Re: comparison operators for user defined types?

[Chris Marrin <chris@...>]

Mike Pall wrote:

> ...
> Lua uses the 'pure' definition for object comparison: there is no
> ordering relationship between objects of different types. This is
> in line with the Lua definition of object equality as object identity.
> So from a language design standpoint this is very consistent.
>
> Because when you allow SFFloat(5.2) < 4.7, then you really should make
> SFFloat(5.2) == 5.2 return a true result, too (difficult, but doable
> with automatic type coercion). But then you better ensure that
> table[SFFloat(5.2)] does the same as table[5.2]. This is where you get
> into real trouble (we had a similar discussion a few weeks ago).
>
> Umm, and automatic type coercion is troublesome, anyway -- have
> a look at the implementation of some languages that do this.
> I personally dislike the implicit string/number coercions in Lua,
> too (but I guess this is a convenience vs. purity issue).
>
> [And I have to add, that for my next bigger project (to be released
>  soon) the 'pure' language definition of Lua makes things an order
>  of magnitude easier.]
>
> So to summarize: the standard Lua operators won't help you here.
> Either use only canonical types or define your own operators,
> e.g. lt(SFFloat(5.2), 4.7). You loose the infix syntax, but
> you'll get used to it.

But those same arguments are true for the arithmetic operators, yet Lua 
supports those operators for different types. For instance:

    local a = SFFloat(4.5) * 5.2

works just fine. And on the subject of "purity", I can agree that 
disallowing string vs. number comparisons is a reasonable (although, in 
my opinion, unnecessary) constraint. But when dealing with user defined 
types that distinction becomes more grey. In my case, what Lua sees as a 
userdata, I see as a number.

I'm not asking Lua to have a type coercion system, just that the 
equality operators behave like the arithmetic ones. I guess I don't 
understand why they are treated differently.

FYI, I patched 5.02 to have luaV_lessthan() and luaV_lessequal() behave 
like Arith, and everything now works for me. I suppose I can patch the 
releases as they come out.

Thanks.

PS - I has been years since anyone has complained about my sig. Back in 
the days when people were using 1200 baud modems those 12 lines made a 
big difference in download time! But I never got accused of being a 
script kiddie. It makes me feel so young :-) If you use a fixed pitch 
font and squint, you will see that it is just my first name in script. 
Sorry about the inconvenience. I will try to remember to delete it after 
this message (to accomodate any squinters reading this) :-)

--
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