map being called for nullable fields

[andremw]

Consider a structure like this:

type internalObj = {name: string}

type obj = {
  id: string,
  internal: option<internalObj>,
}

I already know that the property obj.internal might be missing and I want to decode that to an Option.
I also know that the property internalObj.name can be missing but I want it to always have a default value.

This is how I would write a decoder for the name property with a default value:

let internalObj_decode: Json.Decode.t<internalObj> = {
  open Json.Decode

  map(option(field("name", string)), ~f=name => {
    name: name->Belt.Option.getWithDefault("Default Name"),
  })
}

And this works fine!

But if the decoder for "obj" considers that "internal" is nullable:

let decoder: Json.Decode.t<obj> = {
  open Json.Decode

  map2(field("id", string), field("internal", nullable(internalObj_decode)), ~f=(
    id,
    internal,
  ) => {
    id: id,
    internal: internal,
  })
}

I would expect the decoder for internal to not run if the value is null, like { "id": "123", "internal": null }, but it runs.

Shouldn't nullable prevent running the decoder if the value is null?

[andremw]

Or should I change internalObj_decode to be this instead?

let internalObj_decode: Json.Decode.t<internalObj> = {
  open Json.Decode

  map(field("name", option(string)), ~f=name => {
    name: name->Belt.Option.getWithDefault("Default Name"),
  })
}