Re: Argument checking in Lua?

[Chris Marrin <chris@...>]

Rici Lake wrote:
> On 1-Feb-06, at 2:48 PM, Chris Marrin wrote:
>
> > Lua 5.1 has some nice argument checking functions, accessible from C, 
> > like luaL_checknumber() and luaL_optstring(). But these are not 
> > accessible from Lua, and I have not found anything roughly equivalent. 
> > It would be easy to write such things, of course, by doing type(), 
> > checking against nil and throwing errors. But is there any "standard" 
> > way to do this? I have not been able to find anything...
>
>
> I usually write:
>
>   a = assert(tonumber(a), "number expected")
>
> or, in the case where there is a sensible default:
>
>   a = assert(tonumber(a or sensible_default), "number expected")
>
> This doesn't produce error messages as nice as luaL_*, though.
>
> Also, it doesn't work with strings, since tostring(x) pretty well always 
> produces a string, in constrast with lua_tostring() which only coerces 
> numbers. It would sometimes be nice to have a Lua function which acted 
> like lua_tostring() and a C function which acted like tostring(). Of 
> course, it's easy enough to write both of those.

Ok, thanks. Here is what I am thinking about adding:

    local _check_foo = checkArgs("int, string='hello', number=3.14")
    function foo(a,b,c)
        _check_foo(arg)
        ...
    end

Where checkArgs() returns a function which does all the checking. This 
way, I can "compile" the argument checking function for efficiency. This 
brings up a question. In 5.1, can I use the 'arg' property to pass a 
table of the args to the checker? Or would I have to go:

    function foo(...)
        _check_foo(...)
        ...
    end

instead? And, if I did either of the above, can I change the visible 
params to the foo() function? For instance, can I go either:

    if ...[2] == nil then ...[2] = "hello" end

or:

    if arg[2] == nil then arg[2] = "hello" end

and expect param 'b' in foo to now have the value "hello" if it was nil? 
Or would I need to do something like:

    function foo(a,b,c)
        a,b,c = _check_foo(a,b,c)
        ...
    end

and have the check function do something like:

    local a=...[1], b=...[2], c=...[3]
    if b == nil then b = "hello" end
    return a,b,c

???

--
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