GHC rewrite rules pragma

[Don Stewart]

jakubuv:
> Hi,
> 
> I'm trying to find out how the GHC rewrite rules pragma work, but I'm
> not able to make it working. I have this simple example, where I would
> like to specialize the function gen to spec on strings:
> 
> {-# OPTIONS -O2 -fglasgow-exts #-}
> 
> gen :: [a] -> a
> gen = head
> 
> {-# RULES "gen/Char" gen=spec #-}
> spec :: [Char] -> Char
> spec x = 'x'
> 
> main :: IO ()
> main = putStr (gen "aaa":"\n")
> 
> -- EOF
> 
> I compile it as:
>    ghc -O2 -fglasgow-exts spec.hs
> 
> but as a result, is always prints 'a' while I expect 'x'. Is it right?
> What is the problem here? I would be glad for any answer. I'm using
> GHC 6.8.2.
> 
> Sincerely,
>   Jan.

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.

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