Re: another hat in the ring regarding Lua 5.2: static typing

[Asko Kauppi <askok@...>]

The syntax mod only transforms the constraints into 'assert.*()'  
calls. You can either override 'assert.table' with your own function,  
or use another name.

The sample case you mentioned would be covered roughly by (in a hurry,  
but gives the point):

<<
local function callable( v, field )
	if type(v) == "table" and type(v[field])=="function" then
		return true
	else
		local mt= getmetatable(v)
		if mt.__index and type(v[field])=="function" then
			return true
		end
	end
end

assert.select_callable= function( v )
	assert( callable( v, "select" ), "No .select function", 2 )
	return v
end
<<

That code should be somewhere in your app prior to using the ':  
select_callable' constraint.

There could be more factory support for making such custom asserts  
easier, but these are the rough edges that will be covered during the  
spring. Anything is already possible. :)

Happy weekend!
-asko


Tomas Guisasola Gorham kirjoitti 29.2.2008 kello 14:09:

> On Fri, 29 Feb 2008, steve donovan wrote:
> > On Thu, Feb 28, 2008 at 9:16 PM, Asko Kauppi  
> > <askok@dnainternet.net> wrote:
> > >        local function mydear( t :string_or_table, i :int, j : 
> > > [int], n :
> > > [number] )
> > >            : string, table, [int]
> > >                ...function body with returns...
> > >        end
> >
> > Nice notation!  I was thinking about this general problem the other
> > day: how does one explcitly specify type in a dynamic language? The
> > idea was that types are objects, and one builds them up with
> > expressions:
> >    string_or_table = choice(String,Table)
> >    string_or_nil = choice(String,Nil)
> >    Location = choice({X=Double,Y=Double},{Double,Double})
> 	I like that syntax either, but I have some doubts :-)
> 	Could you accept a proxy table instead of the real table?
> How would you specify that?  How would you specify properties of
> objects?  An example of what I am looking for is:
>
> function get_status (db, id)
> 	local cond = "id="..id.." AND estado_retorno is not null"
> 	return db:select ("status", "processo", cond)()
> end
>
> 	How would you specify that you can call db's select?
> I am not interested in the real type of db: it could be a userdata
> or a table, and in this case it could have the field select or it
> could have a metatable with a __index metamethod; also, the select
> object could be a function or a "callable" object...  How would you
> manage that?
> 	I am not sure if we should check the _type_ or the operations
> we want.  In my case, I only care about db.select being a callable
> object.  One might argue that I would have to check its arguments,
> but I would delegate it for the call itself.
>
> 	Regards,
> 		Tomas