My Common Git Workflow

May 13th, 2010

A recent post that was highly ranked on Hacker News complained about common git workflows causing him serious pain. While I won’t get into the merit of his user experience complaints, I do want to talk about his specific use-case and how I personally work with it in git.

Best I can tell, Mike Taylor (the guy in the post) either tried to figure out a standard git workflow on his own, or he followed poor instructions that tried to bootstrap someone from an svn background while intentionally leaving out important information. In any event, I’ll step through my personal workflow for his scenario, contrasting with subversion as I go.

Cloning the Repository

The very first step when working with a repository is to clone it. In subversion, this is accomplished via svn checkout svn://url/to/repo/trunk. This retrieves the most recent revision of the trunk branch of the repository.

In git, this is accomplished via git clone git://url/to/repo (the http protocol is also possible). This retrieves the entire repository, including other branches and tags.

Making the Change

In both git and subversion, you make the change using a normal text editor.

After Making the Change

In git, you make a local commit, marking the difference between the most recent pulled version (master) and the changes you made. In subversion, the normal workflow does not involve making a change, but apparently some people make manual diffs in order to have a local copy of the changes before updating from the remote. Here’s an example comment from the Hacker News post:

I’ll tell you what happens when I use svn and there’s been an upstream change: I never update my local tree with local modifications. Instead, I extract all my local changes into a diff, then I update my local tree, and then I merge my diff back into the updated tree and commit.

When I need three-way merging, which isn’t often – usually patch can resync simple things like line offsets – it’s handled by a file comparison tool. I have a simple script which handles this

My personal process for making the commit in git almost always involves the gitx GUI, which lets me see the changes for each individual file, select the files (or chunks in the files) to commit, and then commit the whole thing. I sometimes break up the changes into several granular commits, if appropriate.

Updating from the remote

Now that we have our local changes, the next step is to update from the remote. In subversion, you would run svn up. Here, subversion will apply a merge strategy to attempt to merge the remote changes with the local ones that you made. If a merge was unsuccessful, subversion will tell you that a conflict has occurred. If you did not manually save off a diff file, there is no way to get back to the status from before you made the change.

In git, you would run git pull. By default, git applies the “recursive” strategy, which tries to merge your current files with the remote files at the most recent revision. As with subversion, this can result in a conflict. You can also pass the --rebase flag to pull, which is how I usually work. This tells git to stash away your commits, pull the remote changes, and then reapply your changes on top one at a time.

If you use --rebase, you may get a conflict for each of your local commits, which is usually easier to handle than a bunch of conflicts all at once.

I definitely recommend using --rebase which also provides instructions for dealing with conflicts as they arise.

In either case, in my experience, git’s merging capabilities are more advanced than subversion’s. This will result in many fewer cases where conflicts occur.

Resolving Conflicts

From here on, I am assuming you followed my advice and used git pull --rebase.

If a conflict has occurred, you will find that if you run git status, all of the non-conflicting files are already listed as “staged”, while the conflicting files are listed outside the staging area. This means that the non-conflicting files are already considered “added” to the current commit.

To resolve the conflicts, fix up the files listed outside the staging area and git add them. Again, I normally use gitx to move the resolved files into the staging area.

Once you have resolved the conflict, run git rebase --continue. This tells git to use the fixed up changes you just made instead of the original commit it was trying to put on top of the changes you got from the remote.

In subversion, if you got a conflict, subversion will create three files for you: file.mine, file.rOLD, and file.rNEW. You are responsible for fixing up the conflicts and getting back a working file. Once you are done, you run svn resolved.

NOTE: If you had not used git pull --rebase, but instead did raw git pull, you would fix up the files, add the files using git add or gitx, and the run git commit to seal the deal

Yikes! Something went wrong!

In git, if something goes wrong, you just run git reset --hard, which will bring you back to your last local commit.

In subversion, it’s not always possible unless you manually stored off a diff before you started.

Pushing

Now that you’re in sync with the remote server, you push your changes. In git, you run git push. In subversion, you run svn commit.

One Glossed-Over Difference

Subversion allows you to commit changes even if you haven’t svn uped and there have been changes to the remote, as long as there are no conflicts between your local files and the remote files.

Git never allows you to push changes to the remote if there have been remote changes. I personally prefer the git behavior, but I could see why someone might prefer the subversion behavior. However, I glossed over this difference because every subversion reference I’ve found advises running svn up before a commit, and I personally always did that in my years using subversion.

Comparison

Here’s a workflow comparison between git and subversion:

Note that I am not attempting to provide an exhaustive guide to git here; there are many more git features that are quite useful. Additionally, I personally do a lot of local branching, and prefer to be able to think about git in terms of cheap branches, but the original poster explicitly said that he’d rather not. As a result, I didn’t address that here.

I also don’t believe that thinking of git in terms of subversion is a good idea. That said, the point of this post (and the point of the original poster) is that there are a set of high-level version control operations that you’d expect git to be able to handle in simple cases without a lot of fuss.

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The How and Why of Bundler Groups

May 9th, 2010

Since version 0.9, Bundler has had a feature called “groups”. The purpose of this feature is to allow you to specify groups of dependencies which may be used in certain situations, but not in others.

For instance, you may use ActiveMerchant only in production. In this case, you could say:

group :production do
  gem "activemerchant"
end

Specifying groups allows you to do two things. First, you can install the gems in your Gemfile, minus specific groups. For instance, Rails puts mysql and pg in a database group so that if you’re just working on ActionPack, you can bundle install --without db and run the ActionPack tests without having to worry about getting the gems installed.

