Desugaring of array comprehensions
Wadler's desugaring for list comprehensions is not suitable for arrays, as we need to use collective operations to get good parallel code. The build/foldr desugaring, although using collective operations, isn't a good match for how the array operations are implemented. In fact, the naive desugaring from the H98 report is a much better fit:
(1) [: e | :] = [:e:] (2) [: e | b, qs :] = if b then [: e | qs :] else [::] (3) [: e | p <- a, qs :] = let ok p = [: e | qs :] ok _ = [::] in concatMapP ok a (4) [: e | let ds, qs :] = let ds in [: e | qs :] (5) [: e | qs | qss :] = [: e | (XS, XSS) <- zip [: XS | qs :] [: XSS | qss :] :] where XS & XSS are the bound variables in qs & qss
In particular, concatMapP f a essentially implies to apply the lifted version of f directly to a and then the concat strips of one level of segment descriptors; i.e., both the concatP and the mapP vanish due to vectorisation.
Problem with the naive rules
Nevertheless, these rules are not entirely satisfactory. For example, [:e | x <- a, b:] turns into
concatMap (\x -> if b then [:e:] else [::]) a
which is a fairly complicated way to perform
mapP (\x -> e) . filterP (\x -> b) $ a
even when taking vectorisation into account. Under vectorisation, the conditional implies filterP (\x -> b), but adds an expensive, and here useless, merge operation. Maybe these overheads can be optimised away. However, for the moment, we use a desugaring that is based on the above rules, but generates code that should be better suited to array processing.
The idea is to flatten out the processing of comprehensions to some degree by defining a transformation function << . >> that gets two arguments: a pattern pa and a desugared expression ea, where we are guaranteed that ea is array valued and all its elements match pa. The semantics of the transformation function is given by
<<[: e | qs :]>> pa ea = [: e | pa <- ea, qs :] = concatMap (\pa -> [: e | qs :]) ea
We have the second line by applying Rule (3).
Using this definition of << . >>, we can derive a new set of desugaring rules. The derivation proceeds by unfold/fold transformations and some properties of the involved combinators. The resulting rules are the following:
(1') <<[: e | :]>> pa ea = mapP (\pa -> e) ea (2') <<[: e | b, qs :]>> pa ea = <<[: e | qs :]>> pa (filterP (\pa -> b) ea) (3') <<[: e | p <- a, qs :]>> pa ea = let ok p = True ok p = False in <<[: q | qs :]>> (pa, p) (crossMapP ea (\pa -> filterP ok a)) (4') <<[: e | let ds, qs :]>> pa ea = <<[: e | qs :]>> (pa, XS) (mapP (\v@pa -> let ds in (v, XS)) ea) where XS are the variables bound by ds (5') <<[: e | qs | qss :]>> pa ea = <<[: e | qss :]>> (pa, XS) (zipP ea [: XS | qs :]) where XS are the variables bound by qs
The typical array processing comprehensions containing only generators, guards, and parallel comprehensions (but not cross-products and lets) are translated into a straight combination of mapP, filterP, and zipP by these rules, which is exactly what we want.