Re: Loop controls
On Wednesday, May 1, 2002, at 02:27 PM, Aaron Sherman wrote:
> unless my $fh = $x.open {
> die "Cannot open $x: $!";
> } else while $fh.getline -> $_ {
> print;
> } else {
> die "No lines to read in $x";
> }
I think you need a better example--given that the unless clause throws
an exception, the "else while" can just as easily be written as a
"while", no "else". (Which I, personally, would find far clearer.)
- Damien
Re: Loop controls
On Wed, 2002-05-01 at 18:47, Damien Neil wrote:
> On Wednesday, May 1, 2002, at 02:27 PM, Aaron Sherman wrote:
> > unless my $fh = $x.open {
> > die "Cannot open $x: $!";
> > } else while $fh.getline -> $_ {
> > print;
> > } else {
> > die "No lines to read in $x";
> > }
>
> I think you need a better example--given that the unless clause throws
> an exception, the "else while" can just as easily be written as a
> "while", no "else". (Which I, personally, would find far clearer.)
No, I write this sort of thing all the time:
if (foo) {
die $!
} else {
stuff
}
It's clearer, IMHO, to the reader and maintainer of the code, exactly
what is expected. It's taste and style and visual aesthetics, but we
wouldn't have a postfix "if" if those things didn't matter in Perl.
eval {} or carp "blah: $@"
with p5, Ive often written
eval {} or carp "$@ blah";
it seems to work, and it reads nicer (to my eye) than
eval {}; if ($@) {}
but I surmise that it works cuz the return-value from the block is non-zero,
for successful eval, and 0 or undef when block dies, not cuz of magical
treatment of $@.
I gather that ';' is unneeded in p6, and given that $! is the
'exception'al topicalizer,
is this construct no longer reliant on the last value in the eval block ?
put another way, does the 'topicalizer' reflect the exit condition of
the closure ?
Re: eval {} or carp "blah: $@"
On Thu, 2 May 2002, Jim Cromie wrote:
>
> with p5, Ive often written
>
> eval {} or carp "$@ blah";
>
> it seems to work, and it reads nicer (to my eye) than
>
> eval {}; if ($@) {}
>
> but I surmise that it works cuz the return-value from the block is non-zero,
> for successful eval, and 0 or undef when block dies, not cuz of magical
> treatment of $@.
The first one works because the return-value from the eval is undef if the
block fails. The second one is because $@ is true if it failed, and false
if it succeeded.
> I gather that ';' is unneeded in p6, and given that $! is the
> 'exception'al topicalizer,
> is this construct no longer reliant on the last value in the eval block ?
I would imagine that you would do it this way:
try {
...
CATCH { }
}
Only inside the CATCH does $! topicalize; I believe $_ becomes aliased to
that. Remember, C is Perl 6's eval{}.
> put another way, does the 'topicalizer' reflect the exit condition of
> the closure ?
It represents the exception that was thrown, and whatsoever might be
contained in that exception.
Or am I wrong?
Luke
Re: eval {} or carp "blah: $@"
On Thu, May 02, 2002 at 02:33:42PM -0600, Jim Cromie wrote:
>
> with p5, Ive often written
>
> eval {} or carp "$@ blah";
You generally Don't Want To Do That.
If the eval succeeds, but the last statement in the eval happens to come
out as false, then it'll still carp:
$a = 0; eval { 1 < $a } or carp ""
$a = 0; eval { 1 / $a } or carp ""
will both carp
--
My get-up-and-go just got up and went.
Re: Loop controls
> Um... I know it's scary, but I can actually imagine using this (or
> something like this) in development. I'll occasionally work on a section
> of code I'm not ready to integrate yet. It would be nice to be able to
> syntax check it without uncommenting and re-commenting the whole thing.
> :)
Yes. That's what I do: produce modules that are the superposition of:
all('absurd', 'useful', 'scary')
;-)
Damian
Re: eval {} or carp "blah: $@"
At 02:33 PM 5/2/02 -0600, Jim Cromie wrote:
>eval {} or carp "$@ blah";
>
>it seems to work, and it reads nicer (to my eye) than
>
>eval {}; if ($@) {}
% perl -le 'eval { print "No exceptions here"; 0 } or warn "$@ blah"'
No exceptions here
blah at -e line 1.
--
Peter Scott
Pacific Systems Design Technologies
Re: eval {} or carp "blah: $@"
Jim Cromie wrote:
> with p5, Ive often written
>
> eval {} or carp "$@ blah";
>
> it seems to work,
modulo any block that returns false :-(
> and it reads nicer (to my eye) than
>
> eval {}; if ($@) {}
>
> but I surmise that it works cuz the return-value from the block is non-zero,
> for successful eval, and 0 or undef when block dies, not cuz of magical
> treatment of $@.
No. C return the last evaluated value in the block, unless an
exception is thrown, in which case it returns C. The problem with
the C approach is that there are situations in
which the block can evaluate successfully but still return C.
On failure, C sets C<$@>, so careful programmers test it separately.
Of course, you *can* do that in a single statment if you want:
eval {} or @! and carp "$@ blah";
> I gather that ';' is unneeded in p6,
Err...no.
> and given that $! is the 'exception'al topicalizer,
Only inside an explicit CATCH block.
> is this construct no longer reliant on the last value in the eval block ?
In Perl 6 C becomes C and you'll still be able to write:
try { ... } // carp $!;
but that will trip up if the final value of the C block is C.
What you want is:
try {
...
CATCH { carp $! }
}
> put another way, does the 'topicalizer' reflect the exit condition of
> the closure ?
Only in a CATCH block.
Damian
