RULES pragmas

[Simon Peyton-Jones]

I've started a Wiki page, attached to GHC's collaborative documentation,
as a place to collect advice about RULES.
	http://haskell.org/haskellwiki/GHC/Using_Rules

Please feel free to elaborate it.

Simon


| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-bounces at haskell.org]
| On Behalf Of Donald Bruce Stewart
| Sent: 12 July 2006 01:41
| To: Malcolm Wallace
| Cc: glasgow-haskell-users at haskell.org
| Subject: Re: RULES pragmas
| 
| Malcolm.Wallace:
| > I have a question about {-# RULES #-} pragmas.  Here is a very
simple
| > attempt to use them:
| >
| >     module Simplest where
| >     {-# RULES
| >     "simplestRule"       forall x.   id (id x) = x
| >       #-}
| >     myDefn = id (id 42)
| >
| > I want to verify whether ghc-6.4.1 does actually fire this rule, but
| > have so far been unable to do so.  According to the manual (section
| > 7.10.5), the flag -ddump-rules should list "simplestRule" if it has
been
| > parsed correctly, and -ddump-simpl-stats should list the number of
times
| > it has fired.  But it does not appear in either listing.
| >
| > Reasoning that I have the syntax wrong, I have tried numerous
variations
| > on the indentation, added type signatures, etc., all to no avail.
| >
| > So what am I doing wrong?  And is there any way to ask the compiler
to
| > give a warning if the RULES pragma contains errors?
| 
| In this case, it's because it's missing -fglasgow-exts, I think.
| The following works for me with both 6.4 and 6.5 compilers:
| 
|     module Simplest where
| 
|     {-# RULES
|     "simplestRule" forall x. id (id x) = x
|     #-}
| 
|     myDefn = id (id 42)
| 
| when compiled with:
|     $ ghc-6.4.2 -fglasgow-exts -c -ddump-simpl-stats A.hs
| 
|     ==================== Grand total simplifier statistics
|     Total ticks:     11
| 
|     2 PreInlineUnconditionally
|     3 PostInlineUnconditionally
|     1 UnfoldingDone
|     1 RuleFired
|         1 simplestRule
|     4 BetaReduction
|     2 SimplifierDone
| 
| However, in general, you need to be careful that your identifiers
| weren't inlined in the first phase. To control this we add an INLINE
[1]
| pragma to identifiers we want to match in rules, to ensure they
haven't
| disappeared by the time the rule matching comes around.
| 
| Also, you need -O to have rules kick in locally.
| 
| So,
|     module Simplest where
| 
|     {-# RULES
|     "simplestRule" forall x. myid (myid x) = x
|     #-}
| 
|     myDefn = myid (myid 42)
| 
|     myid x = x
|     {-# INLINE [1] myid #-}
| 
| And:
|     $ ghc-6.4.2 -fglasgow-exts -O -c -ddump-simpl-stats A.hs
| 
|     ==================== Grand total simplifier statistics
====================
|     Total ticks:     15
| 
|     6 PreInlineUnconditionally
|     2 UnfoldingDone
|     1 RuleFired
|         1 simplestRule
|     5 BetaReduction
|     1 KnownBranch
|     8 SimplifierDone
| 
| Cheers,
|   Don
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