Function values in a variable have one type #318

A function value stored in a variable has one type, like any variable, so calling it with different argument types is an error:

let id = fn(x) => x;
id(1);
id("a");   // error: id holds a function that doesn't accept String

let f = len;
f("abc");
f([1]);    // error: f holds a function that doesn't accept List<Int>

An overloaded name used as a value is picked by the type expected where it's used (#246, #217), so a list of overloaded functions of different types can't be written either: [len, abs] has to say which abs is meant.

Today, declaring the function with fn (top-level fns are generic) or annotating the variable Any works around it.

Possible directions:

• Generic let lambdas: a let lambda that's never reassigned is generic like a top-level fn (ML-style let-generalisation), reusing the generic-function instances.
• Overload sets as values: a value that keeps all of a name's overloads and picks one at each call, statically where the call's types are known. This would also let [len, abs] and let f = len work, but needs a new kind of value in the VM and the type system, and an answer to what xs.map(f) passes when f is a set.

Found in the review of the type checker (#291).

🤖 Generated with Claude Code