如何在 f# 中创建地图的副本？

Question

   let students =
    Map.empty.
          Add("ABC", "97").
          Add("Jill", "98");;
    printfn "Map - students: %A" students

This I know will create a map named students. I was thinking a way to create a copy of this map, with same key and value. I checked the documentation for f#, and didn't find any methods that clone/create a copy of map.

Answer 1

F# maps are immutable. This means that "changing" a map actually means creating a copy with some slight changes. The copy process uses tricks to make it much faster than a full copy. The new version can actually point back to the old version for most of the data. This is safe to do because the maps are immutable: they can never change.

This means that there is no possible reason to copy a map, except to warm up your CPU 😆

The same applies to List and Set in F#, as they are also immutable. It takes a while to get used to working with immutable data structures but life gets easier once you do.

---

By the way you can create the map with less code like this:

let students =
    [ "ABC", "97"
      "Jill", "98" ]
    |> Map

Answer 2

Here's one way to do it.

let copy =
    students
    |> Seq.map (fun pair -> pair.Key, pair.Value)
    |> Map