Re: [GHC] #14995: QuantifiedConstraints: Incorrect pretty printing

2 Apr 2018


      #14995: QuantifiedConstraints: Incorrect pretty printing
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints, wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

Old description:
...
This code from [https://www.microsoft.com/en-us/research/wp-
content/uploads/2016/02/jfp-outsidein.pdf OutsideIn(X)] compiles
{{{#!hs
{-# Language FlexibleInstances, MultiParamTypeClasses, GADTs,
QuantifiedConstraints #-}
class C a
class           B a b   where op :: a -> b
instance C a => B a [a] where op = undefined
data R a where
  MkR :: C a => a -> R a
k :: (C a => B a b) => R a -> b
k (MkR x) = op x
}}}
but pretty-printing `k` prints `B a b => ..` instead of `(C a => B a b)
=> ..`
{{{
$ ghci -ignore-dot-ghci Test.hs
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( hs/228-bug.hs, interpreted )
Ok, one module loaded.
*Main> :t k
k :: B a b => R a -> b
*Main>
}}}
New description:

 This code from [https://www.microsoft.com/en-us/research/wp-
 content/uploads/2016/02/jfp-outsidein.pdf OutsideIn(X)] compiles

 {{{#!hs
 {-# Language FlexibleInstances, MultiParamTypeClasses, GADTs,
 QuantifiedConstraints #-}

 class C a
 class           B a b   where op :: a -> b
 instance C a => B a [a] where op = undefined

 data R a where
   MkR :: C a => a -> R a

 k :: (C a => B a b) => R a -> b
 k (MkR x) = op x
 }}}

 but pretty-printing `k` prints `B a b => ..` instead of `(C a => B a b) =>
 ..`

 {{{
 $ ghci -ignore-dot-ghci hs/228-bug.hs
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( hs/228-bug.hs, interpreted )
 Ok, one module loaded.
 *Main> :t k
 k :: B a b => R a -> b
 *Main>
 }}}

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14995#comment:1
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