Set of keys from a map

Question

I have a map X and I'm trying to get a set of the keys satisfying a certain condition, something like this:

Map.Keys X
|> Set.filter (fun x -> ...)

...but I cannot find the way to get the keys from F#'s Map collection.

Answer 1

Convert your map to sequence of tuples (key,value) first and then map it to a sequence of just keys:

map |> Map.toSeq |> Seq.map fst

FSI sample:

>Map.ofList[(1,"a");(2,"b")] |> Map.toSeq |> Seq.map fst;;
val it : seq<int> = seq [1; 2]

Or alternatively, as ordering of keys likely does not matter you may use more eager method returning the list of all keys. It is also not hard to make it into extension method keys of Microsoft.FSharp.Collections.Map module:

module Map =
    let keys (m: Map<'Key, 'T>) =
        Map.fold (fun keys key _ -> key::keys) [] m