Second, you can list specific groups to autorequire using Bundler.require. By default, Bundler.require requires all the gems in the default group (which is all the gems that have no explicit group). You can also say Bundler.require(:default, :another_group) to require specific groups.

Note the difference between these operations: bundle install is opt-out, while Bundler.require is opt-in. This is because the common usage of groups is to specify gems for different environments (such as development, test and production) and you shouldn’t need to specify that you want the “development” and “test” gems just to get up and running. On the other hand, you don’t want your test dependencies loaded in development or production.

It is also worth noting that all gems that you installed (i.e. not the ones that you excluded at install time with --without) will be available to require. This has no effect unless you actually require them. This means that in development mode, if you explicitly require rspec, it will work.

Rails 3 defaults to mapping groups to environment names, and explicitly autorequiring the implicit default group and the group named the same as the current environment. For example, in development mode, Rails will require the default group and the development group. The code that does this is in your application.rb:

Bundler.require(:default, Rails.env) if defined?(Bundler)

Consistency

In order to ensure consistency across all environments, bundler resolves the dependencies of your application using the gems listed in all groups, even if you specify --without. This means that while you can skip installing the gems listed in the production group by saying --without production, bundler will still download and examine the gems in order to properly resolve all dependencies.

As a result, the dependencies you install in development mode and test with will be compatible with the gems in other environments. In essence, this policy ensures that if your tests pass and run in development, your app will not fail to run in production because the dependencies resolved differently.

Multiple Inconsistent Configurations

Sometimes, especially when developing gems for wider use, you want to test your code against multiple incompatible configurations. At first glance, you might think that you could use groups for this case, but as described above, groups are designed for cases where all of the gems are compatible, but you don’t always want to have to install them in all situations.

Instead, use multiple Gemfiles, one for each incompatible configuration. When installing, do bundle install --gemfile Gemfile.rails2. This will tell Bundler to use Gemfile.rails2 rather than the default Gemfile. As in all cases in Bundler, you can also specify this option globally with an environment variable (BUNDLE_GEMFILE).

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Ruby 1.9 Encodings: A Primer and the Solution for Rails

May 5th, 2010

UPDATE: The DataObjects drivers, which are used in DataMapper, are now updated to honor default_internal. Let’s keep this moving.

Since Ruby 1.9 announced support for encodings, there has been a flurry of activity to make existing libraries encoding aware, and a tornado of confusion as users of Ruby and Rails have tried to make sense of it.

In this post, I will lay out the most common problems people have had, and what we can do as a community to put these issues to bed in time for Ruby 1.9.2 final.

A Quick Tour

I’m going to simplify some of this, but the broad strokes are essentially correct.

Before we begin, many of you are probably wonder what exactly an “encoding” is. For me, getting a handle on this was an important part of helping me understand the possible solution space.

On disk, all Strings are stored as a sequence of bytes. An encoding simply specifies how to take those bytes and convert them into “codepoints”. In some languages, such as English, a “codepoint” is exactly equivalent to “a character”. In most other languages, there is not a one-to-one correspondence. For example, a German codepoint might specify that the next codepoint should get an ümlaut.

The list of English characters represented by the first seven bits of ASCII (characters 0 through 127 in “ASCII-7″) have the same representation in many (but not all) encodings. This means that if you only use English characters, the on-disk representation of the characters will often be exactly the same regardless of the source encoding.

However, once you start to use other characters, the bytes on disk mean different things in different encodings. Have you ever seen a page on the Internet filled with something like “Führer”? That is the consequence of the bytes of “Führer” stored as UTF-8 being interpreted as Latin-1.

You can trivially see this problem using Ruby 1.9′s encoding support by running this program:

# encoding: UTF-8
 
puts "hello ümlaut".force_encoding("ISO-8859-1").encode("UTF-8")
 
# Output
# hello ümlat

First, we create a String (“hello ümlaut”) in the UTF-8 encoding. Next, we tell Ruby that the String is actually Latin-1. It’s not, so an attempt to read the characters will interpret the raw bytes of the “ü” as though they were Latin-1 bytes. We ask Ruby to give us that interpretation of the data in UTF-8 via encode and print it out.

We can see that while the bytes for “hello ” and “mlat” were identical in both UTF-8 and Latin-1, the bytes for “ü” in UTF-8 mean “ü” in Latin-1.

Note that while force_encoding simply tags the String with a different encoding, encode converts the bytes of one encoding into the equivalent bytes of the second. As a result, while force_encoding should almost never be used unless you know for sure that the bytes actually represent the characters you want in the target encoding, encode is relatively safe to use to convert a String into the encoding you want.

You’ve probably also seen the reverse problem, where bytes encoded in Latin-1 ended up inside a page encoded in UTF-8.

# encoding: ISO-8859-1
 
puts "hello ümlaut".force_encoding("UTF-8")
 
# Output
# hello ?mlat

Here, the sequence of bytes that represents an “ü” in Latin-1 could not be recognized in UTF-8, so they were replaced with a “?”. Note that puts will always simply write out the bytes to your terminal, and the terminal’s encoding will determine how they are interpreted. The examples in this post are all outputted to a terminal using UTF-8 encoding.

As you can imagine, this presents quite the issue when concatenating two Strings of different encodings. Simply smashing together the raw bytes of the two Strings can result in output that is incomprehensible in either encoding. To make matters worse, it’s not always possible to represent all of the characters in one encoding in another. For instance, the characters of the Emoji encoding cannot be represented in the ISO-8859-1 encoding (or even in a standardized way onto the UTF-8 encoding).

