RULES pragmas

[Donald Bruce Stewart]

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