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