Question

I’ve run into this problem a few times and haven’t found any standard solution, so I’m asking here.

For a specific example, imagine I have a list of pairs of words and their part of speech (I’m doing a natural language processing homework assignment), and I’d like to be able to, given a part of speech, look up the counts of the words I’ve encountered.

Is there an accepted solution? Any advice? A template haskell library that solves this for arbitrary depth maps (Hey, I can dream, can’t I)?

Answer 1

If a Map (k1, k2) v is unsuitable (perhaps if you need to extract and manipulate Map k2 vs), then it’s not too hard to define composite Map manipulation functions. e.g.

lookup2 :: (Ord k1, Ord k2) => k1 -> k2 -> Map k1 (Map k2 v) -> Maybe v
lookup2 k1 k2 = lookup k2 <=< lookup k1

But I don’t know of any template haskell library to generate these functions for you, sorry.

EDIT Here’s my analogue of insertWith:

insertWith2 :: (Ord k1, Ord k2) => (v -> v -> v) -> k1 -> k2 -> v -> Map k1 (Map k2 v) -> Map k1 (Map k2 v)
insertWith2 f k1 k2 m = insert k1 (insertWith f k2 v $ fromMaybe empty $ lookup k1 m) m