Add group function for association lists

[emilianobovetti]

I've added three functions: groupAssoc, groupAssocBy and groupAssocWith.

They all take lists in the form List ( k, v ) and output List ( k, List v ).

A group of pairs ( k₁, v₁ ) ... ( kᵢ, vᵢ ) in our input where kᵢ = k₂ = ... = kᵢ will appear in our output as ( k₁, [ v₁, ..., vᵢ ] ). The equality in groupAssocBy and groupAssocWith can be defined as a user function just like in List.sortBy and List.sortWith.

The use case is... well group things of course!

For example:

[ { name = "bob", age = 20 }
, { name = "alice", age = 25 }
, { name = "bob", age = 32 }
]
    |> List.map (\p -> ( p.name, p ))
    |> List.Extra.groupAssoc

gives

[ ( "alice", [ { age = 25, name = "alice" } ] )
, ( "bob", [ { age = 20, name = "bob" }, { age = 32, name = "bob" } ] )
]

I'm sure the same could be done with groupWhile, but I find easier to get my head around groupAssoc EDIT: I've just seen gatherEqualsBy, I guess the use cases collide with this one :smile:

If there is interest I'll happily write some docs and other tests

[Chadtech]

Yeah I appreciate you making the PR, but it does seem like the use case is covered by gatherEqualsBy

[emilianobovetti]

Feel free to close!