Destructure records with optional fields

1

why does destructuring a record with optional fields result in an error:

type myType = {
  a: bool,
  b?: bool,
}
let myFn2 = ({a, b}: myType) => {
  a
}

my expectation would be for b to be destructured with type option<bool>, but instead its type is bool

2

this is because it’s as if such a record can have two shapes:

// shape 1
{
  a: bool,
}

// shape 2
{
  a: bool,
  b: bool,
}

You’re only considering the latter case if you destructure it like you do:

let myFn2 = ({a, b}: myType) => {
  a
}

if you want to take into consideration these two cases (b being defined or not) in the same pattern matching, you have to use ?b:

let myFn2 = ({a, ?b}: myType) => {
  a && Option.getWithDefault(b, false)
}

see playground

2 Likes
3

thanks!
Do you know where this is documented? I tried finding any info in the docs Record | ReScript Language Manual but didn’t find anything

4

here

3 Likes