[GHC] #8021: Multiple constraint classes - the alternative to superclass

[GHC]

#8021: Multiple constraint classes - the alternative to superclass
-------------------------------------+------------------------------------
        Reporter:  wvv               |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------

Comment (by wvv):

 (I) We don't change the type system.

 (II) We have some problems with the type inference. Haskell have 2
 solution if it has an ambiguity:

 1) The error. You must add type signature

 Functions

 {{{
 foo :: Monoid [a] -> [a] -> [a] -> [a]

 bar :: (Arrow a, ArrowPlus a, ArrowZero a) => Monoid a => a b c -> a b c
 -> a b c
 }}}


 2) Default meanings. For example, use class in constraint with minimum
 dependences.


 (III) A lot of work for the type checker.

 Let we have 2 classes:
 {{{
 class C1 => C a0 a1 .. an where
     data ...
     type ...
     f1 :: b1f1 -> b2f2 -> .. -> bn'f1
     f2 :: b1f2 -> b2f2 -> .. -> bn''f2
     ..
     fu :: b1fu -> b2fu -> .. -> bn'''fu

 class C2 => C d0 d1 .. dm where
     data ...
     type ...
     g1 :: b1g1   -> b2g2 -> .. -> bm'g1
     g2 :: b1g2   -> b2g2 -> .. -> bm''g2
     ..
     gu' :: b1gu' -> b2gu' -> .. -> bm'''gu'
 }}}

 Lows:

 1) C1 =/= C2  -- constraint must be different

 2) n == m     -- number of parameters must be the same

 3) u == u'    -- number of functions must be the same

 4) forall i . s(fi) == s(gi)   -- names of functions must be the same

 5) forall i.  n'{i} == m'{i}   -- number of arguments for each functions
 must be the same

 6) forall i j. (bifj :: *) & (bigj :: *) | (bifj :: k) & (bigj :: k)  --
 corresponding arguments must have the same kind

 That's all!

  - forall i. ai :: kia , di :: kid, kia =?= kib - class parameters could
 have different kinds

  - datas and types of C1=>C and C2=>C could be different (by number, kind
 and name)

 Uses in signature:

 {{{
 foo :: C1 => C a => ...

 bar :: C2 => C b => ...

 baz :: C d => ....   <<=|=>>  baz :: () => C d => ...
 }}}


 We use classes like instances of meta-classes

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8021#comment:9>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler