Haskell: converting a list of (a, b) key-value pairs (with possibly repeated keys) to a list of (a, [b]) grouped by key

Question

I\'m a Haskell beginner. Let\'s suppose I want to write a function convertKVList that takes a flat list of key-value pairs, where some of the keys might be rep

Answer 1

That looks like an understandable solution and you can clean it up slightly more:

import Data.Map (toList, fromListWith)
import Control.Arrow (second)

convertKVList :: Ord a => [(a, b)] -> [(a, [b])]
convertKVList = toList . fromListWith (++) . map (second (:[]))

Regarding how you might come up with this on your own: assuming you started with Data.Map, then you want to use the map to combine values with equal keys. The documentation for Data.Map on Hackage says a is the type for values and k for keys.

Knowing this, you can search for a -> a -> a to find functions that might combine two values in a Map k a to produce a new a value. This narrows the API down to a handful of functions like insertWith, fromListWith, and fromAscListWith.

Similarly, to convert your Map k a to [(k, a)], you can search the documentation for Map k a -> [(k, a)] and find only a few functions like assocs, toList, toAscList, and toDescList. Note that in your case, [(k, a)] is instantiated to [(Int, [Int])].

One thing I've found helpful in understanding the standard Haskell libraries is to view the source on Hackage. Seeing which functions are implemented in terms of others helps make the API feel smaller, and I can see which functions are the basic building blocks.