[GHC] #11534: Allow class associated types to reference functional dependencies

[GHC]

#11534: Allow class associated types to reference functional dependencies
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        Reporter:  ekmett            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  FunctionalDependencies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Yes I've often thought of doing this.  An alternative, which no one has
 ever fleshed out, would be to extend System FC somehow to provide evidence
 for functional dependencies.  But what Adam suggests here is more or less
 precisely that; the `Foo` class is a record that has `(j ~ Foo_FD1 i)` as
 one of its fields; that is, evidence that `(j ~ Foo_FD1 i)`.

 I worry a bit about how elaborate the translation might be, and whether
 error messages might mention these strange functions.

 An alternative is just to use the type-family approach in the first place.

 Hmm.  Maybe we could use default declarations to make it even easier:
 {{{
 class j ~ Foo_FD1 i => Foo i j where
   type Foo_FD1 i :: *
   type Foo_FD1 i = j

 instance Foo Int Bool where
   -- Nothing
 }}}
 This is currently rejected because `j` is not bound on the LHS of the
 default decl, but if we allowed it, then we'd get, well, precisely what we
 want.

 Oh, the idea doesn't work at all.  Consider
 {{{
 instance Foo a b => Foo [a] [b] where
   type Foo_FD1 [a] = [F a]  -- This RHS is not so easy to generate!
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11534#comment:25>
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