Btukfyl

I know I can use function(a, b, ...) to solve variable parameters problems. But how can I do if I want to create a function like table.insert (table, [pos,] value) ?
Of cause, not in the following way:

function (table, pos, value)

    if value == nil then

        pos = value 

        value = nil

    end

    -- do something

end

Lua does not have function overloading or typechecking of arguments, if needed those should be implemented by the one needing.
Conveying the usage of your function is also up to you.

If you are not satisfied with your provided snippet, you can rewrite it in other manners for example :

function(arg1,arg2,arg3)

    tab=arg1

    if not arg3 then

        value=arg2            

        index=#tab+1

    else

        assert(type(arg2)=='numer',"bad argument #2 to 'insert' (number expected, got table)")

        value=arg3

        index=arg2

    end

    table.insert(tab,index,value)

end

or:

f2=function(a,b)

    --do smth

end

f3=function(a,b,c) end

f=function(...)

    args={...}

    nargin=#args

    if nargin==2 then 

        f2(args[1],args[2]) --one way to use varied arguments

    elseif nargin==3 then

        f3(...) --other one 

    else error("wrong number of arguments")

    end

end

I don't recommend you making functions with optional middle positional arguments though. At least not in a language without compile time type check, and even in those as well.

If you really want optional arguments, put them in the table:

function(args)

    tab=args.table

    index=args.index or #tab+1

    value=args.value

    --other arguments, options and associated logic here

    table.insert(tab,index,value)

end