Lens: Using items << filtered leads to "missing constraint" compile error without Intellisense error

namespace Test

open FSharpPlus.Lens

type Child = { Id: string }

type Parent = { Children: Child list }

module Test =

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

  let inline _child childId =
    _children << items << filtered (fun c -> c.Id = childId)

The above does not produce an Intellisense error, but compilation fails with the following error on items on the last line:

error FS0193: A type parameter is missing a constraint 'when (FSharpPlus.Control.Traverse or Test.Child Microsoft.FSharp.Collections.list or ^d) : (static member Traverse : Test.Child Microsoft.FSharp.Collections.list * (Test.Child -> ^a) * ^d * FSharpPlus.Control.Traverse -> ^d)'

I’m unable to get it to compile. What’s wrong with the code? And shouldn’t there be an Intellisense error, too?

[gusty]

@cmeeren Here’s a quick workaround:

Since items is just an alias for traverse, you can replace it but using a specific traverse for your case.

So, for instance if you’re working with list

let inline _child childId =
    _children << List.traverse << filtered (fun c -> c.Id = childId)

[gusty]

Data provides additional types, but it also provides some generic extensions to existing types which I reckon it’s a bit confusing since we have also extensions in Extensions.fs

This is like this also for a technical reason, Extensions are used in Invokables, and Data uses Invokables, in this case to restrict to a less generic version.

We’ll need to re-think this for V2.

CC @adz