[Haskell-cafe] SPECIALIZE in the presence of constraints

Max Bolingbroke batterseapower at hotmail.com
Sun Sep 25 20:00:47 CEST 2011

On 26 September 2011 01:37, Nicu Ionita <nicu.ionita at acons.at> wrote:
> 1. how can the compiler (here ghc) know which function to expose as the
> correct generic search function? There must be two search functions
> generated, one generic and its specialization.

Yes, exactly. If you have:

{-# SPECIALISE f :: Int -> Int #-}
f :: Num a => a -> a
f = ...

Then GHC basically generates:

f_spec :: Int -> Int
f_spec = f

f :: Num a => a -> a
f = ...

{-# RULES "f_spec" f = f_spec #-}

> Does the module export both
> and later the linker chooses the correct one, when the client module is
> known?

The RULES mechanism chooses the correct one: when f applied to the
specialised type arguments is seen, the generated RULE rewrites it to

> 2. how can I know that the specialization is really used? When I have
> constraints, will the specializations be generated in the assumption that
> the constraint is matched? When will be this match checked?

The specialisation will be used if GHC can see that the generic
function is applied to the correct type arguments. So for example a
call to f inside another polymorphic function g (say g contains a use
of f like: g = ... (f :: Int -> Int) ...) won't get specialised unless
g it itself specialised or inlined.

> My problem is that, with or without specializations, the performance of the
> code is the same - so I doubt the specializations are used.

GHC tells you which RULEs fired if you use -ddump-simpl or ghc-core.
That might help you diagnose it, since if you see a rule fired for the
specialisation then your code is probably using it. Failing that,
inspecting the core output itself is always useful.


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