Has the entire "switch" feature become experimental? Are there parts of it I can rely on using without future versions of Perl breaking my code? In general, what is the policy toward changing stable features to experimental?
Background
use feature "switch" has been in Perl since 5.10. From 5.10 to 5.14, perlsyn seems to indicate that this is a stable, supported feature.
Starting with 5.16, however, perlsyn begins to call it "an experimental switch feature" and gets a lot more confusing about what's considered experimental.
Parts of the documentation seem to indicate everything about the feature is experimental:
Under the "switch" feature, Perl gains the experimental keywords given, when, default, continue, and break.
There's even an entire section about the Experimental Details on given and when.
However, perlsyn also says that "The foreach is the non-experimental way to set a topicalizer" and gives an example that seems to imply that foreach/when is not experimental.
As far as I can tell, the "experimental" language came from commit c2f1e22 which references RT #90926 which still doesn't give much context, even when paired with RT# 90906.
Has the entire "switch" feature become experimental?
No. It has always been.
Upd: Oh wow, maybe I'm wrong. I can't find a mention of that in 5.10.0 or .1. Maybe it wasn't? Or maybe they forgot to note it? Either way, it seems they messed up worse than I thought if so! But based on what I've seen since, I think the lesson was learned. (e.g. I still think values $ref is a bad idea, but at least it was marked experimental from day
1.)
Are there parts of it I can rely on using without future versions of Perl breaking my code?
Technically, no, although the devs are always careful when it comes to backwards compatibility.
In general, what is the policy toward changing stable features to experimental?
I don't see that ever happening. The deprecation process would be used instead.
Changes so far:
given is changing from creating a lexical $_ to localising $_ like foreach loops in 5.18 (or did it already happen in 5.16?).
5.10.1 saw some big changes in smart-matching*. Don't use (smart-matching in) 5.10.0.
Possible future changes:
The behaviour of smart-matching* is still a hot topic.
* — True, this is a feature distinct from given-when, but it's the same or closely related in most people's minds.
Related
I just installed Perl 5.18, and I get a lot of warnings like this,
given is experimental at .\[...].pl line [...].
when is experimental at .\[...].pl line [...].
Smartmatch is experimental at C:/strawberry/perl/site/lib/[...] line [...].
Looking into these warnings -- which I've never heard mentioned anywhere -- I was only able to find this in two places,
perldelta for 5.18, which only really mentions insofar as to say that the feature has been downgraded to experimental?
this nntp.perl.org post
The Perl Delta still does the most to give mention as to what's happening with those features. It's halfway down buried in the pod,
Smart match, added in v5.10.0 and significantly revised in v5.10.1, has been a regular point of complaint. Although there are a number of ways in which it is useful, it has also proven problematic and confusing for both users and implementors of Perl. There have been a number of proposals on how to best address the problem. It is clear that smartmatch is almost certainly either going to change or go away in the future. Relying on its current behavior is not recommended. Warnings will now be issued when the parser sees ~~, given, or when.
I'm confused at how the most significant change in Perl in the past 10 years could be pulled. I've started using given, when, and smartmatch all over the place. Is there any more information about these futures? How is anyone finding them "confusing?" How are these features likely to change? Is there a plan to implement these features with a module?
There are problems with the design of smart-matching. The decision of what any given TYPE ~~ TYPE should do is most often unobvious, inconsistent and/or disputed. The idea isn't to remove smart matching; it's to fix it.
Specifically, ~~ will be greatly simplified, as you can see in a proposal by the 5.18 pumpking. Decisions as to how two things should match will be done with helpers such as those that already exist in Smart::Match.
... ~~ any(...)
It is much more readable, much more flexible (fully extensible), and solves a number of problems (such as "When should X be considered a number, and when should it be considered a string?").
Some insights might be gained by reading rjbs's proposed changes to smartmatch. He is the pumpking (Perl release manager) after all, so his comments and his view of the future is more relevant than most. There is also plenty of community comment on the matter; see here for instance. The 'experimental' status is in effect because, since things are likely to change in the future, it is responsible to inform users of that fact, even if we don't know what those changes will be.
