[GHC] #8177: Roles for type families

[GHC]

#8177: Roles for type families
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        Reporter:  simonpj           |            Owner:
            Type:  bug               |           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:
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Comment (by simonpj):

 Harump.  You are right.  `type` families are the very source of pattern
 matching so ''of course'' `type` families should have argument of role N.

 You could argue that there might be a parameter of the family that is
 never matched. Thus:
 {{{
 type family F a at R b at N
 type instance F a Int = a -> a
 type instance F a Bool = (a,a)
 }}}
 But it's a bit marginal; in most cases you could re-order the args
 {{{
 type family F b at N :: * -> *
 }}}
 and similarly for the instances.

 I think it's the same for `data` families. For example, it'd be utterly
 wrong to make a newtype coercion between `(D Age)` and `(D Int)`; so `D`'s
 parameter must clearly be role N.

 In short, the main merit of this ticket would be to allow ''parametric''
 (non-indexed) parameters to appear before ''index'' parameters.  That
 seems a fairly marginal gain.

 Simon

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8177#comment:2>
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