Record type spread and coercion blog post

[zth]

All feedback welcome.

[ryyppy]

Really nice post! Added some comments / recommendations. Feel free to ignore if not applicable.

[zth]

@ryyppy if you re-read it now with fresh eyes after the latest changes, how does it look?

[zth]

Let's add 2 short sentences to illustrate:

• Coercing between optional and mandatory fields
• Repeated fields are disallowed

cc @cristianoc

[ryyppy]

As discussed with @zth, I restructured the text a bit.

Coercing between optional and mandatory fields

I don't fully understand how this would look like. The only coercion I got to work was this:

type a = {
  name: string,
  age: option<int>,
}

type b = {
  age?: int,
  name: string,
}

let test = (v: b) => {
  Js.log(v.name)
}

let a: a = {name: "Bambi", age: Some(1)}

test((a :> b))

For me this feels quite natural, so not sure if this example yields any benefit? Or maybe I don't understand what is meant by "coercing between optional and mandatory fields".

[cristianoc]

As discussed with @zth, I restructured the text a bit.

Coercing between optional and mandatory fields

I don't fully understand how this would look like. The only coercion I got to work was this:

type a = {
  name: string,
  age: option<int>,
}

type b = {
  age?: int,
  name: string,
}

let test = (v: b) => {
  Js.log(v.name)
}

let a: a = {name: "Bambi", age: Some(1)}

test((a :> b))

For me this feels quite natural, so not sure if this example yields any benefit? Or maybe I don't understand what is meant by "coercing between optional and mandatory fields".

That is indeed the example of converting from mandatory to optional.

[cristianoc]

"Record Type Coercion is Explicit

Records are still nominally typed, so it is not possible to pass a record a as record b without an explicit type coercion. This is crucial as it prevents accidental dependencies on shapes rather than records, ensuring predictable and more robust type checking results.
"

The explicitness of coercion is a design choice, not an implementation necessity.
We could have implemented implicit subtyping. This has not been done, as that might be "too magic".

[cristianoc]

With current docs.

Is it possible to coerce a mandatory field of option type to an optional field?

No, it is not possible to coerce a mandatory field of an option type to an optional field in ReScript. Coercion does not allow for changing the required/optional status of fields, even if the original field is of type option.

Here's an example to illustrate this:

type a = {
  name: string,
  age: option<int>, // Mandatory field of option type
}

type b = {
  name: string,
  age?: int, // Optional field
}

let aRecord: a = {
  name: "John",
  age: Some(25), // Option field is present
}

let bRecord: b = aRecord :> b // Error: Cannot coerce 'a' to 'b'

In the above example, type a has a mandatory field age of type option<int>, while type b has an optional field age of type int. When trying to coerce aRecord of type a to bRecord of type b, a compilation error will occur because coercion cannot change the required/optional status of fields.

If you need to convert a record with a mandatory field of an option type to a record with an optional field, you will need to create a new record with the desired optional field explicitly set based on the presence or absence of the option value.