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.