Stdlib soundness of option returning functions

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Feature
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Tech-Stack
ocaml
Bereich
backend-api-design

Rechercherichtung

Start by examining the standard library bindings for Array.shift, Array.pop, Array.get, and Map.get, then trace how option-returning functions are represented and compiled. The issue proposes several competing approaches, so first determine which behavior maintainers want; done means the selected approach makes these functions sound while addressing the interop edge cases raised.

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Beschreibung

Array.shift, Array.pop, Array.get, Map.get, and likely others are unsound, see Playground.

Here is what a correct version of shift could be:

let shiftSafe = xs =>
  if xs->Array.isEmpty {
    None
  } else {
    let value = xs->Array.getUnsafe(0)
    let _ = xs->Array.shift
    Some(value)
  }

This is a common mistake when writing JS bindings and can be unexpected to users who expect ReScript to be a safe language.

I couldn't find a previous discussion, but I believe this needs a bit of consideration. The choices I see are:

  1. Implement safe ReScript version of Stdlib functions, rename the zero overhead to shiftUnsafe, getUnsafe, etc.
  2. Fix the compiler's handling of option-returning functions.
  3. Fix the representation of options.

Unfortunately, I think there will always be edge-cases that make (2) a losing battle. Even with correct code generation, people often use imprecise types for interop making compile-time information insufficient to prevent unexpected results. For example, users storing undefined containing data in a JSON type.

Similar to the previous reasoning, (3) is likely going to cause too many issues from users that are relying on a particular option implementation.

So I believe (1) is the correct choice for the standard library. However, this still makes the "obvious" implementation of JS bindings written by users prone to errors. One idea would be to generate a fixture by default unless disabled:

@send external shift: array<'a> => option<'a> = "shift"
@send @noInterop external shiftUnsafe: array<'a> => option<'a> = "shift"

// Stdlib_Array.bs.js
let shift = t => Primitive_option.some(t.shift())
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