Well, that's what's said in the description of the patch that downgraded this set of features to experimental:
The behavior of given/when/~~ are likely to change in perl 5.20.0:
either smart match will be removed or stripped down. In light of this,
users of these features should be warned. A category
"experimental::smartmatch" warning should be issued for these features
when they are used.
So while you can indeed turn these warnings off, with something like this (source):
no if $] >= 5.018, warnings => "experimental::smartmatch";
... it's just turning your eyes off the problem.
perl5140delta says that localized tied variables are no long tied. This change was implemented in 5.13.1 but reverted in 5.13.2. Is this back in 5.14 (from my testing it does not appear to be) or is the delta for 5.14.0 incorrect?
I care because I believe this would break File::chdir which I use regularly.
The perldelta says that localized tied "hashes and arrays" are no longer tied. Scalars are unaffected.
The doc you linked, perl5132delta says:
localised tied scalars are tied again.
The change in behaviour in 5.13.1 of localising tied scalar values has been reverted to the existing 5.12.0 and earlier behaviour (the change for arrays and hashes remains).
The really useful thing I have to say is that you can check your favorite module's status on many versions of Perl using the CPAN Testers service.
Check out the report for File::chdir 0.1004. It passes all of the 5.14 RC tests on different platforms. According to CPANTS you are in good shape.
Of course you could download and build a test version of 5.14 and try it yourself, just to be sure.
I know I risk asking a speculative question, however, inspired by this recent question I wonder which editor does the best job of syntax highlighting Perl. Being well aware of the difficulties (impossibilities) of parsing Perl I know there will not be a perfect case. Still I wonder if there is a clear leader in faithful representation.
N.B. I use gedit and it works fine, but with known issues.
Komodo Edit does a good job and also scans your modules (including those installed via CPAN) and does well at generating autocomplete data for them.
I'm a loyal vim user and rarely encounter anything odd with the native syntax.vim, except for these cases (I'll edit in more if/when I find them; others please feel free also):
!!expression is better written !!!!expression (everything after two ! is rendered as a comment quoted string; four ! brings everything back to normal)
m## or s### renders everything after the # as a comment; I usually use {} as a delimiter when avoiding / for leaning toothpick syndrome
some edge cases for $hash{key} where key is not a simple alphanumeric string - although it's safer to enclose such key names in '' anyway so as to not have to look up the exact cases for when a bareword is treated as a key name
I haven't used it, but Padre should be good since it's written in Perl. IIRC It uses PPI for parsing
jEdit...with the tweaks that I have amassed over the years. It's got the most customizable syntax highlighting I've ever seen.
I use Emacs in CPerl mode. I think it does a terrific job, although similar to Ether's answer, it's not perfect. What's more, I usually use Htmlize to publish highlighted code to the web. It's kind of annoying to use fancier forums like this one that do their own syntax highlighting, since it's not really any easier and the results aren't as good.
I recently installed a module from CPAN and noticed one of its dependencies was common::sense, a module that offers to enable all the warnings you want, and none that you don't. From the module's synopsis:
use common::sense;
# supposed to be the same, with much lower memory usage, as:
#
# use strict qw(vars subs);
# use feature qw(say state switch);
# no warnings;
# use warnings qw(FATAL closed threads internal debugging pack substr malloc
# unopened portable prototype inplace io pipe unpack regexp
# deprecated exiting glob digit printf utf8 layer
# reserved parenthesis taint closure semicolon);
# no warnings qw(exec newline);
Save for undef warnings sometimes being a hassle, I've usually found the standard warnings to be good. Is it worth switching to common::sense instead of my normal use strict; use warnings;?
While I like the idea of reducing boiler-plate code, I am deeply suspicious of tools like Modern::Perl and common::sense.
The problem I have with modules like this is that they bundle up a group of behaviors and hide behid glib names with changeable meanings.
For example, Modern::Perl today consists of enabling some perl 5.10 features and using strict and warnings. But what happens when Perl 5.12 or 5.14 or 5.24 come out with great new goodies, and the community discovers that we need to use the frobnitz pragma everywhere? Will Modern::Perl provide a consistent set of behaviors or will it remain "Modern". If MP keeps with the times, it will break existing systems that don't keep lock-step with its compiler requirements. It adds extra compatibility testing to upgrade. At least that's my reaction to MP. I'll be the first to admit that chromatic is about 10 times smarter than me and a better programmer as well--but I still disagree with his judgment on this issue.
common::sense has a name problem, too. Whose idea of common sense is involved? Will it change over time?
My preference would be for a module that makes it easy for me to create my own set of standard modules, and even create groups of related modules/pragmas for specific tasks (like date time manipulation, database interaction, html parsing, etc).
I like the idea of Toolkit, but it sucks for several reasons: it uses source filters, and the macro system is overly complex and fragile. I have the utmost respect for Damian Conway, and he produces brilliant code, but sometimes he goes a bit too far (at least for production use, experimentation is good).
I haven't lost enough time typing use strict; use warnings; to feel the need to create my own standard import module. If I felt a strong need for automatically loading a set of modules/pragmas, something similar to Toolkit that allows one to create standard feature groups would be ideal:
use My::Tools qw( standard datetime SQLite );
or
use My::Tools;
use My::Tools::DateTime;
use My::Tools::SQLite;
Toolkit comes very close to my ideal. Its fatal defects are a bummer.
As for whether the choice of pragmas makes sense, that's a matter of taste. I'd rather use the occasional no strict 'foo' or no warnings 'bar' in a block where I need the ability to do something that requires it, than disable the checks over my entire file. Plus, IMO, memory consumption is a red herring. YMMV.
update
It seems that there are many (how many?) different modules of this type floating around CPAN.
There is latest, which is no longer the latest. Demonstrates part of the naming problem.
Also, uni::perl which adds enabling unicode part of the mix.
ToolSet offers a subset of Toolkit's abilities, but without source filters.
I'll include Moose here, since it automatically adds strict and warnings to the calling package.
And finally Acme::Very::Modern::Perl
The proliferation of these modules and the potential for overlapping requirements, adds another issue.
What happens if you write code like:
use Moose;
use common::sense;
What pragmas are enabled with what options?
I would say stick with warnings and strict for two main reasons.
If other people are going to use or work with your code, they are (almost certainly) used to warnings and strict and their rules. Those represent a community norm that you and other people you work with can count on.
Even if this or that specific piece of code is just for you, you probably don't want to worry about remembering "Is this the project where I adhere to warnings and strict or the one where I hew to common::sense?" Moving back and forth between the two modes will just confuse you.
There is one bit nobody else seems to have picked up on, and that's FATAL in the warnings list.
So as of 2.0, use common::sense is more akin to:
use strict;
use warnings FATAL => 'all'; # but with the specific list of fatals instead of 'all' that is
This is a somewhat important and frequently overlooked feature of warnings that ramps the strictness a whole degree higher. Instead of undef string interpolation, or infinite recursion just warning you and then keeping on going despite the problem, it actually halts.
To me this is helpful, because in many cases, undef string interpolation leads to further more dangerous errors, which may go silently unnoticed, and failing and bailing is a good thing.
I obviously have no common sense because I going more for Modern::Perl ;-)
The "lower memory usage" only works if you use no modules that load strict, feature, warnings, etc. and the "much" part is...not all that much.
Not everyone's idea of common sense is the same - in that respect it's anything but common.
Go with what you know. If you get undef warnings, chances are that your program or its input is incorrect.
Warnings are there for a reason. Anything that reduces them cannot be useful. (I always compile with gcc -Wall too...)
I have never had a warning that wasn't something dodgy/just plain wrong in my code. For me, it's always something technically allowed that I almost certainly don't want to do. I think the full suite of warnings is invaluable. If you find use strict + use warnings adequate for now, I don't see why you'd want to change to using a non-standard module which is then a dependency for every piece of code you write from here on out...
