Section 8.13.2 of the GHC manual[1] states:
GHC keeps trying to apply the rules as it optimises the program. For example, consider:
let s = map f t = map g in s (t xs)
The expression s (t xs) does not match the rule "map/map", but GHC will substitute for s and t, giving an expression which does match. If s or t was (a) used more than once, and (b) large or a redex, then it would not be substituted, and the rule would not fire.
The part I'm interested in here is (a); if an expression is used more than one then it cannot be substituted for. Is there any way to work around this or force it? The reason I ask is that as a bit of fun (and inspired by Joachim Breitner's blog post [2]) I was going to try writing a rewrite rule for the first time. What I had in mind was this: {-# RULES "index cycled list" forall list n. cycle list !! n = list !! (n `mod` length list) #-} However, in the case he has written about this won't fire, since the LHS cannot be substituted as `cycle list` is used more than once:
let rlist = cycle list print ( rlist !! (10^9), rlist !! 0 )
I can get it to fire again if I write it like this:
{-# RULES "!!/cycle" forall list. (!!) (cycle list) = (\n -> list !! (n `mod` length list)) #-}
...
let rlist = (!!) (cycle list) print (rlist (10^9), rlist 0)
But this is non-obvious and I'd rather have it fire in the first case (i.e. when used naïvely). So, back to my question; is there a workaround or force for this... or does it break too many things if done? [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/rewrite-rules.html#i... [2] http://www.joachim-breitner.de/blog/archives/308-guid.html