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- Subject: Re: semantics
- From: Will Leshner <will@...>
- Date: Sat, 10 Feb 2001 10:28:14 -0700
Scheme, for example, is adamant (I sat through many classes with Dan
Friedman making this point over and over again) that order of evaluation is
and should be undefined.
> From: Luiz Henrique de Figueiredo <email@example.com>
> Reply-To: firstname.lastname@example.org
> Date: Sat, 10 Feb 2001 11:18:09 -0200 (EDT)
> To: Multiple recipients of list <email@example.com>
> Subject: Re: semantics
>> 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
> 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?).