[GHC] #10116: Closed type families: Warn if it doesn't handle all cases

[GHC]

#10116: Closed type families: Warn if it doesn't handle all cases
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        Reporter:  andrewthad        |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Changes (by jstolarek):

 * cc: jan.stolarek@… (added)
 * type:  task => feature request


Old description:

> Right now, I can do the following:
>
> data Foo = A | B
> type family Bar (n :: 'Foo) where
>   Bar 'A = Int
>
> I would like to be warned if I write a partial type function like this
> because partial functions are almost always a mistake.

New description:

 Right now, I can do the following:

 {{{#!hs
 data Foo = A | B
 type family Bar (n :: 'Foo) where
   Bar 'A = Int
 }}}

 I would like to be warned if I write a partial type function like this
 because partial functions are almost always a mistake.

--

Comment:

 Checking pattern exhaustiveness for closed type families could also allow
 us to detect unreachable equations:

 {{{#!hs
 data Foo = A | B
 type family Bar (n :: Foo) where
   Bar 'A = Int
   Bar 'B = Char
   Bar a = Double
 }}}
 Here the last equation is unreachable but GHC does not discover this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10116#comment:1>
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