Hi Xiaolan!
On Sunday 24 Jan 2010 05:37:49 Xiao Lan (小兰) wrote:
> Hi,
>
> what's the difference between do a block and eval a block?
>
> sub test {
> my $bl = shift;
> eval $bl;
> # do $bl;
> }
>
> test { print "hello\n" };
Well, if I run:
<<<<<<<<<<<<<<<<
#!/usr/bin/perl
use strict;
use warnings;
sub test {
my $bl = shift;
eval $bl;
# do $bl;
}
test { print "hello\n" };
>>>>>>>>>>>>>>>>
I'm getting:
<<<<<<
$ perl Test.pl
hello
Odd number of elements in anonymous hash at Test.pl line 12.
>>>>>>
This is a hint that the "print" gets evaluated at the line of the test, and
then perl thinks it is an anonymous hash-ref. If you are more explicit and use
a << sub { ... } >>, like so:
<<<<<
#!/usr/bin/perl
use strict;
use warnings;
sub test {
my $bl = shift;
eval $bl;
# do $bl;
}
test(sub { print "hello\n" });
>>>>>
Then it prints nothing, because the eval tries to evaluate the sub {...}
reference as a string. (And do $bl will try to read a file and process it for
code). What you really want is a:
<<<<
#!/usr/bin/perl
use strict;
use warnings;
sub test {
my $bl = shift;
print "Before\n";
eval { $bl->() };
print "After\n";
# do $bl;
}
test(sub { print "hello\n" });
>>>>
Now, in this case, eval { ... } and do { ... } will work the same. But there
would be changes:
1. If you throw an exception, then eval will catch it and set $@ to it. If no
exception was thrown, it will set "$@" to the empty string. Also look at
Object-Oriented exceptions:
http://www.shlomifish.org/lecture/Perl/Newbies/lecture4/exceptions/
2. If you do something funky like << do { ... } while (COND()) >> then Perl
has some magic to execute the contents of the do { ... } once before
evaluating the COND() (as opposed to << while (COND()) { BODY } >> ).
--------------
So if you want to do something like << my $var = RESULT_OF_BLOCK; >> use do {
... } and if you want to trap an exception use eval { ... } (and check $@
afterwards).
Regards,
Shlomi Fish
>
> from the code, I see both do and eval work the same.
>
> Thanks.
--
-----------------------------------------------------------------
Shlomi Fish http://www.shlomifish.org/
http://www.shlomifish.org/humour/ways_to_do_it.html
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