Re: [GHC] #12863: Associated data families don't use superclasses when deriving instances

22 Nov 2016

      #12863: Associated data families don't use superclasses when deriving instances
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 This is just what I'd expect.

 The `deriving( Functor )` clause is trying to infer a suitable context
 `???` for
 {{{
 instance ??? => Functor (TotalMap (Identity key))
 }}}
 Let's call this "deriving context inference" (DCI).

 To do that it needs `Functor (TotalMap key)`.  But DCI insists on a simple
 context with a class applied to type variables, otherwise it could infer
 stupid context like `Num (Tree a)`.  So it insists on being able to
 simplify `Functor (TotalMap key)` and it can't.

 It's true that `Key key` is also OK for `???`, since `Functor (TotalMap
 key)` is a superclass, but DCI will never guess that.  Apart from anything
 else it might be too restrictive.

 Now I think you are pointing out that DCI for an ''associated'' data type
 could include the context of the parent instance declaration.  It would
 add a bit more code and complexity (to the compiler and the user manual).
 And it might be much more restrictive than necessary.  E.g.
 {{{
 instance (Key key) => Key (Identity key) where
   data TotalMap (Identity key) val = Id val deriving( Functor )
 }}}
 I suppose we ''could'' specify that the derived instance is
 {{{
 instance (Key key) => Functor (TotalMap (Identity key))
 }}}
 but today we'll get the superior
 {{{
 instance Functor (TotalMap (Identity key))
 }}}
 But it would be a valid alternative design.

 My suggestion: use a standalone deriving clause to specify the context
 yourself.  As you point out, it's not hard.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12863#comment:2
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