given and for

GlitchMr has asked for the wisdom of the Perl Monks concerning the following question:

After you will type at least use 5.010 you can use given statement. It allows you to use when inside it, but for/foreach also allows it. I would like to know why Perl has given when for can be used for this same thing (at least I think so).

For background, in this example, both code samples are equivalent.

for ($value) {
    when (1) { say "One!" }
    when (2) { say "Two!" }
    default  { say "Something!" }
}
[download]

given ($value) {
    when (1) { say "One!" }
    when (2) { say "Two!" }
    default  { say "Something!" }
}
[download]

Comment on given and for Select or Download Code

Replies are listed 'Best First'.
Re: given and for by moritz (Cardinal) on Aug 21, 2012 at 09:02 UTC
First of all `for` and `given` are not equivalent, they differ subtly in the scoping of their $_ variable: `use strict; use warnings; use 5.010; $_ = 42; sub f { say } given (23) { f } for (23) { f } __END__ 42 23` [download] So `given` creates a new lexical $_, while `for` localizes the existing $_, which results in dynamic scoping. But the real reason is that code must be readable to humans, and `for` has the connotation of a loop, whereas `given` makes it implicit that it's one value that is talked about. So it's all about intent. Perl 6 - the future is here, just unevenly distributed	[reply] [d/l] [select]
Re^2: given and for by Monk::Thomas (Friar) on Aug 24, 2012 at 09:20 UTC
"Switch Statements Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work right), you can say [...]" -- http://search.cpan.org/~rjbs/perl-5.16.1/pod/perlsyn.pod#Switch_Statements	[reply]
Re: given and for by Arunbear (Prior) on Aug 21, 2012 at 09:41 UTC
Also see: Use for() instead of given()	[reply]
Re: given and for by MidLifeXis (Monsignor) on Aug 21, 2012 at 12:58 UTC
Why does Perl have `while` when we have `if / goto`? They can both do the same thing, right? Just because you can shoehorn some construct into doing something else does not mean that that it is the best tool for the job. It may not be the clearest indication of the programmers intent, may cause confusion with a person later reading the code, and so on. To me, there would be a disconnect between the `for / when`, and every time I would read it, there would be a short time where I would have to do a mental switch between "this is a loop" and "this is a set of choices based on a value". For the `given / when`, I would immediately be expecting a set of choices. There is also a false assertion in your question - that the two constructs do the same thing. See the output for `perl -MO=Terse ...` for the two samples given. moritz has gone into some of the differences between them in his post. --MidLifeXis	[reply] [d/l] [select]
Re: given and for by 2teez (Vicar) on Aug 21, 2012 at 10:13 UTC
Hi, given and when are analogous to switch and case in other languages. Please, see subheading " Switch Statements " under documentation perlsyn for more details.	[reply]
Re^2: given and for by GlitchMr (Sexton) on Aug 21, 2012 at 10:17 UTC
I know that they are analogous, but that wasn't what I asked. I asked why Perl has `given` when `for` can also be used for this.	[reply] [d/l] [select]