[GHC] #11766: Lazy application gives "No instance" error while strict application works

29 Mar 2016

      #11766: Lazy application gives "No instance" error while strict application works
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           Reporter:  MichaelK       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This might make sense if there were any IO. Here's the code:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE OverlappingInstances #-}
 {-# LANGUAGE UndecidableInstances #-}

 import Data.Maybe (isJust)

 data Wrapper a = Wrapper a deriving (Show)

 class Resolution a
 instance Resolution (Wrapper a)

 class (Resolution b, Resolution d) => C a b c d | a -> b, c -> d, a d ->
 c, b c -> a where
   cfun :: (b -> d) -> a -> c

 instance (Resolution b, Resolution d, a ~ b, c ~ d) => C a b c d where
   cfun = ($)

 instance (Eq a, C b c d e) => C (Maybe a -> b) c (Maybe a -> d) e where
   cfun f b = \x -> cfun f (b x)

 foo :: Maybe a -> Wrapper Bool
 foo = Wrapper . isJust
 }}}

 Applying `Nothing` strictly or in a `let` clause gives the expected answer
 (I expect that `cfun id foo` would be equivalent to `foo`):
 {{{#!hs
 *Main> cfun id foo $! Nothing
 Wrapper False

 *Main> let f = cfun id foo in f Nothing
 Wrapper False
 }}}
 But regular application (or just `(cfun id foo) Nothing`) returns the
 following error:
 {{{#!hs
 *Main> cfun id foo $ Nothing
 <interactive>:6:1:
     No instance for (Resolution (Maybe a0 -> Wrapper Bool))
       (maybe you haven't applied enough arguments to a function?)
       arising from a use of ‘cfun’
     In the expression: cfun id foo
     In the expression: cfun id foo $ Nothing
     In an equation for ‘it’: it = cfun id foo $ Nothing
 }}}

 In case it helps, the purpose of this code is for `cfun` to have the
 effective type of
 {{{#!hs
 cfun :: (Eq a0, Eq a1, .., Eq an) => (Wrapped b -> Wrapped c)
      -> (Maybe a0 -> Maybe a1 -> .. -> Maybe an -> Wrapped b)
      -> (Maybe a0 -> Maybe a1 -> .. -> Maybe an -> Wrapped c)
 }}}
 i.e. apply a function to the "wrapped" return value of another function of
 the above form.

 Tested on GHC 8.0.1-rc2 (most recent OSX binary as of now) and 7.10.3.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11766
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