Use ... of a variadic function in "normal" function

[atticus-sullivan]

Use-case: Writing a variadic function wich uses a binary function to reduce its arguments.

Example:

local function foo(a:string, b:string): string
    print(a,b)
end

local function bar(...: string): string
    return foo(...)
end

(playground currently not showing the error until I do a change like removing and adding one parenthesis for example).

I'm not sure if this always should compile as ... might be more/less than (in this case) two values, but commonly it is guarded (via select('#', ...)) that the argument count is >= (and usually in lua the additional arguments are just discarded).

As I think about this while writing this, maybe this is not quite a bug report but more seeking for guidance how to write something like this in current teal (just upgraded from 0.15.3 where the above was still working) and maybe a feature request (still not sure how to make this work in a type-safe manner though).

For now the only alternative I'm aware of is putting ... in a table (this is not safe in case nil is involved though) and then access as an array.

[hishamhm]

There are some alternatives.

If you know that bar might be called with fewer than 2 arguments but your foo is fine with it, then you may want to specify that the arguments in foo are optional, and that will compile:

local function foo(a?: string, b?: string): string
    print(a,b)
end

local function bar(...: string): string
    return foo(...)
end

If you know that bar will always be called with at least 2 entries (assuming you also use ... for other things in your code), then you may want to unpack the first two entries of ... in the signature:

local function foo(a: string, b: string): string
    print(a,b)
end

local function bar(a: string, b: string, ...: string): string
    return foo(a, b)
end

If you want a more 0.15-like behavior, this still works:

local function foo(a: string, b: string): string
    print(a,b)
end

local function bar(...: string): string
    local a, b = ...
    return foo(a, b)
end

[atticus-sullivan]

Oh I really like your second solution. Combined with using ?string and checking for nil in order to test if only one or two arguments were given.

Thanks for the suggestions 👍