Type-constrained variadic function annotation

[kylechui]

Hi there, I'm seeing if I can implement a version of Haskell's bind operator for the Maybe monad but in Lua. I have a function that takes in some number of parameters, say f has type fun(param1: T1, param2: T2, ...): T?, i.e. f takes in some number of parameters T1, ..., Tn (the number of which is unknown) and returns some optional type T?. I'm essentially writing an evaluate function bind(f, ...) that uses the arguments of ... and passes them into f, returning the result.

Is there a way to use annotations to force ... to be the same arguments/type as f's input? In other words, ... should have type T1?, T2?, ..., Tn?. Furthermore, bind should have return type T?, the same as f.

[C3pa]

Does this suffice?

---@generic T 
---@param f fun(param1: T, param2: T, ...: T): T?
---@param ... T
---@return T?
local function bind(f, ...) end

[kylechui]

Ah, I didn't realize generics could be applied to .... Perhaps it will work for my purposes

[kylechui]

Hmm I think one issue that still remains is that by using a generic type T, we have that T itself could be type nil, which somewhat defeats the type system. Would there be a way to say that T must be non-nil?

[C3pa]

Would there be a way to say that T must be non-nil?

None that I know of.

Yeah, there is some room for improvement on the LLS's side.