[Haskell-cafe] SPECIALIZE in the presence of constraints

Nicu Ionita nicu.ionita at acons.at
Sun Sep 25 18:37:18 CEST 2011

Hallo List,

I can't understand how pragma SPECIALIZE works in the presence of 

I have 2 modules, one which defines a general search framework, and one 
which implements it in a concrete context. The general module defines 
functions like:

{-# SPECIALIZE pvQSearch :: Node (Game m) Move Int => Int -> Int -> Int 
-> Search (Game m) Move Int Int #-}
pvQSearch :: Node m e s => s -> s -> Int -> Search m e s s
pvQSearch a b c = do ...

while the implementation uses concrete data types Move and Int (for e 
and s respectively) and defines instances and other types (like the 
polimorphic type Game m, where m is a monad).

Node m e s is a class and the instance Node (Game m) Move Int is defined 
in the implementation module. From outside this construct is used by 
calling a generic search function, but giving parameters which match the 
implementation (i.e. with Move and Int for e and s, for example).

Now what I don't understand:

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

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?

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



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