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Type narrowing in case expressions #2060

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despairblue opened this issue Mar 10, 2023 · 1 comment · May be fixed by #3647
Open

Type narrowing in case expressions #2060

despairblue opened this issue Mar 10, 2023 · 1 comment · May be fixed by #3647
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help wanted Contributions encouraged priority:low

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@despairblue
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Was wondering if this could be added to the compiler:

pub type Being {
  Person(name: String, age: Int, is_happy: Bool)
  // imagine this type having a gazillion fields
  Dog(name: String, age: Int, is_happy: Bool)
}

pub fn have_birthday(being) {
  // It's a birthday, so increment their age and make them happy
  case being {
    Dog(..) as dog -> Dog(..dog, age: dog.age + 1, is_happy: True)
    Person(..) as person ->
      Person(..person, age: person.age + 1, is_happy: True)
  }
}

This leads to

[gleam] error: Unsafe record update
   ┌─ ./src/playground.gleam:10:23
   │
10 │     Dog(..) as dog -> Dog(..dog, age: dog.age + 1, is_happy: True)
   │                       ^^^ I can't tell this is always the right constructor

This type has multiple constructors so it cannot be safely updated.
If this value was one of the other variants then the update would be
produce incorrect results.

Consider pattern matching on it with a case expression and then
constructing a new record with its values.

Now I already pattern-matched on it and the compiler could know it's a dog. If the Dog would have lot's of fields then writing out every field becomes tedious.
@lpil
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lpil commented Mar 10, 2023 •
edited
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A fab suggestion! We would need to implement #34 first before this type narrowing feature.

I'm sure we had an existing issue for this but I can't find it now unfortunately.

@lpil lpil added help wanted Contributions encouraged area:analysis labels Mar 10, 2023
@lpil lpil changed the title Permit updating records with multiple constructors if the constructor is known Type narrowing in case expressions Jul 22, 2024
@GearsDatapacks GearsDatapacks linked a pull request Sep 25, 2024 that will close this issue
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Type narrowing in case expressions GearsDatapacks/gleam
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@despairblue @lpil