We now have three kinds of canned tree traversals, all of which are smart about
which types they're applied to: explicit-descent transformations,
implicit-descent transformations, and implicit-descent checks. I've only
provided depth-first application of the latter two, but we could do
breadth-first in the future if necessary.
This provides gmapMFor and gmapMFor2, which are like gmapM, but know what
they're looking for, and can therefore avoid going down branches of the tree
that won't contain any interesting types.
The basic approach is quite similar to Uniplate's PlateData: there's a function
(containsType) that'll tell you whether one type is contained somewhere within
another. However, unlike Uniplate, we build a static IntMap IntSet of the types
we need to know about, which allows rather quicker lookups. (I did try using
PlateData first.)
The result is that applyDepthM is now much quicker than it was before.
applyDepthM2 is a bit less impressive, which I assume is because it can't
really prune the tree much if it's looking for two types.
Future enhancements:
- convert more passes to use applyDepthM*;
- make gmapMFor* aware of constructors rather than just types, which should
allow a bit more pruning.
