Re: Argument checking in Lua?

[Chris Marrin <chris@...>]

Rici Lake wrote:
> On 1-Feb-06, at 6:46 PM, Chris Marrin wrote:
>
> > 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.
>
>
> That sounds like a good idea, although it would be a fair amount of 
> work. However, there is no arg table in 5.1, and ... is not a table, or 
> indeed any other first-class object. So, for example,
>
>    ...[2]
>
> does not mean what you think it does. (It would apply the index 2 to the 
> first argument of the function.)
>
> All is not lost, however, since you can compile a check function with 
> named arguments; after all, the prototype tells you how many arguments 
> there will be. And instead of explicitly calling _check_foo, you could 
> pass foo itself to checkArgs and have it return a wrapped function for you.

Interesting approach. That would change the syntax of the function from:

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

to:

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

which looks pretty good.

> Personally, rather than trying to parse a prototype string, I'd go with 
> a table of individual prototypes, something like this:
>
> function foo(a, b, c) ... end
> checked_foo = checkArgs{"int", "string:hello", "number:3.14"}(foo)

I was both trying to avoid so many quotes, which are error prone, and to 
 match the way I will be generating these strings. Emma is a 
declarative system and an author will be able to do something like:

    <function name="foo" params="int, string='hello', number=3.14">
        function(a,b,c) ... end
    </function>

and from this, I can generate the above form. The generated function 
would look something like:

    function(a,b,c)
        ...
        if b == nil then
            b = "hello"
        else
            assert(type(b) == "string")
        end
        ...
        return f(a,b,c)
    end

> Let me know if that wasn't enough of a hint :)

thanks for the help!

--
chris marrin              ,""$, "As a general rule,don't solve puzzles
chris@marrin.com        b`    $  that open portals to Hell" ,,.
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