Add more useful functions to the fields plugin

[7sharp9]

[TysonMN]

For convenience in this issue, here is OCaml's ppx_fields_conv plugin.

[TysonMN]

I would like the get functions named as such. Though, I suppose both could exist.

let getOne r = r.One

I would like individual (copy-and-update) set functions. Unlike ppx_fields_conv, I don't think it is a good idea to generate a function that mutates a field. That way, there is more friction when deciding to go with a mutable approach. However, if mutating functions are going to be generated, then I would prefer that they get a worse name than the copy-and-update function. So the copy-and-update function is called setOne and the mutating function is called mutateOne. I think this name matches well with F#'s mutable keyword.

let setOne v r = { r with One = v }

I would like individual map functions.

let mapOne f = { r with One = r.One |> f }

I recently decided that I can no longer live without these functions, so I started manually implementing them in field-specific modules. That way, they are easy to copy-paste and find-replace when I add a new field. It also makes reading the implementation of map a bit easier because then map is defined via get and get instead of having mapOne defined in terms of getOne and setOne.

let map get set f a =
  a |> get |> f |> Func.flip set a

type MyRecord =
  { MyField1: int
    MyField2: int }

module MyRecord =
  module MyField1 =
    let get m = m.MyField1
    let set v m = { m with MyField1 = v }
    let map f = f |> map get set
  module MyField2 =
    let get m = m.MyField2
    let set v m = { m with MyField2 = v }
    let map f = f |> map get set

Then calling code can open MyRecord and write r |> MyField1.map increment.

I don't have a strong preference for field-specific modules when generating the code though. Also, a name like mapOne is more idiomatic (than One.map) in F# because of examples like Result.mapError.

I also think it would be interesting to consider defining set via create since this would allow a type parameter of a generic record to change because of a call to set or map. However, this gets complicated fast. For example, consider

type RoseTreeLike<'a, 'b> =
  { A: 'a
    B: 'b
    Children: RoseTreeLike<'a, 'b> list }

No individual field can change its type in a mapping, but Children and either A or B can vary their type (somewhat) at the same time.

I looked through the functions generated by ppx_fields_conv. I don't see the value in them (yet!).

I currently see the value in individual get, set, and map functions though, and I would prefer to not have to keep doing copy-paste and find-replace. I am interested in improving the fields plugin to make this happen.

[7sharp9]

Im happy to take a PR adding anything that's useful as long as there is an example of usage or some documentation added.