Hi Leonhard, the monoidal-containers package should be interesting to you: http://hackage.haskell.org/package/monoidal-containers Cheers, Simon Am Mo., 18. Nov. 2019 um 23:22 Uhr schrieb Leonhard Applis :

Hello Francesco,

...

`Map k v` is already an instance of `Monoid` (when `v` is an instance of `Ord`), are you sure you need to write another one?

Thanks for your answer! My "Values" is a Monoid, however it seems that the Graph cannot properly be seen correctly as a Monoid

using mconcat does not (properly) work, meaning that <> is not correctly applied to the someMonoids so instead of

type G = Map Text someMonoid mconcat [gs]

I have to write:

foldr (Map.unionWith (<>)) Map.empty [gs]

This passes the tests. Same Problem with g1 <> g2 --(Does not work properly) and Map.unionWith (<>) g1 g2 --(Does work)

I have declared someMonoid myself, do I need to declare something special about it? I feel like my G is messing up with <> being about him, or about someMonoid

A broken down piece of code is:

import Data.Map as Map

data Sum = Sum Int deriving (Eq,Show)

instance Semigroup Sum where (<>) (Sum a) (Sum b)= Sum(a + b) instance Monoid Sum where mempty = Sum 0

type G = Map.Map String Sum

And to verify my problem: GHCI > v = Map.singleton "A" (Sum 1) GHCI > u = Map.singleton "A" (Sum 1) GHCI> c= v <> u GHCI> Map.lookup "A" c Just (Sum 1)

but I want Just (Sum 2)

thanks Leonhard _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners