Getopt::Long - why \$ ?

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

I've been happily using Getopt for sometime, but it only recently crossed my mind to wonder what the hell is going on when options are assigned to variables.

GetOptions(
  "header=s" => \$default_header
)
[download]

Basically, what's going on with "header=s" => \$"? If I wasn't familiar with Getopt, I'd be scratching my head over why I'm passing a weird string ("header=s") and a reference to $default_header to a function called GetOptions, and using a hash assignment to do it.

map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2*$d*$e+$a,$e**2
-$d**2+$b);$c=$d**2+$e**2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20
[download]

Tom Melly, pm (at) cursingmaggot (stop) co (stop) uk

Comment on Getopt::Long - why \$ ? Select or Download Code

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Re: Getopt::Long - why \$ ? by Corion (Patriarch) on Oct 29, 2018 at 14:25 UTC
How would you define an interface where you specify a string and a place where to store the value if that string is matched somewhere? You pass a list of pairs to `GetOptions(...)`, and that list of pairs is 1) the specification of the switch (`"name\|alias=value type"`) and 2) the place where the value should be written to. The way to pass a place around in Perl is usually to pass a reference to the target around, which is what happens with the `\$foo` thing. An alternative approach is to make `GetOptions(...)` store all things it finds in a hash. This will give you a plain hash without weird syntax, but on the downside, you lose the typo checking that `strict.pm` provides.	[reply] [d/l] [select]
Re^2: Getopt::Long - why \$ ? by Melly (Chaplain) on Oct 29, 2018 at 14:39 UTC
Ah, got it. I think my conceptual problem was that I don't think I ever use (or think of) references as a way to set the referenced variable's value. A reference to me tends to mean that either I need a complex data-structure, or I've got a fussy return from a subroutine (e.g. two separate variable-length lists that I need to keep separate). Anyway, many thanks your holiness. `map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2$d$e+$a,$e2 -$d2+$b);$c=$d2+$e2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20` [download] Tom Melly, pm (at) cursingmaggot (stop) co (stop) uk	[reply] [d/l]
Re: Getopt::Long - why \$ ? by ikegami (Patriarch) on Oct 30, 2018 at 02:21 UTC
There's no hash in there. "`=>`" is just a fancy way of writing "`,`", and it has nothing to do with hashes. (An identifier on the LHS of `=>` is also treated a string literal, but that's not relevant here.)	[reply] [d/l] [select]