[GHC] #11623: Wrong error message for type family definition that is wrong in multiple ways

[GHC]

#11623: Wrong error message for type family definition that is wrong in multiple
ways
-------------------------------------+-------------------------------------
           Reporter:  rwbarton       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 GHCi, version 8.1.20160212: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -XTypeFamilies
 Prelude> type family T where { Maybe T = Int }

 <interactive>:2:23: error:
     • Too many parameters to T:
         T is unexpected;
         expected only no parameters
     • In the type family declaration for ‘T’
 }}}
 This error message is wrong ("Too many parameters to `T`").

 Seems like GHC checks whether the equation `Maybe T = Int` has the right
 number of arguments before it checks whether it is actually headed by `T`.
 Then the error message is worded in a way that assumes the instance head
 actually was `T`.

 The 7.10 error message was better in this regard:
 {{{
 <interactive>:3:23:
     Number of parameters must match family declaration; expected 0
     In the type family declaration for ‘T’
 }}}
 but it would be even better to generate the error below (that is generated
 once the number of arguments is correct) for the original program:
 {{{
 Prelude> type family T a where { Maybe (T ()) = Int }

 <interactive>:5:25: error:
     • Mismatched type name in type family instance.
         Expected: T
           Actual: Maybe
     • In the type family declaration for ‘T’
 }}}
 We should generate that error before kind-checking the definition too.

 I don't really understand the difference between these two cases though,
 so maybe my guess is wrong.
 {{{
 Prelude> type family T where { T () = Int }

 <interactive>:10:23: error:
     • Too many parameters to T:
         () is unexpected;
         expected only no parameters
     • In the type family declaration for ‘T’
 Prelude> type family T :: * -> * where { T () = Int }

 <interactive>:11:33: error:
     • Number of parameters must match family declaration; expected 0
     • In the equations for closed type family ‘T’
       In the type family declaration for ‘T’
 }}}

 (As an aside, "expected only no parameters" is awkward, if there is some
 legitimate way to generate this message. It should just be "expected no
 parameters".)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11623>
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