A new episode in the series:
"Yes, Perl can do!"
I recently had a discussion with a fellow perlmonger about a 3h video¹ about meta-programming in Python thanks to so called decorators. (something like advice in LISP)
So my reply was
Great, but what exactly are the benefits over attributes and type-glob manipulation in Perl?
His long reply can be summarized with something like
- Attributes are very difficult to handle
- "Nobody" understands them
- no good tutorials
- In Python decorators are first class objects with very short elegant code
- glob manipulations in Perl are cryptic
- Attributes are rarely used in Perl
- While typical use cases in Python are
- memoization,
- debugging,
- logging
- CLI wrapping,
- route-wrapper for web-applications
- ...
So I accepted the challenge to prove him wrong:
Taking this (rather stupid) example from SO:
def makebold(fn):
def wrapped():
return "<b>" + fn() + "</b>"
return wrapped
def makeitalic(fn):
def wrapped():
return "<i>" + fn() + "</i>"
return wrapped
@makebold
@makeitalic
def hello():
return "hello world"
print hello() ## returns <b><i>hello world</i></b>
I hacked this code
use strict;
use warnings;
use Data::Dumper qw'Dumper';
use Attribute::Handlers;
use feature 'say';
sub wrap {
my ($glob,$c_wrapper) = @_;
no warnings 'redefine';
*$glob = $c_wrapper;
}
sub BOLD :ATTR {
my ($pkg,$glob,$ref) = @_;
wrap $glob => sub { "<b>" . $ref->() . "</b>" };
}
sub ITALIC :ATTR {
my ($pkg,$glob,$ref) = @_;
wrap $glob => sub { "<i>" . $ref->() . "</i>"};
}
sub hello :ITALIC :BOLD {
return "hello world";
}
say hello();
__END__
<b><i>hello World</i></b>
IMHO the Perl code is already better readable and can even be further improved with more syntactic sugar.
Not every decorator really needs to wrap code, abstracting it into a function wrap from an imported module seems reasonable (Python-folks always return the identical function-ref to handle this)
My friend was impressed, but maybe he wasn't critical enough.
Here my questions :
- Did I miss something?
- Does Attribute::Handlers have traps to be avoided?
- Why do we ignore attributes?
- Why don't we adapt (i.e. steal) good use cases from Python or LISP
Actually I wanted to meditate more about the issue and refine the code before posting, but I'm quite busy at the moment and the risk to forget this task in a drawer is quite high.
So following the release often paradigm I just posted my raw results...
At least I hope you have now good arguments, if someone claims Python was superior because of decorators!
Cheers Rolf
( addicted to the Perl Programming Language)
¹) OMG 8-|
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