As a result, when you attempt to concatenate two Strings of different encodings in Ruby 1.9, Ruby displays an error.

# encoding: UTF-8
 
puts "hello ümlaut".encode("ISO-8859-1") + "hello ümlaut"
 
# Output
# incompatible character encodings: ISO-8859-1 and UTF-8 (Encoding::CompatibilityError)

Because it’s extremely tricky for Ruby to be sure that it can make a lossless conversion from one encoding to another (Ruby supports almost 100 different encodings), the Ruby core team has decided to raise an exception if two Strings in different encodings are concatenated together.

There is one exception to this rule. If the bytes in one of the two Strings are all under 127 (and therefore valid characters in ASCII-7), and both encodings are compatible with ASCII-7 (meaning that the bytes of ASCII-7 represent exactly the same characters in the other encoding), Ruby will make the conversion without complaining.

# encoding: UTF-8
 
puts "hello umlat".encode("ISO-8859-1") + "hello ümlaut"
 
# Output
# hello umlathello ümlaut

Since Ruby does not allow characters outside of the ASCII-7 range in source files without a declared encoding, this exception eliminates a large number of potential problems that Ruby’s strict concatenation rules might have introduced.

Binary Strings

By default, Strings with no encoding in Ruby are tagged with the ASCII-8BIT encoding, which is an alias for BINARY. Essentially, this is an encoding that simply means “raw bytes here”.

In general, code in Rails applications should not encounter BINARY strings, except for Strings created in source files without encodings. However, since these Strings will virtually always fall under the ASCII-7 exception, Ruby programmers should never have to deal with incompatible encoding exceptions where one of the two encodings is ASCII-8BIT (i.e. BINARY).

That said, almost all of the encoding problems reported by users in the Rails bug tracker involved ASCII-8BIT Strings. How did this happen?

There are two reasons for this.

The first reason is that early on, database drivers generally didn’t properly tag Strings they retrieved from the database with the proper encoding. This involves a manual mapping from the database’s encoding names to Ruby’s encoding names. As a result, it was extremely common from database drivers to return Strings with characters outside of the ASCII-7 range (because the original content was encoded in the database as UTF-8 or ISO-8859-1/Latin-1).

When attempting to concatenate that content onto another UTF-8 string (such as the buffer in an ERB template), Ruby would raise an incompatible encoding exception.

# encoding: UTF-8
 
puts "hello ümlaut" + "hello ümlaut".force_encoding("BINARY")
 
# Output
# incompatible character encodings: UTF-8 and ASCII-8BIT (Encoding::CompatibilityError)

This is essentially identical to the scenario many people encountered. A UTF-8 String was presented to Ruby as a BINARY String, since the database driver didn’t tag it. When attempting to concatenate it onto UTF-8, Ruby had no way to do so reliably, so it raised an exception.

One reason that many people didn’t encounter this problem was that either the contents of the template or the text from the database were entirely in the ASCII-7 subset of their character set. As a result, Ruby would not complain. This is deceptive, because if they made a small change to their template, or if a user happened to enter non-ASCII-7 data (for instance, they got their first user named José), they would suddenly start seeing an incompatible encoding exception.

When people see this incompatible encoding exception, one common reaction is to call force_encoding("UTF-8") on the BINARY data. This will work great for Strings whose bytes actually are encoded in UTF-8. However, if people whose Strings were encoded in ISO-8859-5 (Russian) followed this instruction, they would end up with scrambled output.

Additionally, it’s impossible to simply encode the data, since Ruby doesn’t actually know the source encoding. In essence, a crucial piece of information has been lost at the database driver level.

Unfortunately, this means that well-meaning people who have solved their problem by force_encoding their Strings to UTF-8 (because the bytes actually did represent UTF-8 characters) become baffled when their solution doesn’t work for someone working on a Russian website.

Thankfully, this situation is now mostly solved. There are updates for all database drivers that map the encodings from the database to a Ruby encoding, which means that UTF-8 text from the database will be UTF-8 Strings in Ruby, and Latin-1 text from the database will be ISO-8859-1 Strings in Ruby.

Unfortunately, there is a second large source of BINARY Strings in Ruby. Specifically, data received from the web in the form of URL encoded POST bodies often do not specify the content-type of the content sent from forms.

In many cases, browsers send POST bodies in the encoding of the original document, but not always. In addition, some browsers say that they’re sending content as ISO-8859-1 but actually send it in Windows-1251. There is a long thread on the Rack tracker about this, but the bottom line is that it’s extremely difficult to determine the encoding of a POST body sent from the web.

As a result, Rack handlers send the raw bytes through as BINARY (which is reasonable, since handlers shouldn’t be in the business of trying to wade through this problem) and no middleware exists (yet) to properly tag the String with the correct encoding.

This means that if the stars align, the raw bytes are UTF-8, end up in a UTF-8 database, and end up coming back out again tagged as UTF-8. If the stars do not align, the text might actually be encoded in ISO-8859-1, get put into a UTF-8 database, and come out tagged as UTF-8 (and we know what happens when ISO-8859-1 data is mistakenly tagged as UTF-8).

In this case, because the ISO-8859-1 data is improperly tagged as UTF-8, Ruby happily concatenates it with other UTF-8 Strings, and hilarity ensues.

