Re: [Haskell-beginners] Python's collections.defaultdict(list) in Haskell?

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 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 :

...

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