Hi all, some time ago me and my friend had a discussion about optimizations performed by GHC. We wrote a bunch of nested list comprehensions that selected objects based on their properties (e.g. finding products with a price higher than 50). Then we rewrote our queries in a more efficient form by using tree-based maps as indices over the data. My friend knows how to turn nested list comprehensions into queries that use maps and claims this could be automated. He argued that it would be fine if compiler did such an optimization automatically, since we can guarantee that both versions have exactly the same semantics and if they have the same semantics we are free to use faster version. From a functional point of view this is a good point, but nevertheless I objected to his opinion, claiming that if compiler performed such a high-level optimization - replace underlying data structure with a different one and turn one algorithm into a completely different one - programmer wouldn't be able to reason about space behaviour of a program. I concluded that such a solution should not be built into a compiler but instead turned into an EDSL. I would like to hear your opinion on this. How far a compiler can go with transforming code? I don't want to concentrate on discussing whether such an optimization is possible to implement from a technical point of view. What I'm interested in is whether such kind of high-level optimization could be accepted. Janek