Because English characters have the same byte representation in all commonly used encodings, this problem is not as common as you might imagine. Unfortunately, this simply means that people who do encounter it are baffled and find it hard to get help. Additionally, this problem doesn’t manifest itself as a hard error. it can go unnoticed and dismissed as a minor annoyance if the number of non-ASCII-7 characters are low.

In order to properly solve this problem for Ruby 1.9, we need a very good heuristic for properly determining the encoding of web-sent POST bodies. There are some promising avenues that will get it right 99.9% of the time, and we need to package them into up a middleware that will tag Strings correctly.

Incompatible Encodings

If you’ve been paying attention, you’ve probably noticed that while the database drivers have solved one problem, they actually introduced another one.

Imagine that you’re using a MySQL database encoded in ISO-8859-1 (or ISO-8859-5, popular for Russian applications, or any other non-UTF-8 encoding). Now that the String coming back from the database is properly tagged as ISO-8859-1, Ruby will refuse to concatenate it onto the ERB buffer (which is encoded in UTF-8). Even if we solved this problem for ERB, it could be trivially reintroduced in other parts of the application through regular concatenation (+, concat, or even String interpolation).

Again, this problem is somewhat mitigated due to the ASCII-7 subset exception, which means that as long as one of the two incompatible Strings uses only English characters, users won’t see any problems. Again, because this “solution” means that the Ruby developer in question still may not understand encodings, this simply defers the problem to some uncertain point in the future when they either add a non-ASCII-7 character to their template or the user submits a non-ASCII-7 String.

The Solution

If you got this far, you’re probably thinking “Holy shit this encoding stuff is crazy. I don’t want to have to know any of this! I just want to write my web app!”

And you’d be correct.

Other languages, such as Java and Python, solve this problem by encodeing every String that enters the language as UTF-8 (or UTF-16). Theoretically, it is possible to represent the characters of every encoding in UTF-8. By doing this, programmers only ever deal with one kind of String, and concatenation happens between UTF-8 Strings.

However, this solution does not work very well for the Japanese community. For a variety of complicated reasons, Japanese encoding, such as SHIFT-JIS, are not considered to losslessly encode into UTF-8. As a result, Ruby has a policy of not attempting to simply encode any inbound String into UTF-8.

This decision is debatable, but the fact is that if Ruby transparently transcoded all content into UTF-8, a large portion of the Ruby community would see invisible lossy changes to their content. That part of the community is willing to put up with incompatible encoding exceptions because properly handling the encodings they regularly deal with is a somewhat manual process.

On the other hand, many Rails applications work mostly with encodings that trivially encode to UTF-8 (such as UTF-8 itself, ASCII, and the ISO-8859-1 family). For this rather large part of the community, having to manually encode Strings to solve incompatible encoding problem feels like a burden that belongs on the machine has been inappropriately shifted onto Rails application developers.

But there is a solution.

By default, Ruby should continue to support Strings of many different encodings, and raise exceptions liberally when a developer attempts to concatenate Strings of different encodings. This would satisfy those with encoding concerns that require manual resolution.

Additionally, you would be able to set a preferred encoding. This would inform drivers at the boundary (such as database drivers) that you would like them to convert any Strings that they tag with an encoding to your preferred encoding immediately. By default, Rails would set this to UTF-8, so Strings that you get back from the database or other external source would always be in UTF-8.

If a String at the boundary could not be converted (for instance, if you set ISO-8859-1 as the preferred encoding, this would happen a lot), you would get an exception as soon as that String entered the system.

In practice, almost all usage of this setting would be to specify UTF-8 as a preferred encoding. From your perspective, if you were dealing in UTF-8, ISO-8859-* and ASCII (most Western developers), you would never have to care about encodings.

Even better, Ruby already has a mechanism that is mostly designed for this purpose. In Ruby 1.9, setting Encoding.default_internal tells Ruby to encode all Strings crossing the barrier via its IO system into that preferred encoding. All we’d need, then, is for maintainers of database drivers to honor this convention as well.

It doesn’t require any changes to Ruby itself, and places the burden squarely on the few people who already need to deal with encodings (because taking data from outside of Ruby, via C, always already requires a tagging step). I have spoken with Aaron Patterson, who has been working on the SQLite3 driver, and he feels that this change is simple enough for maintainers of drivers dealing with external Strings to make it a viable option. He has already patched SQLite3 to make it respect default_internal.

However you feel about Ruby’s solution to expose String encodings directly in the language, you should agree that since we’re stuck with it for the forseeable future, this solution shifts the burden of dealing with it from the unwashed masses (most of whom have no idea what an encoding is) to a few maintainers of C extensions and libraries that deal in binary data. Getting this right as soon as possible will substantially ease the transition from Ruby 1.8 to Ruby 1.9.

Postscript: What Happened in 1.8!?

When people first move to 1.9 and encounter these shenanigans, they often wonder why everything seemed so simple in Ruby 1.8, and yet seemed to work.

There are a few reasons for this.

First, keep in mind that in Ruby 1.8, Strings are simple sequences of bytes. Ruby String operations just concatenate those byte sequences together without any kind of check. This means that concatenating two UTF-8 Strings together will just work, since the combined byte sequence is still valid UTF-8. As long as the client for the Ruby code (such as the browser) is told that the bytes are encoded in UTF-8, all is well. Rails does this by setting the default charset for all documents to UTF-8.

Second, Ruby 1.8 has a “UTF-8″ mode that makes its regular expression engine treat all Strings as UTF-8. In this mode (which is triggered by setting $KCODE = “UTF-8″), the regular expression engine correctly matches a complete UTF-8 character for /./, for instance. Rails sets this global by default, so if you were using Rails, regular expressions respect unicode characters, not raw bytes.

