[GHC] #9701: GADTs not specialized properly

[GHC]

#9701: GADTs not specialized properly
-------------------------------------+-------------------------------------
              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.9
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  Runtime     |       Blocked By:
  performance bug                    |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
Description changed by dfeuer:

Old description:

> This has probably been raised before, but I can't find a ticket. If I
> write
>
> {{{#!hs
> data Silly a where
>   Silly :: Ord a => a -> Silly a
>
> isItSilly :: a -> Silly a -> Bool
> isItSilly a (Silly x) = a < x
>
> isItSillyInt :: Int -> Silly Int -> Bool
> isItSillyInt = isItSilly
> }}}
>
> then I get
>
> {{{
> isItSilly
> isItSilly =
>   \ @ a_aBq eta_B2 eta1_B1 ->
>     case eta1_B1 of _ { Silly $dOrd_aBs x_aAy ->
>     < $dOrd_aBs eta_B2 x_aAy
>     }
>
> isItSillyInt
> isItSillyInt = isItSilly
> }}}
>
> Although GHC knows that `eta_B2` is an `Int`, and `x_aAy` therefore must
> be one as well, it looks up the `<` method in the `eta1_B1` dictionary
> instead of just using `ltInt`.

New description:

 This has probably been raised before, but I can't find a ticket. If I
 write

 {{{#!hs
 data Silly a where
   Silly :: Ord a => a -> Silly a

 isItSilly :: a -> Silly a -> Bool
 isItSilly a (Silly x) = a < x

 isItSillyInt :: Int -> Silly Int -> Bool
 isItSillyInt = isItSilly
 }}}

 then I get

 {{{
 isItSilly
 isItSilly =
   \ @ a_aBq eta_B2 eta1_B1 ->
     case eta1_B1 of _ { Silly $dOrd_aBs x_aAy ->
     < $dOrd_aBs eta_B2 x_aAy
     }

 isItSillyInt
 isItSillyInt = isItSilly
 }}}

 Although GHC knows that `eta_B2` and `x_aAy` are `Int`, it looks up the
 `<` method in the `eta1_B1` dictionary instead of just using `ltInt`.
 Note: it does not help to `INLINE` `isItSilly`.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9701#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler