Defining new optics with `<&>` does not seem to work like in the documentation

Description

I wanted to define optics over a custom data type. Failed to do it with the approach given by the documentation, and then succeeded by doing it differently. I don’t fully understand the consequences of this, but I get the sense something is wrong – either with what I do, or the documentation.

Repro steps

> open FSharpPlus.Lens ;;
> type Person<'TName> = { Name : 'TName; Age : int } ;;
type Person<'TName> =
  {Name: 'TName;
   Age: int;}

> (* Let's do it like in the documentation; that looks neat! *) ;;
> let inline _name f { Name = n; Age = a } = f n <&> fun n' -> { Name = n'; Age = a } ;;
val inline _name :
  f:('a ->  ^b) -> Person<'a> ->  ^_arg4
    when ( ^b or ('c -> Person<'c>)) : (static member ( <&> ) :  ^b *
                                                                ('c ->
                                                                   Person<'c>)
                                                                  ->  ^_arg4)

> (* Okay, so that looks like it might have succeeded. *) ;;
> (* Let's find a person try to it on. *) ;;
> let me = { Name = "Chris"; Age = 39 } ;;
val me : Person<string> = {Name = "Chris";
                           Age = 39;}

> (* Oops, I meant to be slightly more anonymous. *) ;;
> setl _name "kqr" me ;;

  setl _name "kqr" me ;;
  -----^^^^^

/home/kqr/stdin(33,6): error FS0001: Expecting a type supporting the operator '<&>' but given a function type. You may be missing an argument to a function.

On the other hand, if I re-create the optic with the lens helper function, things seem to work better:

> let inline _name f = lens (fun p -> p.Name) (fun p n -> { Name = n; Age = p.Age }) f ;;
val inline _name :
  f:('a ->  ^b) -> (Person<'a> ->  ^c)
    when (FSharpPlus.Control.Map or  ^b or  ^c) : (static member Map : ( ^b *
                                                                        ('d ->
                                                                           Person<'d>)) *
                                                                       FSharpPlus.Control.Map
                                                                         ->
                                                                          ^c)

> setl _name "kqr" me ;;                                                                                   
val it : Person<string> = {Name = "kqr";
                           Age = 39;}

Related information

• Debian Stretch
• Version 1.0.0 of FSharpPlus
• Microsoft ® F# Interactive version 4.1 – .NET Core SDK 3.0.100-preview8-013656

[gusty]

Ideally both. A quick update: unfortunately we’re still trying to get back the doc generation and trying to understand why from one day to another it started failing. @wallymathieu is doing some efforts in order to get it working again.

[gusty]

I added a note in the docs, sorry again for the confusion. Hopefully no more devs fall in the trap.