Third, very little non-English content in the wild is actually encoded in ISO-8859-1. If you were expecting to deal with content that was not English, you would probably set your MySQL database to use a UTF-8 encoding. Since Rails sets UTF-8 as the charset of outbound documents, most browsers will in fact return UTF-8 encoded data.

Fourth, the problems caused when an ISO-8859-1 String is accidentally concatenated into a UTF-8 String are not as jarring as the errors produced by Ruby 1.9. Let’s try a little experiment. First, open up a text editor, create a new file, and save it in the ISO-8859-1 encoding.

$KCODE = "UTF-8"
 
iso_8859_1 = "ümlaut"
 
# the byte representation of ümlaut in unicode
utf8 = "\xC3\xBCmlat"
 
puts iso_8859_1
puts utf8
 
puts iso_8859_1 + utf8
puts utf8 + iso_8859_1
 
# Output
# ?mlat
# ümlaut
# ?mlatümlaut
# ümlaut?mlat

If you somehow get ISO-8859-1 encoded content that uses characters outside of the ASCII-7 range, Ruby doesn’t puke. Instead, it simply replaces the unidentified character with a “?”, which can easily go unnoticed in a mostly English site with a few “José”s thrown into the mix. It could also easily be dismissed as a “weird bug that we don’t have time to figure out right now”.

Finally, Rails itself provides a pure-Ruby UTF-8 library that mops up a lot of the remaining issues. Specifically, it provides an alternate String class that can properly handle operations like split, truncate, index, justify and other operations that need to operate on characters, not bytes. It then uses this library internally in helpers like truncate, transparently avoiding a whole other class of issue.

In short, if you’re dealing mostly with English text, and you get unlucky enough the get ISO-8859-1 input from somewhere, the worst case is that you get a “?” instead of a “é”. If you’re dealing with a lot of non-English text, you’re probably being not using ISO-8859-1 sources. In either case, English (ASCII) text is compatible with UTF-8, and Rails provides solid enough pure-Ruby UTF-8 support to get you most of the rest of the way.

That said, anyone dealing with encodings other than UTF-8 and ISO-8859-1 (Japanese and Russian Rubyists) were definitely not in a good place with Ruby 1.8.

Thanks

I want to personally thank Jay Freeman (aka saurik), who in addition to being a general badass, spent about 15 hours with me patiently explaining these issues and working through the Ruby 1.9 source to help fully understand the tradeoffs available.

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The Web Doesn’t Suck. Browsers Are Innovating.

April 30th, 2010

This week saw a flurry of back-and-forth about the future of the web platform. In the “web sucks” camp were Sachin Agarwal of Posterous (The Web Sucks. Browsers need to innovate) and Joe Hewitt (Sachin summarized some of his tweets at @joehewitt agrees with me).

Chris Blizzard responded with a few narrow examples of what Firefox has been doing lately (geolocation, orientation, WebGL).

On Hacker News, most of the comments took Sachin to task for his argument that standards don’t matter, pointing people back to the “bad old days” of the browser wars.

In my opinion, both camps kind of miss the point.

Sachin made some very pointed factual claims which are just complete, pure hokum. Close to the top of his post, he says:

Web applications don’t have threading, GPU acceleration, drag and drop, copy and paste of rich media, true offline access, or persistence. Are you kidding me?

Really? Are you kidding me. In fact, browsers have implemented, and the WHAT-WG has been busily standardizing, all of these things for quite a number of years now. Threading? Check. GPU acceleration? Check and check. Drag and drop? Check. Offline access and persistence? Check and check.

Almost universally, these features, which exist in shipping browsers today, were based on experiments conducted years ago by browsers (notably Firefox and Safari, more recently Chrome), and did not wait for approval from the standards bodies to begin implementing.

In fact, large swaths of HTML5 are based on proposals from browsers, who have either already implemented the feature (CSS-based animations and transitions) or who then build the feature without waiting for final approval. And this is transparently obvious to anyone, since HTML5 has not not yet been approved, and some parts are the subject of internal W3C debate, yet the browsers are implementing the parts they like and innovating in other areas.

You can see this explicitly in the features prefixed with -moz or -webkit, because they’re going ahead and implementing features that have not gotten consensus yet.

In 2010, the WHAT-WG is a functioning place to bring a proposal for further review once you’re conducting some experiments or have a working implementation. Nobody is sitting on their hands waiting for final approval of HTML5. Don’t confuse the fact that people are submitting their ideas to the standards bodies with the misguided idea that only approved standards are being implemented.

And in fact, this is basically never how it’s worked. Apple implemented the <canvas> tag and <input type="search" /> in 2004 (here’s a 2004 rant from the editor of the then-brand-new HTML5 spec). Opera and Apple worked on the <video> tag in 2007. The new CSS flexible box model is based on XUL, which Firefox has had for over a decade. HTML5 drag and drop support is based on implementations shipped by the IE team over a decade ago. Firefox has been extending JavaScript for almost its entire history (most recently with JavaScript 1.8).

CSS transition, transforms and animations were built by Apple for the iPhone before they were a spec, and only later ported to the desktop. Firefox did the initial experimentation on WebGL in 2006, back when they were calling it Canvas 3d (here‘s a working add-on by Mozilla developer Vladimir Vukićević).

Google Gears shipped what would become the Application Cache, Web Storage, and Web Workers, proving out the concepts. It also shipped geolocation, which is now part of the HTML5 spec.

@font-face originally shipped in Internet Explorer 5.5. Apple has added new mobile events (touchstart, touchmove, touchend, onorientationchange) with accompanying JavaScript APIs (Apple developer account needed) without any spec activity that I can discern. Firefox, at the same time, added support for hardware accelerometers.

Even Microsoft bucked the standards bodies by shipping its own implementation of the W3C’s “cross-origin-resource-sharing” spec called Cross domain request, and shipped it in IE8.

It’s perfectly understandable that people who haven’t been following along for the past few years might have missed all of this. But the fact remains that browser vendors have been moving at a very rapid clip, implementing all kinds of interesting features, and then sharing them with other browsers for feedback, and, one hopes, consensus.

Some of these implementations suck (I’ve heard some people say not-nice things about WebGL and drag-and-drop, for instance). But that’s quite a bit different from saying that browsers are sitting on their hands waiting for the W3C or WHAT-WG to tell them what to do.

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Named Gem Environments and Bundler

April 21st, 2010

In the beginning, Rubygems made a decision to allow multiple versions of individual gems in the system repository of gems. This allowed people to use whatever versions of gems they needed for individual scripts, without having to partition the gems for specific purposes.

This was a nice starting place. Being able to just install whatever gems into one place and have scripts just work, without having to partition your gems into buckets made Rubygems an extremely pleasant tool to work with. In my opinion, it was a good decision.

As the ecosystem has matured, and more people build gems with dependencies (especially loose dependencies, like “rack”, “>= 1.0″), this original sin has introduced the dreaded activation error:

can't activate rspec-rails (= 1.3.2, runtime) for [], 
already activated rspec-rails-2.0.0.beta.6

This error occurs because of the linear way that gems are “activated”. For instance, consider the following scenario:

# On your system:
thin (1.2.7)
- daemons (>= 1.0.9)
- eventmachine (>= 0.12.6)
- rack (>= 1.0.0)

actionpack (2.3.5)
- activesupport (= 2.3.5)
- rack (~> 1.0.0)

rack (1.0.0)
rack (1.1.0)
activesupport (2.3.5)
daemons (1.0.9)
eventmachine (0.12.6)

Quickly glancing at these dependencies, you can see that these two gems are “compatible”. The gems have the rack dependency in common, and the >= 1.0.0 is compatible with ~> 1.0.0.

However, there are two ways to require these gems. First, you could require actionpack first.

require "action_pack"
# will result in "activating" ActiveSupport 2.3.5 and Rack 1.0.0
 
require "thin"
# will notice that Rack 1.0.0 is already activated, and satisfies
# the >= 1.0.0 requirement, so just activates daemons and eventmachine

Second, you could require thin first.

require "thin"
# will result in "activating" Rack 1.1.0, Daemons 1.0.9, 
# and EventMachine 0.12.6
 
require "action_pack"
# will notice that Rack 1.1.0 is already activated, but that
# it is incompatible with ~> 1.0.0. Therefore, it will emit:
# ---
# can't activate rack (~> 1.0.0, runtime) for ["actionpack-2.3.5"], 
# already activated rack-1.1.0 for ["thin-1.2.7"]

In this case, because thin pulled in Rack 1.1, it becomes impossible to load in actionpack, despite the fact that a potentially valid combination exists.

This problem is fundamental to the approach of supporting different versions of the same gem in the system and activating gems linearly. In other words, because no single entity ever has the opportunity to examine the entire list of dependencies, the onus is on the user to make sure that the gems requires are ordered correctly. Sometimes, it means that the user must explicitly require the right version of a child dependency just to make sure that the right versions are loaded.

There are two possible solutions to this problem.

Multiple Named Environments

One solution to this problem is to catch potential conflicts at installation time and ask the user to manage multiple named environments, each with a fully consistent, non-conflicting view of the world.

The best way to implement this looks something like this (I’ll use a fictitious gemenv command to illustrate):

$ gemenv install thin
- Installing daemons (1.0.10)
- Installing eventmachine (0.12.10)
- Installing rack (1.1.0)
- Installing thin (1.2.7)

$ gemenv install actionpack -v 2.3.5
- Uninstalling rack (1.1.0)
- Installing rack (1.0.1)
- Installing actionpack (2.3.5)

$ gemenv install rack -v 1.1.0
- rack (1.1.0) conflicts with actionpack (2.3.5)
- if you want rack (1.1.0) and actionpack (2.3.5)
  you will need a new environment.

$ gemenv create rails3
$ gemenv install actionpack -v 3.0.0.beta3
- Installing abstract (1.0.0)
- Installing builder (2.1.2)
- Installing i18n (0.3.7)
- Installing memcache-client (1.8.2)
- Installing tzinfo (0.3.20)
- Installing activesupport (3.0.0.beta3)
- Installing activemodel (3.0.0.beta3)
- Installing erubis (2.6.5)
- Installing rack (1.1.0)
- Installing rack-mount (0.6.3)
- Installing rack-test (0.5.3)
- Installing actionpack (3.0.0.beta3)

$ gemenv use default
$ ruby -e "puts Rack::VERSION"
1.0.1
$ gemenv use rails3
$ ruby -e "puts Rack::VERSION"
1.1.0

Essentially, the single entity with full knowledge of all dependencies is the installer, and the user creates as many environments as he or she needs for the various non-conflicting sets of gems in use.

This works nicely, because it guarantees that once using an environment, all gems available are compatible. Note that the above command is fictitious, but it bears similarity to rip.

Virtual, Anonymous Environments

Another solution, the one bundler uses, is to allow multiple, conflicting versions of gems to exist in the system repository of packages, but to ensure a lack of conflicts by resolving the dependencies used by individual applications.

First, install conflicting gems. In this case, Rails 2.3 and Rails 3.0 require different, incompatible versions of Rack (1.0.x and 1.1.x).

$ gem install rails
... output ...
$ gem install rails -v 3.0.0.beta3
... output ...

Next, specify which version of Rails to use in your application’s Gemfile:

gem "rails", "2.3.5"

When running script/server in an app using Bundler, Bundler can determine that Rails needs Rack 1.0.x, and pulls in Rack 1.0.1 from the system. If you had specified gem "rails", "3.0.0.beta3", bundler would have pulled in Rack 1.1.0 from the system.

In essence, we have the same kind of isolation as the fictitious command above, but instead of manually managing named environments, Bundler creates virtual isolated environments based on the list of gems used in your application.

Why Did We Use Virtual, Anonymous Environments?

When considering the tradeoffs between these two solutions, we realized that applications already typically have a list (executable or not) of its dependencies. The gem install command already works great for installing dependencies, and introducing a dependency resolution step there feels awkward and out of place.

Additionally, as an application evolves, it’s natural to continue updating its list of gems, keeping a record of the changes as you go. You could keep a separate named environment for each application, but you’d probably want to keep a list of the dependencies in the application anyway so that it’s possible to get up and running on a second machine.

In short, since a list of an application’s dependencies makes sense anyway, why burden the end-user with the need to maintain separate named environments and manually install gems.

And once we’re doing this, why not build a toolchain around installing gems and keeping records of not only the top-level installed gems, but the exact versions of all gems used at a given time? Why not build in support for “edge” gems on your local machine or in git repositories? Why not create workflows for sharing your application with other developers and deploying your application?

As application and gem developers ourselves, we wanted a tool that managed application’s dependencies across the lifecycle of an application, in the context of an application.

That said, there is absolutely room for experimentation in this space. Tools that enforce consistency at install time might be extremely appropriate for managing the gems that you use in scripts, but which you don’t share with other machines. Context matters.

Postscript

We commonly hear something to the effect of “but why add all this complexity? Without all these features, bundler could be so much smaller!”. The truth is that the bundler code itself is under 2,000 lines of code, and hasn’t grown a whole lot in the past few major revisions.

The new rpg tool, recently released by the venerable Ryan Tomayko, is also in that range. The original rip (currently in the “classic” branch) was also in that ballpark. The new rip (the current master branch), may well demonstrate that a fully-featured dependency management system for Ruby can be written in many fewer lines of code. If so, I’m excited to see the abstractions that they use to make it so.

But the bottom line is that all of the new package management solutions have done a good job of packing features into a small number of new lines of code, bundler included. By starting with good abstractions, it’s often possible to add rich new features without having to add a whole lot of new code (or even by deleting code!). We’ve certainly found that in our journey with bundler.

Posted in Other | 7 Comments »

Ruby Require Order Problems

April 17th, 2010

Bundler has inadvertantly exposed a number of require order issues in existing gems. I figured I’d take the opportunity to talk about them. There are basically two kinds of gem ordering issues:

Missing Requires

Imagine a gem that uses nokogiri, but never requires it. Instead, it assumes that something that is required before it will do the requiring. This happens relatively often with Rails plugins (which were previously able to assume that they were loaded at a particular time and place).

A well-known example of this problem is gems that didn’t require “yaml” because Rubygems required it. When Rubygems removed its require in 1.3.6, a number of gems broke. In general, bundler assumes that gems require what they need. If gems do so, this class of ordering problems is eliminated.

Constant Definition as API

This is where a gem checked for defined?(SomeConstant) to decide whether to activate some functionality.

This is a bit tricky. Essentially, the gem is saying that the API for using it is “require some other gem before me”. Personally, I consider that to be a problematic API, because it’s very implicit, and can be hard to track down exactly who did what require.

A better solution is to provide an explicit hook for people to activate the optional functionality. For instance, Haml
provides: Haml.init_rails(binding) which you can run after activating Haml.

This is slightly more manual than some would like, but the API of “make sure you require the optional dependencies before me” is also manual, and more error-prone.

Even if Bundler “respected require order”, which we plan to do (in some way) in 0.10, it’s still up to the user of the gem to ensure that they listed the optional gem above the required gem. This is not ideal.

A workaround that works great in Bundler 0.9 is to simply require the order-dependent gems above Bundler.require. We do this in Rails, so that gems can test for the existence of the Rails constant to decide whether to add optional Rails dependencies.

require "rails/all" 
Bundler.require(:default, Rails.env)

In the case of shoulda and mocha, a better solution could be:

# Gemfile 
group :test do 
  gem "shoulda" 
  gem "mocha" 
  # possible other gems where order doesn't matter 
end
 
# application.rb 
Bundler.require(:default) 
Bundler.require(Rails.env) unless Rails.env.test? 
 
# test_helper.rb 
# Since the order matters, require these gems manually, in the right order 
require "shoulda" 
Bundler.require(:test)

In my opinion, if you have gems that specifically depend on other gems, it is appropriate to manually require them first before automatically requiring things using Bundler.require. You should treat Bundler.require as a shortcut
for listing out a stack of requires.

Possible solutions?

One long-term solution, if we get gem metadata in Rubygems 1.4 (or optional dependencies down the line), would be to specify that a gem has optional dependencies on another gem and specify a file to run both gems are
available.

