Re: [Haskell-cafe] fmap vs. liftM

On Tue, 5 Feb 2008, Felipe Lessa wrote:

On Feb 5, 2008 6:06 PM, Dan Weston wrote:

...

Can you do this with a GHC rule? Something like:

{-# RULES "join_dot_fmap_return/id" forall x . join (fmap return x) = x #-}

Dan

I guess this would make use of the rule (otherwise the transformation would change the code's semantic) but would not enforce that the rule itself is valid (which is undecidable).

I have already thought about (ab)using GHC rules for forcing programmers to take care. :-) That is, if the rule would be stated as above, then programmers _have_ to ensure that the law is satisfied, and the optimizer will penalize violations of the rules with non-working code.