[GHC] #8669: Closed TypeFamilies regression

[GHC]

#8669: Closed TypeFamilies regression
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        Reporter:  merijn                       |            Owner:
            Type:  bug                          |           Status:  new
        Priority:  high                         |        Milestone:
       Component:  Compiler                     |          Version:  7.7
      Resolution:                               |         Keywords:
Operating System:  Unknown/Multiple             |     Architecture:
 Type of failure:  Incorrect result at runtime  |  Unknown/Multiple
       Test Case:                               |       Difficulty:
        Blocking:                               |  Unknown
                                                |       Blocked By:
                                                |  Related Tickets:
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Comment (by simonpj):

 I know why this happens.  If I add the kinds to the definition, I get
 this:
 {{{
 type family Restrict k (a :: k) (as :: [k]) :: Constraint where
     Restrict Symbol (a::Symbol) (a ': as) = (a ~ "Oops! Tried to apply a
 restricted type!")
     Restrict k (x::k) (a ': as) = Restrict x as
     Restrict k (x::k) '[] = ()
 }}}
 The first equation only applies to types of kind `Symbol` because the RHS
 has an equality that forces `a :: Symbol` since `"Oops" :: Symbol`.

 The constraint `(Restricted (a:*) [(),Int])` can't match the first
 equation (because the first equation matches only things of kind `Symbol`,
 so it correctly chooses the second.

 What to do?  For your (2) how about this:
 {{{
 class Error (s::Symbol)    -- No instances
 }}}
 Now `Error :: Symbol -> Constraint`, as you wanted, and you can write
 {{{
 type instance where
     Restrict a (a ': as) = Error "Oops! Tried to apply a restricted type!"
     Restrict x (a ': as) = Restrict x as
     Restrict x '[] = ()
 }}}
 And away you go.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8669#comment:1>
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