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Postings from August 2001
> More questions regarding the new 'given when' construct.
More answers (modulo Larry :-)
> 1) Is a 'when' clause required? Is it:
>
> [ LABEL: ] given ( expr_1 ) {
> when expr_2 : block
> [ when expr_3 : block ...]
> expr_n [;]
> }
>
> or
>
> [ LABEL: ] given ( expr_1 ) {
> [ when expr_2 : block ...]
> expr_n [;]
> }
Not required. It's:
[ LABEL: ] given ( expr_1 ) {
[statement ; ...]
[statement]
}
A C<when> statement is just a particular kind of statement.
> 2) In expressions 2 through n-1 - the equivalent to 'case' - you
> may refer to the return value of expression 1 with $^_. Does
> this imply that expression 1 will always (and only) be evaluated
> in scalar context? If not, can you mix contexts across 'when'
> clauses? Is the value of $^_ then a reference to the return
> value?
The C<given> expression is evaluated in scalar context.
> 3) Can you refer to $^_ within expression n - the equivalent to 'default:'?
> given ( a() ) {
> print $^_; # No when clause? See #1
> }
$^_ isn't an alias for the C<given>'s expression.
It's a higher-order function placeholder
(see: http://www.yetanother.org/damian/Perl5+i/higherorder.html)
So the answer is "no".
> 4) Can you refer to $^_ within the code blocks of each 'when' clause?
> given ( a() ) {
> when $^_ == 1 : { foo($^_) }
> when $^_ == 2 : { bar( 5 + $^_ ) }
> when $^_ == 3 : { ++$^_ and print $^_ } # Mutable? See #5
> }
No. See the previous answer.
If the gvien value is needed within the block, the most likely mechanism
would be:
given ( my $a = a() ) {
when $^_ == 1 : { foo($a) }
when $^_ == 2 : { bar( 5 + $a ) }
when $^_ == 3 : { ++$a and print $a } # Mutable? See #5
}
> 5) Is $^_ an alias, reference (See #2), or a copy of the return value of
> expression 1, and is it mutable? Does it have the appropriate magic and
> overloading capabilities?
None of the above. See above.
> 6) How many times is expression 1 evaluated?
Once.
> $a = 0;
> given ( ++$a ) {
> when $^_ > 5 : { #foo }
> when $^_ > 4 : { #bar }
> when $^_ > 3 : { #baz }
> #foobar
> }
>
> I would expect that $a would equal '1' after #foobar executed. But if
> multiple contexts are allowed (see question 2), how would
N/A.
> 7) The 'when' blocks are not fall-through, a la C's switch statement. You
> can obtain a similar result by invoking 'next' within the 'when' block.
>
> Is this regardless of whether expr2 would have also evaluted true? (IOW,
> does it truly jump to the next block?) If not, does it jump to the
> expression after that (if it exists) and try again, or does it default
> expression (since the original next expression evaluated false)?
A C<next> within a C<when>'s block causes control to jump to the next
statement in the innermost surrounding <given>'s block.
If that is a C<when> statement, the C<when> test is then evaluated as normal.
> 8) Is the order of evaluation guaranteed? (I assume so, but just thought
> I'd check.)
I certainly hope so! ;-)
> 9) It has a lexical construct similar to a conditional block, so that
> lexical variables declared in the 'given' expression are within the inner
> scope, correct?
Correct.
> 10) I've already asked Damian this question on the side, but I'll
> repeat (and expand) it here, to give me an even ten. Should it
> be a single default expression, multiple default expressions,
> or a default block?
Any of the above. A C<given> block is just a block. A C<when> statement
is just a statement. You can put any statement inside any block, so you
can put both C<when> and non-C<when> statements inside a C<given> (in any
sequence and multiplicity you like).
Damian
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