Re: [GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition")

22 Jul 2014

      #8634: Relax functional dependency coherence check ("liberal coverage condition")
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              Reporter:  danilo2     |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:  7.10.1
              Priority:  high        |           Version:  7.7
             Component:  Compiler    |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:  Phab:D69    |   Type of failure:  None/Unknown
          Architecture:              |         Test Case:
  Unknown/Multiple                   |          Blocking:
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:  #1241,      |
  #2247, #8356, #9103, #9227         |
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Comment (by danilo2):

 Replying to [comment:37 rwbarton]:
 It still would not work:
 1) the following script does not compile
 2) I DO want to determine the output type based on the input types - I
 really want to introduce there fundep. I mean - I want to be able to write
 `tst a = property (property a)` and use it as `tst X` and get `Z` as the
 result.

 Additional - this is only one of the exampels we are using the extension.
 There are some other places where it works quite well. I do not know if
 here is the best place to discuss possible workarounds (but I'm very happy
 and thankfull to hear suggestions)? Anyway, I post the code I mentioned
 above:

 {{{#!haskell
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}

 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE AllowAmbiguousTypes #-}

 class Property a b where
    property :: a -> b

 data X = X
 data Y = Y deriving Show
 data Z = Z deriving Show

 instance (y ~ Y) => Property X y where
     property _ = Y

 instance (z ~ Z) => Property Y z where
     property _ = Z

 tst :: forall a s b. (Property a s, Property s b) => a -> b
 tst a = property (property a :: s)

 main = do
     print (property $ property X)

 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:38
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