id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5059	Pragma to SPECIALISE on value arguments	batterseapower		"I've sometimes found myself wishing for this pragma to get some ""partial evaluation on the cheap"". The idea is to allow something like:

{{{
defaultOpts :: Options
defaultOpts = ...

{-# SPECIALISE f defaultOpts :: Int -> Int #-}
f :: Options -> Int -> Int
f opts x = ... f opts ...
}}}

This would desugar into this additional code:

{{{
{-# RULES ""f/spec"" f defaultOpts = f_spec_1 #-}
f_spec_1 = (\opts x -> ... ... f opts ...) defaultOpts -- NB: body of f duplicated
}}}

The hope is that the simplifier and RULE matcher will tidy this up so we get a nice loop back to f_spec_1 with the body of the function specialised for the particular opts.

This is useful when functions are called often with particular arguments. An example would be where ""f"" is an edit-distance function which takes costs to be assigned to each edit, strings to be compared and returns an integer distance. In my library, the costs are given almost always going to be the default ones so I want to make that case fast, but I want to allow the user to supply their own set.

This pragma is somewhat subsumed by:

  1. SpecConstr, if the options are algebraic data/literals that are also scrutinised by the body of f

  2. Static argument transformation, except that the RULE based strategy achieves more code sharing compared to SAT

I think that pragma might be a relatively simple to implement nice-to-have feature."	feature request	new	low	7.6.2	Compiler	7.0.3			johan.tibell@…	Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown