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- Subject: Re: Argument checking in Lua?
- From: Chris Marrin <chris@...>
- Date: Wed, 01 Feb 2006 15:46:05 -0800
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")
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:
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 ... == nil then ... = "hello" end
if arg == nil then arg = "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:
a,b,c = _check_foo(a,b,c)
and have the check function do something like:
local a=..., b=..., c=...
if b == nil then b = "hello" end
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