CODEWITHSUNDEEP
haskell
Kevin Meredith
Kevin Meredith

Reputation: 41909

Group Elements by Equality

I'm looking for a standard library function that groups elements by equality.

Example:

f [1,2,3,1,2,3] == [ [1,1], [2,2], [3,3] ]

Composing group and sort does the job:

Prelude Data.List> group . sort $ [1,2,3,1,2,3]
[[1,1],[2,2],[3,3]]

But, is there a native function in Haskell's library that can perform the above work in a single function?

I looked at Data.List, but did not find such a function.

Upvotes: 0

Views: 127

Answers (1)

Julia Path
Julia Path

Reputation: 2366

There is no such function in base. However there is the discrimination package. In Data.Discrimination you will find the following function:

group :: Grouping a => [a] -> [[a]]

You don't have to write those Grouping instances yourself, as there are default methods in place for types that have an instance for Generic. For example:

{-# LANGUAGE DeriveGeneric #-}
import Data.Discrimination

data A = A deriving Generic
instance Grouping A
instance Sorting A -- Not necessary for what you are trying to do.

Upvotes: 3

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