| This is the main wibble people forget when writing rules -- inlining. | In your example, 'gen' is so cheap, it is immediately | inlined, so it won't be available to match on in your rule. I'll add a note in the user manual about this. In general, GHC tries RULES before inlining. In this particular example that fails for a tiresome internal reason -- but in any case the solution Don mentions is noticeably more robust. Simon | | Anything you want to match should have its inlining delayed: | | {-# OPTIONS -O2 -fglasgow-exts #-} | | gen :: [a] -> a | gen = head | {-# INLINE [1] gen #-} | | {-# RULES | "gen/Char" gen = spec | #-} | | spec :: [Char] -> Char | spec x = 'x' | | main :: IO () | main = putStr (gen "aaa":"\n") | | Running this in ghc-core, we see: | | RuleFired | 1 gen/Char | | Good. | And the program prints out: | | $ ./A | x | | There was some discussion of adding a warning for any function | used in the left hand side of a rewrite rule, that doesn't have | precise inlining control added. That would have caught this. | | -- Don | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users