Re: The meaning of 'sugar' [Forked from Re: Why do we have ipairs?]

[Ross Bencina <rossb-lists@...>]

On 12/06/2014 10:44 AM, Paige DePol wrote:
> Perhaps you could elaborate on this point a bit as I feel I am
> missing something here. Under C switch statements require literal
> values yet switch statements are widely used and, as far as I know,
> the use of which is not considered bad code.

Hopefully I'm not putting words into Roberto's mouth here. In any case 
it's not my intention. My impression is as follows:

In C there are two mechanisms for symbolic constants: #defines and enums.

A switch like this:

switch(x){
	case 3: foo(); break;
	case 42: bar(); break;
	case 66: baz(); break;
};

Would be considered bad style. The following (using enum or #defined 
constants) would not:

switch(x){
	case MOUSE_UP: foo(); break;
	case MOUSE_DOWN: bar(); break;
	case MOUSE_MOVE: baz(); break;
}

The problem is in Lua (unless I'm missing something), you don't have 
symbolic constants, so it's not possible to "compile" the second form to 
anything better than a sequence of comparisons. If you use Lua 
variables, their values are not constant, and so could change each time 
the switch is executed (hence the equivalence to a linear if/else chain).


> In which way does adding symbolic constants allow the optimising of
> switch statements?

It allows for a balanced tree, perfect hash and/or static lookup table 
to be constructed at compile time. This can also be done if the cases 
are numeric or string literals (perhaps that's what your patch does, 
sorry I haven't looked at it). But it can not be done if the cases are 
symbolic (because the values can change), which I would suggest is by 
far the most common use-case (maybe you have others though).


> How would adding a switch optimised for literals
> badly impact the code standards for Lua?

By people writing code as in my first example above.


> Obviously, as I have created a switch patch for Lua (based on
> computed gotos) I have an interest in understanding the points being
> raised here! ;)

For the record: I started writing my earlier email to defend the utility 
of a computed goto, and then realized what Roberto was talking about.

Maybe your patch has a solution that resolves the above.

Ross.