[Haskell-cafe] Compiler's bane

[John Meacham]

On Wed, Aug 27, 2008 at 08:59:28PM +0100, Andrew Coppin wrote:
>  let y = 5 in (f x) y
>
> ...and x is now a free variable. OOPS!
>
> Trying to tease out exactly under which conditions you can and cannot  
> perform the substitution is utterly maddening. Since this is a Haskell  
> mailing list and as such it is populated by vast numbers of people with  
> PhDs and so forth... does anybody happen to know the *correct* solution  
> to this conundrum? Before I become clinically insane...? o_O

A simple solution is to separate your 'substitution' and
'simplification' passes. As in, perform any substitutions you like, but
_keep_ the original binding for x.  let the simplifier/optimizer
recognize when bindings are dead and remove them. It can do so simply by
keeping track of free variables in the subterms, which is also useful
for things like let floating transformations.

        John


-- 
John Meacham - ⑆repetae.net⑆john⑈