Hi Andrew, my probably dodgy reason for mentioning deforestation is that sharing of intermediate values is a major stumbling block; code that uses data linearly is possibly well suited for deforesting. See Frankau's SASL for a language that deforests all lists simply by not letting you copy them! (IIRC there is another constraint that forbids accumulating parameters too.)
Similarly, there are recursion patterns for which fusion isn't very easy.
Yep, I suspect you're right.
That's why most array-based code is explicitly in-place. wouldn't it be nice if it didn't have to be?
I agree. As an aside, DiffArray looks quite nice: http://www.haskell.org/haskellwiki/Modern_array_libraries ``if a diff array is used in a single-threaded style, ..., a!i takes O(1) time and a//d takes O(length d).'' Notice the use of "seq" in 2nd example to enforce a kind of single-threaded behaviour. Seems nasty! I wonder if this could be avoided by providing a (*!*) such that arr *!* i = seq x (arr, x) where x = arr ! i It returns a new array which the programmer should use if they want single-threaded behaviour. Matt.