On Tuesday 15 Dec 2009 15:53:28 Philip Potter wrote:
> 2009/12/15 Shlomi Fish <[email protected]>:
> > On Tuesday 15 Dec 2009 14:25:28 Xiao Lan (小兰) wrote:
> >> On Tue, Dec 15, 2009 at 7:50 PM, Xiao Lan (小兰) <[email protected]>
> >
> > wrote:
> >> > I did have tried that, but this will get a runtime error.
> >>
> >> Sorry this is exactly a compile-time error.
> >>
> >> # cat except.pl
> >>
> >> eval { $x=12/0 };
> >> print "0 div error" if $@;
> >>
> >> # perl -c except.pl
> >> Illegal division by zero at except.pl line 2.
> >>
> >> So, can't we capture it at runtime when meeting such error?
> >
> > No, you cannot capture compile-time errors at runtime. You'll need to
> > make sure your code compiles before you run it. Note that you can capture
> > such errors if you do perldoc -f require, perldoc -f do (for a filename),
> > string eval "", etc. at run-time, in which case perl 5 invokes its
> > compiler to compile some code at run-time.
>
> How can "Illegal division by zero" be a compile-time error? It seems
> clear to me that it's a run-time error, which the optimizer has
> (wrongly) decided to raise at compile-time.
>
Well, the Perl compiler tends to collapse constants. So if it sees something
like:
<<<
... ( CONSTANT_EXPR() / 0 ) ...
>>>
it will try to find the value of "CONSTANT_EXPR() / 0" so it can optimise it
as a constant for the interpreter, and then get a fault there. It's a standard
optimisation technique that also exists in other compilers such as gcc that
are collapsing such constant expressions. So it does get evaluated at compile-
time and as such should be avoided.
Regards,
Shlomi Fish
> Phil
>
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