You can use insertWith: http://hackage.haskell.org/package/containers-0.5.6.3/docs/Data-Map-Strict.html#g:6

E.g.:
let xs = [("Item0", ["a","b","c"]), ("Item1", ["x","y"])]
let m = Map.fromList xs
Map.insertWith (++) "Item0" ["d"] m

"d" is cons'ed from the left, so it shouldn't be too slow. I don't know why insertWith has this type instead of:
Ord k => (b -> a -> a) -> k -> b -> Map k a -> Map k a

which would allow you to write Map.insertWith (:) "Item0" "d" m

Maybe you should report it.

--

Sylvain

2015-11-10 6:23 GMT+01:00 Dan Stromberg <strombrg@gmail.com>:

These are some good leads.

I'll be adding values one at a time, and yes, my keys aren't necessarily unique.

Is there a way of cons'ing on the single values one at a time, that will avoid the slowness of ++ ?

Thanks.

On Mon, Nov 9, 2015 at 7:45 PM, Sylvain Henry <hsyl20@gmail.com> wrote:

import qualified Data.Map as Map

-- if your keys are unique

let xs = [("Item0", ["a","b","c"]), ("Item1", ["x","y"]), ("Item2", ["abc","def"])]

Map.fromList xs

-- if you want to combine values for keys that are equal
let xs = [("Item0", ["a","b","c"]), ("Item1", ["x","y"]), ("Item0", ["abc","def"])]

Map.fromListWith (++) xs

--

Sylvain

2015-11-10 3:07 GMT+01:00 Dan Stromberg <strombrg@gmail.com>:

I'm spending a little time here and there to learn some Haskell.  I'm coming from a chiefly Python/C/bash background.

I want to build a Data.Map where the keys are strings, and the values are lists of strings.

In Python, collections.defaultdict(list) makes this pretty straightforward. It gives a hash table ("dict") that has values that default to an empty list, since list() produces an empty list. More info here: https://docs.python.org/3/library/collections.html#collections.defaultdict

Is there an equivalent in Haskell?

Thanks!

--

Dan Stromberg

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Dan Stromberg

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