘g’ is a rigid type variable bound by
         the inferred type of
         traverseCompose :: (a -> f b) -> g a -> f (h a')
         at Bug.hs:7:1-46
       Expected type: (a -> f b) -> g a -> f (h a')
         Actual type: (a -> f b) -> Compose f'0 g'1 a -> f (h a')
     • In the expression: _Wrapping Compose . traverse
       In an equation for ‘traverseCompose’:
           traverseCompose = _Wrapping Compose . traverse
     • Relevant bindings include
         traverseCompose :: (a -> f b) -> g a -> f (h a')
           (bound at Bug.hs:7:1)
   |
 7 | traverseCompose = _Wrapping Compose . traverse
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 So for some reason, this bug only seems to pop up when `lens` is installed
 as a library. I'm stumped.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14172#comment:2
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