One of the things haskell can do with lazy sequences is fuse away intermediate data structures, e.g. take map f . map g and substitute map (f . g) autom

One of the things haskell can do with lazy sequences is fuse away intermediate data structures, e.g. take map f . map g and substitute map (f . g) automatically (as a compiler optimization). I don't think that's a safe transformation in general in a non-pure language; not sure about a pure strict language.