simple rewrite rule example

[Don Stewart]

jac:
> Hi,
>
> I'd like to learn how to use ghc rewrite rules. I simply want to replace 
> a function called f by a function called g. I do not unterstand why the 
> rule f->g does not fire.
>
> Cheers, Jan
>
>
> module Main where
>
>
> {-# RULES
>   "f->g"    forall x. f x = g x
>   #-}
>
>
> main :: IO ()
> main = print (f 1)
>
>
> {-# NOINLINE f #-}
> f :: a -> Bool
> f _ = False
>
>
> g :: a -> Bool
> g _ = True
>
>

Did you enable rewrite rules?

    $ ghc -fenable-rewrite-rules A.hs --make

    $ time ./A                              
    True
    ./A  0.01s user 0.00s system 88% cpu 0.011 total

Using ghc-core:

    1 RuleFired
        1 f->g


-- Don