[GHC] #12466: Typechecker regression: Inaccessible code in a type expected by the context

[GHC]

#12466: Typechecker regression: Inaccessible code in a type expected by the context
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 mpickering, I'm a bit confused too why `foo 5 'a'` itself doesn't
 typecheck, but that's not how `lens` is using this style of code. The way
 `lens` is using it, it's closer in style to something like this:

 {{{#!hs
 bar :: Foo a => a -> a
 bar = foo 5
 }}}

 Now `bar 'a'` will typecheck.

 Similarly, if you want a more complete `lens`-based piece of code to test
 against, you can use the following:

 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}
 module Lens where

 indexing :: Indexable Int p => ((a -> Indexing f b) -> s -> Indexing f t)
 -> p a (f b) ->
 s -> f t
 indexing l iafb s = snd $ runIndexing (l (\a -> Indexing (\i -> i `seq` (i
 + 1, indexed ia
 fb i a))) s) 0

 newtype Indexing f a = Indexing { runIndexing :: Int -> (Int, f a) }

 instance Functor f => Functor (Indexing f) where
   fmap f (Indexing m) = Indexing $ \i -> case m i of
     (j, x) -> (j, fmap f x)

 instance Applicative f => Applicative (Indexing f) where
   pure x = Indexing $ \i -> (i, pure x)
   Indexing mf <*> Indexing ma = Indexing $ \i -> case mf i of
     (j, ff) -> case ma j of
        ~(k, fa) -> (k, ff <*> fa)

 type IndexedTraversal i s t a b = forall p f. (Indexable i p, Applicative
 f) => p a (f b)
 -> s -> f t

 class Conjoined p where
   conjoined :: ((p ~ (->)) => q (a -> b) r) -> q (p a b) r -> q (p a b) r
   conjoined _ r = r

 class Conjoined p => Indexable i p where
   indexed :: p a b -> i -> a -> b

 ---------------------------------------------------------------------

 newtype Indexed i a b = Indexed { runIndexed :: i -> a -> b }

 -- You'll need to comment out the instance below to typecheck on HEAD
 instance Conjoined (Indexed i)

 ---------------------------------------------------------------------

 -- This now typechecks:
 traversed :: Traversable f => IndexedTraversal Int (f a) (f b) a b
 traversed = conjoined traverse (indexing traverse)
 }}}

 -----

 The fact that `foo 5 'a'` doesn't typecheck might be part of the bug. As
 Edward mentioned above, I suspect that we are too eagerly discharging an
 obligation somewhere, and that explains:

 1. Why `foo 5 'a'` doesn't typecheck but `bar 'a'` does
 2. Why a manually implemented `instance Foo Char where foo _ a = a`
 typechecks but a default `instance Foo Char` doesn't

 But these are just my suspicions.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:21>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler