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Query about Perl syntax

From Henry Law@21:1/5 to All on Mon Nov 30 12:35:44 2020

I'm just getting into Mojolicious. Within the tutorial, in one of the
sections on testing I find this code (abbreviated a little - it's not runnable):

use Test::More;
use Test::Mojo;
...
# Test if the HTML login form exists

get_ok('/')

->status_is(200)
->element_exists('form input[name="user"]')
->element_exists('form input[name="pass"]')
->element_exists('form input[type="submit"]');

The syntax of the last five lines surprises me. It looks like a chain of
five method calls, where the result of the "get_ok" method has itself a "status_is" method, which returns a result which has an "element_exists"
method ... all the way down until the last "element_exists" method
returns a status to the $t object, which then talks to Test::More.

Much has changed in Perl, and in other things like HTML for that matter,
since I started programming, and I'm an amateur so I don't get exposed to continuing education: in short I'm horribly out of date. So is my interpretation of that last statement correct, in terms of its functional steps? Or is there some change in Perl syntax which means that it works
in some other way?

--
Henry Law n e w s @ l a w s h o u s e . o r g
Manchester, England

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From Henry Law@21:1/5 to All on Mon Nov 30 13:35:11 2020

On Mon, 30 Nov 2020 19:16:18 +0000, hymie! wrote:

The short and simple answer is that the method call both changes the
object and returns the changed object.

Ah! I see. So the returned object is of the same class (indeed, it's
the same object) and has the same methods, which can be called again and
again. Clever!

--
Henry Law n e w s @ l a w s h o u s e . o r g
Manchester, England

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From hymie!@21:1/5 to who on Mon Nov 30 19:16:18 2020

In our last episode, the evil Dr. Lacto had captured our hero,
Henry Law <[email protected]>, who said:

# Test if the HTML login form exists

get_ok('/')

->status_is(200)
->element_exists('form input[name="user"]')
->element_exists('form input[name="pass"]')
->element_exists('form input[type="submit"]');

The short and simple answer is that the method call both changes the object
and returns the changed object. So the consecutive method calls all
change the underlying object $t . It's basically a shorthand for

$t = $t->get_ok('/');
$t = $t->status_is(200);
$t = $t->element_exists('form input[name="user"]');
$t = $t->element_exists('form input[name="pass"]');
$t = $t->element_exists('form input[type="submit"]');

--hymie! http://nasalinux.net/~hymie [email protected]

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From Rainer Weikusat@21:1/5 to Henry Law on Wed Dec 2 16:33:18 2020

Henry Law <[email protected]> writes:

On Mon, 30 Nov 2020 19:16:18 +0000, hymie! wrote:

The short and simple answer is that the method call both changes the
object and returns the changed object.

Ah! I see. So the returned object is of the same class (indeed, it's
the same object) and has the same methods, which can be called again and again. Clever!

Well, "sort-of". This kind of chaining is very bad if one ever needs to
debug this code: The substatements making up this complex statement are
neither breakable nor is it possible to insert code between them without breaking up the expression first.

IMHO, that's a "classic" example of "optimizing for elegance at the
expense of maintainability".

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From Randal L. Schwartz@21:1/5 to All on Wed Dec 2 11:36:24 2020

"Henry" == Henry Law <[email protected]> writes:

Henry> Ah! I see. So the returned object is of the same class (indeed,
Henry> it's the same object) and has the same methods, which can be
Henry> called again and again. Clever!

That's only by design choice of the programmer. Methods that do this
can be chained:

sub enable_shipping {
my $self = shift;
[act on $self for the shipping side-effect]
return $self;
}

In other words, the object itself needs to be returned from the method
for the next method in the chain to have something to bite on.

Dart does this better, having stolen the idea from Smalltalk. Methods
are invoked with a single dot for normal behavior (return from method
becomes return from overall expression) or with two dots, where the
current value is input to the method (which typically has side-effects),
but also remains the return value from the expression. In Dart and
Smalltalk, this is called a "cascade", and can come in very handy.

So that original text would look like

t.get_ok('/') this value is input to all the following
..status_is(200)
..element_exists('form input[name="user"]')
..element_exists('form input[name="pass"]')
..element_exists('form input[type="submit"]');

The return value is the result of t.get_ok(). And none of the other
methods have to be built specially. It's very slick.

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <[email protected]> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/Dart consulting, Technical writing, Comedy, etc. etc.
Still trying to think of something clever for the fourth line of this .sig

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From Randal L. Schwartz@21:1/5 to All on Thu Dec 3 01:52:17 2020

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> Well, "sort-of". This kind of chaining is very bad if one ever needs to Rainer> debug this code: The substatements making up this complex statement are Rainer> neither breakable nor is it possible to insert code between them without
Rainer> breaking up the expression first.

I use them extensively in my File::Finder. Then again, I don't debug
much. :)

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <[email protected]> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/Dart consulting, Technical writing, Comedy, etc. etc.
Still trying to think of something clever for the fourth line of this .sig

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From Rainer Weikusat@21:1/5 to Randal L. Schwartz on Thu Dec 3 18:33:09 2020

[email protected] (Randal L. Schwartz) writes:

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> Well, "sort-of". This kind of chaining is very bad if one ever needs to
Rainer> debug this code: The substatements making up this complex statement are
Rainer> neither breakable nor is it possible to insert code between them without
Rainer> breaking up the expression first.

I use them extensively in my File::Finder. Then again, I don't debug
much. :)

I don't see any examples in

https://metacpan.org/source/MERLYN/File-Finder-0.53/lib%2FFile%2FFinder.pm

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From Randal L. Schwartz@21:1/5 to All on Fri Dec 4 12:54:45 2020

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> I don't see any examples in

Rainer> https://metacpan.org/source/MERLYN/File-Finder-0.53/lib%2FFile%2FFinder.pm

at the very bottom: "show 202 lines of pod". You must've noot clicked
that. How long have you been using metacpan? :)

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <[email protected]> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/Dart consulting, Technical writing, Comedy, etc. etc.
Still trying to think of something clever for the fourth line of this .sig

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From Rainer Weikusat@21:1/5 to Randal L. Schwartz on Fri Dec 4 21:46:37 2020

[email protected] (Randal L. Schwartz) writes:

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> I don't see any examples in

Rainer> https://metacpan.org/source/MERLYN/File-Finder-0.53/lib%2FFile%2FFinder.pm

at the very bottom: "show 202 lines of pod". You must've noot clicked
that. How long have you been using metacpan? :)

There's no code in the documentation. And I never use metacpan, at least
not routinely.

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From Randal L. Schwartz@21:1/5 to All on Sat Dec 5 15:15:17 2020

"Rainer" == Rainer Weikusat <[email protected]> writes:

at the very bottom: "show 202 lines of pod". You must've noot clicked
that. How long have you been using metacpan? :)

Rainer> There's no code in the documentation.

Wow. You're blinded, or something. Take a look at the formatted
version:

https://metacpan.org/pod/File::Finder

I see about 50 lines of code.

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <[email protected]> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/Dart consulting, Technical writing, Comedy, etc. etc.
Still trying to think of something clever for the fourth line of this .sig

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From Rainer Weikusat@21:1/5 to Randal L. Schwartz on Sun Dec 6 11:21:12 2020

[email protected] (Randal L. Schwartz) writes:

"Rainer" == Rainer Weikusat <[email protected]> writes:

at the very bottom: "show 202 lines of pod". You must've noot clicked
that. How long have you been using metacpan? :)

Rainer> There's no code in the documentation.

Wow. You're blinded, or something. Take a look at the formatted
version:

https://metacpan.org/pod/File::Finder

I see about 50 lines of code.

There's no example of expression chaining in there either. And after the
"I use it extensively ..." I had expected to see this in the code of the
module itself, meaning, this amusing arrangments of statements a la

if (state[-3] < 3.144 / 22.5) {
state[25] = state[-139]
} else {
etas[-52] = etas[931]
}

etc

NB: The code doesn't really contain this statement which is a bit of an exaggeration.

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From Randal L. Schwartz@21:1/5 to All on Mon Dec 7 08:03:53 2020

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> There's no example of expression chaining in there either.

In the pod for File::Finder:

Synopsis:

use File::Finder;
## simulate "-type f"
my $all_files = File::Finder->type('f');

## any rule can be extended:
my $all_files_printer = $all_files->print;

## traditional use: generating "wanted" subroutines:
use File::Find;
find($all_files_printer, @starting_points);

## or, we can gather up the results immediately:
my @results = $all_files->in(@starting_points);

## -depth and -follow are noted, but need a bit of help for find:
my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}'); find($deep_dirs->as_options, @places);

Examples in pod:

my $files_print = $files->print;

my $files_depth_print = $files->depth->print;

my $big_or_old_files =
$files->left->size("+50")->or->atime("+30")->right;

my @names = File::Finder->in("/tmp");

in File::Finder::Steps:

my $big_or_old = File::Finder
->type('f')
->left
->size("+100")->or->mtime("+90")
->right;
find($big_or_old->ls, "/tmp");

# list all non-files in /tmp
File::Finder->not->type('f')->ls->in("/tmp");

# list all files and dirs, but don't descend into CVS dir contents: File::Finder->type('d')->name('CVS')->prune->comma->ls->in('.');

my $pm_files = File::Finder->name('*.pm')->in('.');
my $pm_files_too = File::Finder->name(qr/pm$/)->in('.');

my $files = File::Finder->type('f');
# find files that are exactly mode 644
my $files_644 = $files->perm(0644);
# find files that are at least world executable:
my $files_world_exec = $files->perm("-1");
# find files that have some executable bit set:
my $files_exec = $files->perm("+0111");

my $blaster = File::Finder->atime("+30")->eval(sub { unlink });

my $old = File::Finder->atime("+30");
my $big = File::Finder->size("+100");
my $old_or_big = File::Finder->eval($old)->or->eval($big);
my $killer = File::Finder->eval(sub { unlink });
my $kill_old_or_big =
File::Finder->eval($old_or_big)->ls->eval($killer); $kill_old_or_big->in('/tmp');

my $plugh_files = File::Finder->type('f')->contains(qr/plugh/);

Truly, Rainer, I sincerely doubt your ability to read a perldoc page
now. {sigh}

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <[email protected]> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/Dart consulting, Technical writing, Comedy, etc. etc.
Still trying to think of something clever for the fourth line of this .sig

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From Rainer Weikusat@21:1/5 to Randal L. Schwartz on Wed Dec 9 19:46:41 2020

[email protected] (Randal L. Schwartz) writes:

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> There's no example of expression chaining in there either.

[...]

## -depth and -follow are noted, but need a bit of help for find:
my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}'); find($deep_dirs->as_options, @places);

[...]

my $files_depth_print = $files->depth->print;

my $big_or_old_files =
$files->left->size("+50")->or->atime("+30")->right;

Truly, Rainer, I sincerely doubt your ability to read a perldoc page
now. {sigh}

I wasn't particularly interested in the contents as I still had expected
to find this in the code.

So, File::Finder emulates a domain specific mini-language via chained
method calls. Out of my head, this seems like a bit of a weird idea but
as I've never tried to solve this problem myself, this isn't exactly an
"in depth" assessment.

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From Randal L. Schwartz@21:1/5 to All on Wed Dec 9 12:30:24 2020

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> I wasn't particularly interested in the contents as I still had expected
Rainer> to find this in the code.

Find *what* in the code?

Rainer> So, File::Finder emulates a domain specific mini-language via
Rainer> chained method calls. Out of my head, this seems like a bit of a Rainer> weird idea but as I've never tried to solve this problem myself, Rainer> this isn't exactly an "in depth" assessment.

I learned Smalltalk Cascades almost a decade before I started with
Perl. It felt natural to do this. Also, File::Find::Rule also did it
this way, but I found that the non-equivalence to File::Find always
threw me off, so I wrote my own that did relate better.

print "Just another Perl hacker," # the original

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <[email protected]> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/Dart consulting, Technical writing, Comedy, etc. etc.
Still trying to think of something clever for the fourth line of this .sig

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From Rainer Weikusat@21:1/5 to Randal L. Schwartz on Thu Dec 10 16:48:13 2020

[email protected] (Randal L. Schwartz) writes:

"Rainer" == Rainer Weikusat <[email protected]> writes:

Rainer> I wasn't particularly interested in the contents as I still had expected
Rainer> to find this in the code.

Find *what* in the code?

Chaining of methods with side-effects via returned object. As that's
just in the interface, "I use it extensivly in ..." should perhaps
rather have been "I imagine users of ... to use it extensively".

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From Randal L. Schwartz@21:1/5 to All on Thu Dec 10 23:35:33 2020

"Rainer" == Rainer Weikusat <[email protected]> writes:

Find *what* in the code?

Rainer> Chaining of methods with side-effects via returned object. As
Rainer> that's just in the interface, "I use it extensivly in ..."
Rainer> should perhaps rather have been "I imagine users of ... to use
Rainer> it extensively".

Ahh, but that's for the users of the module to use! I just provide the implementation and the API, except for a few uses in the docs and the
tests.

And there's the two magazine articles I wrote on it.

http://www.stonehenge.com/merlyn/LinuxMag/col73.html http://www.stonehenge.com/merlyn/PerlJournal/col08.html

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <[email protected]> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/Dart consulting, Technical writing, Comedy, etc. etc.
Still trying to think of something clever for the fourth line of this .sig

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Query about Perl syntax

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