How to implement filter for custom type?

[cmeeren]

I have a custom type Skippable<'a> that's just like Option (but semantically different). It's defined in a separate library.

In my code, I want to extend this type to support filter. With that in mind, I have tried the following, but none of them work (I have also tried switching the arguments):

type Skippable<'a> with
  static member Filter(s, p) = Skippable.filter p s

type Skippable<'a> with
  member this.Filter(p) = Skippable.filter p this

type FSharpPlus.Control.Filter with
  static member Filter(s, p) = Skippable.filter p s

In all cases, when I try to use filter on a Skippable<_> value, I get a compiler error saying

No overloads match for method 'ToSeq'.

Is this because I am using extension members? Is it possible to add support to custom types using extension members?

[cannorin]

F#+ currently does not support orphan instances i.e. does not allow type class instantiation if neither

1. the instance is defined along with the type class itself (e.g. built-in with F#+), nor
2. the instance is defined along with the target type (in your case, inside the FSharp.JsonSkippable)

This is a restriction coming from the F# compiler. However, dotnet/fsharp#6805 would enable us to define orphan instances.

[cmeeren]

I see, thanks for the clarification.