lua-users home
lua-l archive

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index]

>Are you saying we shouldn't program with side effects?  That is ironic for a
>language where variables default to the global scope.

I'm just saying that if you change global variables in functions or tag methods,
then you should be careful. But that applies to any language, even C. (Let's
not even mention C++ in this respect.)

>I think any language that allows side effects should be very clear about
>evaluation order.

Any language that has global variables allows these kind of side effects,
but not even C is clear about evaluation order (it only says it's undefined).
For instance, in b=a+f(), if a is global and f changes a, then the value of b
is undefined. Even without global variables there are things like b=a+(a++),
which C books are careful to say it's undefined.

Ultimately, the compiler is the only authority for how expressions are
evaluated. In the case of C, they may give different results, because there
are many different compilers for many different platforms. In the case of Lua,
there's only one compiler, which generates code to a fixed virtual machine and
thus it's easy to guarantee consistency. Of course, this does not avoid the
need to say something about evaluation order in the manual, but at least when
in doubt you can always run luac on the code and see how the expressions is

On the other hand, if we want other Lua implementations to exist, the manual
should be more specific. Other implementations might not use the same VM
(or should they?).