On Wed, 8 Dec 2004, Derek Elkins wrote:
> > Is it possible and senseful for a compiler to extract common > > sub-expressions? Naively I think that for a given tree of an > > expression it is efficiently possible to find all common sub-trees. > > Referential transparency would assure that equal expressions have the > > same value, so they can be replaced by the same object. E.g. the > > example above could be automatically transformed to: > > > > ms as = > > let tmp0 = zipWith (+) (zipWith (*) as tmp1) (0:tmp1) > > tmp1 = 1:tmp0 > > in tmp0 > > It's possible and will preserve semantics, but isn't always an > optimization. > > http://www.haskell.org/pipermail/haskell-cafe/2003-December/005574.html I see the problem but I doubt that it is solved satisfyingly by letting the user choose whether he prefers storage or re-computing of data structures. What about the function double x = x++x ? Here 'x' may be a list that is better re-computed instead of being stored completely. But the replacement of common sub-expression is already performed by the one who calls 'double'. Though, GHC seems to have no problems particularly with this function. _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
