[GHC] #12792: Wrong error message when using a data type as a class instance head

[GHC]

#12792: Wrong error message when using a data type as a class instance head
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        Reporter:  nomeata           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by nomeata):

 * priority:  normal => low


Comment:

 Related, but still minor: GHC would happily suggest a data type
 constructor where a class name is expected.
 {{{#!hs
 Prelude> data Bar a
 Prelude> instance Baz a

 <interactive>:3:10:
     Not in scope: type constructor or class ‘Baz’
     Perhaps you meant ‘Bar’ (line 2)
 }}}

 I read more through the code and see now why this is not easily done. The
 renamer does not know which names have methods, but for methods it knows
 what name they belong to (their parent, also just a name), so this can be
 caught early in the renamer.

 But since this parent information is “untyped”, we can actually trick the
 compiler in passing through the renamer, by using a record label (whose
 parent is the data type) as a class name:
 {{{#!hs
 data List a = List { bar :: a}
 instance List Int where bar = bar
 }}}
 produces
 {{{
 [1 of 1] Compiling Main             (.hs -> .o)

 /tmp/T12792.hs:2:10: error:
     • Expected a constraint, but ‘List Int’ has kind ‘*’
     • In the instance declaration for ‘List Int’
 }}}

 I get one way of getting saner behavior and early error reporting here
 would be to change the `NameSpace` type to distinguish between TyCons and
 classes, and at every use of namespaces pay close attention where one, the
 other, or both are valid. Since a `Name` knows it `NameSpace`, this would
 fix the problem at hand. But it would be quite a bit of work, and without
 other obvious benefits, not worth it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12792#comment:5>
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