#5326: Polymorphic instances aren't automatically specialised
---------------------------------+------------------------------------------
Reporter: reinerp | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.0.3 | Keywords: specialisation
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
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Related to #255. Given (roughly) the example from that ticket:
{{{
f :: (Storable a, Eq a) => T a
g :: T (Ptr a)
g = f
}}}
we find that g is not specialised. Adding a SPECIALISE pragma fixes this,
but it ought not to be necessary.
The following module is a complete example:
{{{
module C where
class C a where f :: a -> a
newtype Id a = Id a
instance C (Id a) where f = id
g :: C a => Int -> a -> a
g 0 a = a
g n a = g (n-1) (f a)
h :: Int -> Id a -> Id a
h = g
j :: Int -> Id Int -> Id Int
j = g
}}}
We find that h passes a dictionary to g:
{{{
C.h =
\ (@ a_alq) (w_smq :: Int) (w1_smu :: C.Id a_alq) ->
case w_smq of _ { I# ww_sms ->
C.$wg @ (C.Id a_alq) (C.$fCId @ a_alq) ww_sms w1_smu
}
}}}
whereas j is specialised as desired, getting its own worker and no
dictionaries:
{{{
C.j =
\ (w_smg :: Int) (w1_smk :: C.Id Int) ->
case w_smg of _ { I# ww_smi ->
(C.$wj ww_smi w1_smk)
`cast` (sym (C.NTCo:Id Int)
:: Int ~ C.Id Int)
}
}}}
If we add
{{{
{-# SPECIALISE g :: Int -> Id a -> Id a #-}
}}}
then h is specialised as desired.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5326>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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