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.
