#9288 Pass undefined for an optional external argument or JSX key that is None

e941d27298f5f7dcbc9a42cce0d5d7b349d27f7d..pr-9288

c90750038 · mununki · Nov 09, 2026, 15:21 UTC

Pass undefined for an optional external argument or JSX key that is None

Why

An optional argument of an external passed as ~x=?e, where e is a call, a dict or array lookup, a conditional or anything else that cannot be read twice to test it first, is compiled to Primitive_option.valFromOption(e). valFromOption reads a property of its argument to find a nested None, so it threw a TypeError when e was None, though its doc comment says that it keeps undefined. #8380 replaced Primitive_option.toUndefined, which tested for undefined before calling valFromOption, on that premise. The same call is emitted for a JSX key, also with -bs-jsx-preserve, for the argument of an inlined function that forwards ~x? to an external, and for an optional @int argument, or @string argument with @as, which switches on it. valFromOption now returns undefined for undefined, as it does for null, which fixes all of them.

modifiedJS Primitive_option.valFromOption

js

Before

After

function valFromOption(x) {function valFromOption(x) { if (x === null || x.BS_PRIVATE_NESTED_SOME_NONE === undefined) { if ((x == null) || x.BS_PRIVATE_NESTED_SOME_NONE === undefined) { return x; return x; } } … …}}

modifiedOutput optional argument that is None

%%raw(`function greet(name) { return name === undefined ? "Hello" : "Hello " + name }`)@val external greet: (~name: string=?, unit) => string = "greet" let greetUser = (names: dict<string>, id) => greet(~name=?names->Dict.get(id), ())Console.log(greetUser(dict{"1": "Ann"}, "2"))

js

Before and After are equal

function greetUser(names, id) {  return greet(Primitive_option.valFromOption(names[id]));}

text

Before

After

TypeError: Cannot read properties of undefined (reading 'BS_PRIVATE_NESTED_SOME_NONE')Hello

Review

Why change the runtime and not the compiler? Every path that emits the unchecked call goes through valFromOption: Arg_optional in ocaml_to_js_eff, the JSX key in Lam_compile_primitive, inlined functions, and @int and @as arguments, which switch on its result. get_default_undefined_from_optional returns an expression, so a test in the compiler needs the argument bound to a temporary: a statement for each such argument, through hold_arg for calls and separately for JSX keys, plus temporaries for the arguments to its left (see sequence_arguments). Bringing back toUndefined would undo #8380 for one comparison. This way the generated code stays as it is, and code that 13.0.0-alpha.5 or later already compiled works once the runtime is updated.
Does the extra test cost the other callers anything? They pass a value known to be Some: a match on a nested option (Pval_from_option), and the branch after x !== undefined of an @obj assignment or of an optional argument that can be read twice. For them x == null replaces x === null, still one comparison; in a micro-benchmark of 2×10^8 calls the difference is within noise for objects or numbers alone and under 0.3 ns a call for mixed inputs. The runtime grows by one character, and no other compiled module of the runtime, the stdlib or the tests changes.
Could an argument that can be read twice now be compiled to valFromOption(x) alone, without x !== undefined ? … : undefined? It could, but that would change much output and call the runtime for None. The next commit does it only for a JS property read, which may run a getter.