如何使用可选参数对PHP函数进行类型推断

Question

The thing with PHP is that it doesn't have a specific syntax when using a function with optional arguments. This:

foo(10);

can either be this

function foo($a) {}

or this

function foo($a = 0) {}

or this

function foo($a, $b = 0, ...) {}

(or even function foo() {}, but disregard that for now).

So how do we type-infer it when a function is used before it's defined?

One solution is to record all usages until the definition comes, and then see if they all can unify with it. The example above would then store

int -> unit

and later check if it's compatible with

int -> (typ list) -> unit

or whatever (where typ list is the list of optional arguments).

You think this could work? Are there other conventional ways to deal with this?

Answer 1

You'll probably have to either insert "weak" type variables and keep them until function definition is found, or first build a complete AST and then run it through the type inference and raise an error if there are functions whose definitions are unknown (I wonder how would you about built-in functions, perhaps use pre-baked type signature database?).