Re: [GHC] #13306: Problems with type inference for static expressions

25 May 2017

      #13306: Problems with type inference for static expressions
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        Reporter:  edsko             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
                                     |  StaticPointers
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 edsko):

 Here's another much simpler test case:

 {{{#!hs
 {-# LANGUAGE GADTs          #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE StaticPointers #-}

 {-# OPTIONS_GHC -Wall #-}

 import GHC.StaticPtr

 data U :: * -> * where
   UBool :: U Bool
   UInt  :: U Int

 toDouble :: U a -> StaticPtr (a -> Double)
 toDouble UBool = static (\x -> if x then 1 else 0)
 toDouble UInt  = static fromIntegral
 }}}

 The first line yields "Couldn't match expected type `Bool` with actual
 type `a`", and the second line yields "No instance for (`Integral a`)".
 Writing

 {{{#!hs
 toDouble UInt  = static (fromIntegral :: Int -> Double)
 }}}

 instead yields "Couldn't match type `a` with `Int`; but

 {{{#!hs
 toDouble UInt  = static fromIntegral :: StaticPtr (Int -> Double)
 }}}

 ''is'' accepted. If we instead use

 {{{#!hs
 fakeStatic :: a -> StaticPtr a
 fakeStatic = undefined
 }}}

 then of course

 {{{#!hs
 toDouble :: U a -> StaticPtr (a -> Double)
 toDouble UBool = fakeStatic (\x -> if x then 1 else 0)
 toDouble UInt  = fakeStatic fromIntegral
 }}}

 is accepted as is.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13306#comment:7
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