#5059: Pragma to SPECIALISE on value arguments
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 7.0.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5059>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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