Using setl and view for the same optic in one function gives error "The type Data.Identity does not match the type Data.Const"

I’m trying to use optics to set a record field together with the record’s UpdatedAt: DateTimeOffset field only if the field value has changed.

I have created the following generic function, which doesn’t compile:

let inline private setField valueOptic value updatedAtOptic now source =
  let oldVal = view valueOptic source
  if value = oldVal then source
  else
    setl valueOptic value source
    |> setl updatedAtOptic now

What am I doing wrong?

[gusty]

That was a tip for @wallymathieu / @gusty for a change in FSharpPlus, right? Not in user code?

We can consider using those functors for lensing but of course, we’ll need to make sure all other optics don’t break. Testing should be added although there is some lens coverage already, but we’ll need to add test for stuff that was previously not allowed, like your use case. Feel free to open PR or a draft PR to play with it.

Would be great it FSharpPlus could solve this natively,

We can’t fix type system limitations (like rank-n types) but we can certainly explore tricks like the suggested one that makes a better user experience.

or illuminate whether lenses is the wrong tool for this kind of job

TBH I’m not an expert in lenses, it is an abstraction that it’s not specific to this library or to F# so I would try to find guidance in other places as well, for instance I asked your question in the FP slack, in the #Lens channel as I’m also curious to see if/how lens would help in that situation.

My feeling is that we need a lens that encapsulate that behavior, we can certainly create it like this:

let inline _nameWithUpdate now f p =
    map (fun v -> if v = p.Name then { p with Name = v } else { p with Name = v; UpdatedAt = now; NameHistory = p.Name::p.NameHistory }) (f p.Name)

but I understand that you want to derive it in a general way, and I think that would involve using other functors in anycase. So if you’re happy with the mutable solution stick to it. I personally like it because yes it does a mutation but it’s encapsulated inside the function, it doesn’t leak outside, as opposed to the solution of passing twice the lens.

[gusty]

A similar technical problem was reported by @reinux on our gitter 2 weeks ago:

Hey, hopefully quick question: is there a type alias I can use for lenses? I’m trying to pass a lens as an argument, but it results in a type error when I try to use view and setl together. Something like this:

let f lens x =
  printfn "%s" (view lens x)
  setl lens "abc" x

The fix suggested by @nikolamilekic should apply here as well.