Re: Concatenating strings with '+'

[Chris Marrin <chris@...>]

Ben Sunshine-Hill wrote:
> On 7/20/05, Ignacio Burgueño <ignacio@tecnolink.com.uy> wrote:
>
> > Hi. I'd like to modify the parser so I could concatenate strings using '+'.
> > I guess that I need to modify lparser.c, function subexpr. My initial
> > attempt was to change the operand from OPR_ADD to OPR_CONCAT, but only if
> > the operands are strings. This last part is what I couldn't figure out. Is
> > this possible ?
> >
> > Thanks in advance,
> > Ignacio
>
>
> No, it isn't possible. Consider this construction:
>
> function foo(a,b)
>     return a+b
> end
>
> Q: Are a and b strings or not? A: There's no way to know. This is
> what's known as "static type analysis", and is only possible for
> dynamically typed languages in special cases.
>
> There's a good reason why + doesn't concatenate in Lua: to avoid
> accidentally concatenating when you really meant to add. Since numbers
> can be used in a string context and vice versa, this would generate
> quite a few problems. Stick to .. for concatenation.

This is an old-time issue from Javascript, which overloads '+' to do 
both concat and numeric add. It is solvable by having certain rules 
about when to do concat and when to do add. For instance, Javascript 
will use concat if either operand is a string. Otherwise it will try to 
convert both operands to numbers and do an add (from the ECMAScript 3 
spec). I'm not saying that is a good rule, and it certainly does get you 
into trouble. But it is at least a consistent rule :-)

Adding this logic to Lua would be very easy. You would simply go into 
lvm.c and change the Arith() function to test for strings if op is 
TM_ADD and then jump out to luaV_concat() if so. This adds a bit of 
logic to Arith() which slows down every arithmetic operation. But, at 
worst, this is something like:

    if (op == TM_ADD &&
            (ttype(rb) == LUA_TSTRING || ttype(rc) == LUA_TSTRING))
        luaV_concat(...);

For all ops other than ADD this adds one integer compare and for ADD it 
adds an additional one or two integer compares. Not bad for what it 
gives you IMHO...

--
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