[GHC] #11439: Request for comments: Allow duplicate type signatures

[GHC]

#11439: Request for comments: Allow duplicate type signatures
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I do think something like this would be useful. But recall that type
 signatures and function definitions need not be next to each other. And
 I'm very suspicious of a proposal that cares about top-to-bottom ordering
 of definitions (nowhere else in Haskell is that done).

 On the other hand, this is a good idea for a pragma. Perhaps

 {{{
 foo :: a -> a
 foo x = x

 {-# TYPECHECK
 foo :: Bool -> Bool
 foo :: Int -> Int
 foo :: Maybe a -> Maybe a
 foo True :: Bool
 map id []
   #-}
 }}}

 The idea here is that the `TYPECHECK` pragma allows to you list a bunch of
 expressions. GHC checks that the expressions are well-typed. If one isn't,
 an error is issued. You could imagine Haddock looking for examples where
 the expression is a type signature for a bare variable and then using the
 types as extra documentation for that variable.

 While we're at it, I'd also love

 {{{
 {-# ILL_TYPED
 map True
 False - 'x'
   #-}
 }}}

 If an expression in `ILL_TYPED` is actually well-typed, an error is
 issued. This would be very useful in putting in unit tests for fancy type-
 level features.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11439#comment:1>
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