Question

Given a sequence of a group id/value tuples, it was easy to calculate group totals (pretty much the same way I would do it with C# and LINQ):

let items = ["g1",5; "g2",10; "g1",20]

let groupsums = 
    items  
    |> Seq.groupBy (fun x -> fst x) 
    |> Seq.map (fun (g, s) -> Seq.fold (fun acc x -> acc + snd x) 0 s)

But being new to F#, I can’t see a way to so the same with lists. Do I have to use mutable variables, or is there a functional way to do the same with lists?

Answer 1

There is no built in List.groupBy. A number of F# built in types have functions that are assigned the seq version of said function. e.g. from list.fs

let inline sumBy f (list : list<_>) = Seq.sumBy f list

I’m pretty sure the designers of F# had many discussions about what to duplicate for the sake of consistency and what to omit for for sake of DRY. I personally wish they stuck with DRY.

If you want to make your own “functional” List.groupBy I’d use map and list.

let groupBy list =
    list 
    |> List.fold (fun group (g, x) -> 
        match group |> Map.tryFind g with
        | Some(s) -> group |> Map.remove g |> Map.add g (x::s)
        | None -> group |> Map.add g [x]
        ) Map.empty
    |> Map.toList 

let groupsums = groupBy >> List.map (snd >> List.sum)

You can skip keeping lists if you only need the sum.

let groupAndSumBy list =
    list 
    |> List.fold (fun group (g, x) -> 
        match group |> Map.tryFind g with
        | Some(s) -> group |> Map.remove g |> Map.add g (x + s)
        | None -> group |> Map.add g x
        ) Map.empty
    |> Map.toList
    |> List.map snd

Output

> groupsums items;;
val it : int list = [25; 10]

> groupAndSumBy items;;
val it : int list = [25; 10]