Fsharpplus: How to compose Map._item prism?

[cmeeren]

I want to update a specific item in a map if it exists, similar to what you'd do with items << filtered in a list or similar.

I have tried using Map._item, which AFAIK is a prism, but I can't get it to compose because of a type mismatch between Child and Child option:

```f#
open FSharpPlus
open FSharpPlus.Lens

type Child =
{ Value: int }

type Parent =
{ Children: Map }

let inline _value f (c: Child) =
f c.Value <&> fun v -> { c with Value = v }

let inline _children f (p: Parent) =
f p.Children <&> fun v -> { p with Children = v }

let setChildValue childKey value parent : Parent =
parent |> setl (_children << Map._item childKey << _value) value
// ^^^^^^ Type mismatch

![image](https://user-images.githubusercontent.com/7766733/92100203-c790d580-eddb-11ea-966d-66360e3ebf87.png)


Is there a simple way to achieve what I want?

For comparison, here is a working example that does the same with a list of children with IDs:

```f#
open FSharpPlus
open FSharpPlus.Lens


type Child =
  { Id: string
    Value: int }

type Parent =
  { Children: Child list }


let inline _value f (c: Child) =
  f c.Value <&> fun v -> { c with Value = v }

let inline _children f (p: Parent) =
  f p.Children <&> fun v -> { p with Children = v }

let setChildValue childId value parent : Parent =
  parent |> setl (_children << items << filtered (fun x -> x.Id = childId) << _value) value

[cmeeren]

Oh, I got it, it's subtly demonstrated here. I was just missing _Some:

f# let setChildValue childKey value parent : Parent = parent |> setl (_children << Map._item childKey << _Some << _value) value

I still have a lot to learn about prisms. The documentation shows the syntax, but doesn't really explain. For example, I'm confused about what _Some actually does (what the "traversal" is and how it impacts optics).

[gusty]

I recommend the "lens over tea" series for advanced stuff.

Be aware that some stuff from there might break here due to the weaker type system. Specially with traversals.