GHC would be a better example, I think. It performs stream fusion. This means it can turn 'map f (map g xs)' into 'map (f . g) xs', and of course it gets more complex than that, but that's the basics. It directly optimises lists (which, this being Haskell, are lazy sequences).
Is it for built-in map or it would work in a general way for, say, `myMap f (myMap g xs)` ?
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Is it for built-in map or it would work in a general way for, say, `myMap f (myMap g xs)` ?
Just the stdlib `map`, but it's all ordinary library code. You can easily add your own rewrite rules.