When it comes to warnings, I support the use of any module or built-in language feature that gives you the level of warnings that helps you make your code as solid and reliable as it can possibly be. An ignored warning is not helpful to anyone.
But if you're cozy with the standard warnings, stick with it. Coding to a stricter standard is great if you're used to it! I wouldn't recommend switching just for the memory savings. Only switch if the module helps you turn your code around quicker and with more confidence.
Many of peoples argues in a comments with what if the MP changes, it will break your code. While this can be an real threat, here is already MUCH things what are changes over time and break the code (sometimes after a deprecation cycle, sometimes not...).
Some other modules changed the API, so breaks things, and nobody care about them. E.g. Moose has at least two things what are deprecated now, and probably will be forbidden in some future releases.
Another example, years ago was allowed to write
for $i qw(some words)
now, it is deprecated. And many others... And this is a CORE language syntax.
Everybody survived. So, don't really understand why many of people argues againist helper modules. When they going to change, (probably) here will be a sort of deprecation cycle... So, my view is:
if you write programs to yourself, use any module you want ;)
if you write a program to someone, where someone others going to maintnanece it, use minimal nonstandard "pragma-like" modules (common::sense, modern::perl, uni::perl etc...)
in the stackoverflow questions, you can safely use common::sense or Modern::Perl etc. - most of users who will answer, your questions, knows them. Everybody understand than it is easier to write use 5.010; for enable strict, warnings and fearures with 10 chars as with 3 lines...
When it come to saying what version of Perl we need for our scripts, we've got options, oh, brother, we've got options:
use 5.010;
use 5.010_001;
use 5.10.0;
use v5.10;
use v5.10.0;
All seem to work. perlcritic complains about all but the first two. (It's unfortunate that the v strings seem to have such flaws, since Perl 6 expects you to do use v6; for your Perl 6 scripts...)
So, what should we be doing to indicate that we want to use a particular version of perl?
There are really only two options: decimal numbers and v-strings. Which form to use depends in part on which versions of Perl you want to "support" with a meaningful error message instead of a syntax error. (The v-string syntax was added in Perl 5.6.) The accepted best practice -- which is what perlcritic enforces -- is to use decimal notation. You should specify the minimum version of Perl that's required for your script to behave properly. Normally that means declaring a dependency on language features added in a major release, such as using the say function added in 5.10. You should include the patch level if it's important for your script to behave properly. For example, some of my code specifies use 5.008001 because it depends on the fix for a bug that 5.8.0 had which was fixed in 5.8.1.
I just use something like 5.010_001. I've grow weary of dealing with version string problems for something that should be mind-numbingly simple.
Since I mostly deal with build systems, I have the constant struggle of Module::Build's internal version.pm which is out of sync with the version.pm on CPAN. I think that's mostly better now, but I have better things to think about.
The best practice should always be to do the thing that commands the least of your attention, and certainly not take more attention than the value it gives back. In my opinion, v-strings and dotted decimals were a huge distraction with no additional benefit, wasting a lot of valuable programmer time just to get back to the starting point.
I should also note that Perl::Critic has often pushed questionable practices for the higher purpose of reducing the ways that people do things. However, those practices often cause problems, make them un-best. This is one of those cases. A more realistic best practice is to not make Perl::Critic compliance your goal. Use it where it is useful, but in cases like this, don't waste mental time on it.
The "modern" way is to use the forms starting with v. However, that may not necessarily be what you really want to do.
Critic complains because older versions of Perl won't understand and play nicely with the forms that start with v. However, if your version of Perl supports it, v is nicer to read because you can say:
use v5.10.1;
... rather than ...
use 5.010_001;
So, in the documentation for use, the following workaround is offered:
use 5.006; use v5.6.1;
NB: I think the documenation is in error here, as the v is omitted from the example at perldoc use.
Since the versions of Perl that don't support the v syntax will fail at the first use, they won't get to the second more specific and readable one.