For instance, the Haml gem could say:

# gemspec 
s.integrates_with "rails", "~> 3.0.0.beta2", "haml/rails" 
 
# haml/rails.rb 
require "haml" 
require "rails" 
Haml.init_rails(binding)

Bundler would handle this in Bundler.require (or possibly Bundler.setup).

If this feature existed in Rubygems, it would work via gem activation. If the Haml gem was activated after the Rails gem, it would require “haml/rails” immediately.

If the Haml gem was activated otherwise, it wouldn’t do anything until the Rails gem was activated. When the Rails gem was activated, it would require the file.

Posted in Other | 15 Comments »

Using .gemspecs as Intended

April 2nd, 2010

When you clone a repository containing a Unix tool (or download a tarball), there’s a standard way to install it. This is expected to work without any other dependencies, on all machines where the tool is supported.

$ autoconf
$ ./configure
$ make
$ sudo make install

This provides a standard way to download, build and install Unix tools. In Ruby, we have a similar (little-known) standard:

$ gem build gem_name.gemspec
$ gem install gem_name-version.gem

If you opt-into this convention, not only will it simplify the install process for your users, but it will make it possible for bundler (and other future automated tools) to build and install your gem (including binaries, proper load path handling and compilation of C extensions) from a local path or git repository.

What to Do

Create a .gemspec in the root of your repository and check it in.

Feel free to use dynamic code in here. When your gem is built, Rubygems will run that code and create a static representation. This means it’s fine to pull your gem’s version or other shared details out of your library itself. Do not, however, use other libraries or dependencies.

You can also use Dir[] in your .gemspec to get a list of files (and remove files you don’t want with -; see the example below).

Here’s bundler’s .gemspec:

# -*- encoding: utf-8 -*-
lib = File.expand_path('../lib/', __FILE__)
$:.unshift lib unless $:.include?(lib)
 
require 'bundler/version'
 
Gem::Specification.new do |s|
  s.name        = "bundler"
  s.version     = Bundler::VERSION
  s.platform    = Gem::Platform::RUBY
  s.authors     = ["Carl Lerche", "Yehuda Katz", "André Arko"]
  s.email       = ["carlhuda@engineyard.com"]
  s.homepage    = "http://github.com/carlhuda/bundler"
  s.summary     = "The best way to manage your application's dependencies"
  s.description = "Bundler manages an application's dependencies through its entire life, across many machines, systematically and repeatably"
 
  s.required_rubygems_version = ">= 1.3.6"
  s.rubyforge_project         = "bundler"
 
  s.add_development_dependency "rspec"
 
  s.files        = Dir.glob("{bin,lib}/**/*") + %w(LICENSE README.md ROADMAP.md CHANGELOG.md)
  s.executables  = ['bundle']
  s.require_path = 'lib'
end

If you didn’t already know this, the DSL for gem specifications is already pretty clean and straight-forward, there is no need to generate your gemspec using alternative tools.

Your gemspec should run standalone, ideally with no additional dependencies. You can assume its __FILE__ is located in the root of your project.

When it comes time to build your gem, use gem build.

$ gem build bundler.gemspec

This will spit out a .gem file properly named with a static version of the gem specification inside, having resolved things like Dir.glob calls and the version you might have pulled in from your library.

Next, you can push your gem to Rubygems.org quickly and painlessly:

$ gem push bundler-0.9.15.gem

If you’ve already provided credentials, you’ve now published your gem. If not, you will be asked for your credentials (once per machine).

You can easily automate this process using Rake:

$LOAD_PATH.unshift File.expand_path("../lib", __FILE__)
require "bundler/version"
 
task :build do
  system "gem build bundler.gemspec"
end
 
task :release => :build do
  system "gem push bundler-#{Bunder::VERSION}"
end

Using tools that are built into Ruby and Rubygems creates a more streamlined, conventional experience for all involved. Instead of trying to figure out what command to run to create a gem, expect to be able to run gem build mygem.gemspec.

A nice side-effect of this is that those who check in valid .gemspec files can take advantage of tools like bundler that allow git repositories to stand in for gems. By using the gem build convention, bundler is able to generate binaries and compile C extensions from local paths or git repositories in a conventional, repeatable way.

Try it. You’ll like it.

Posted in Other | 33 Comments »

Ruby’s Implementation Does Not Define its Semantics

February 25th, 2010

When I was first getting started with Ruby, I heard a lot of talk about blocks, and how you could “cast” them to Procs by using the & operator when calling methods. Last week, in comments about my last post (Ruby is NOT a Callable Oriented Language (It’s Object Oriented)), I heard that claim again.

To be honest, I never really thought that much about it, and the idea of “casting” a block to a Proc never took hold in my mental model, but when discussing my post with a number of people, I realized that a lot of people have this concept in their mental model of Ruby.

It is not part of Ruby’s semantics.

In some cases, Ruby’s internal implementation performs optimizations by eliminating the creation of objects until you specifically ask for them. Those optimizations are completely invisible, and again, not part of Ruby’s semantics.

Let’s look at a few examples.

Blocks vs. Procs

Consider the following scenarios:

def foo
  yield
end
 
def bar(&block)
  puts block.object_id
  baz(&block)
end
 
def baz(&block)
  puts block.object_id
  yield
end
 
foo { puts "HELLO" } #=> "HELLO"
bar { puts "HELLO" } #=> "2148083200\n2148083200\nHELLO"

Here, I have three methods using blocks in different ways. In the first method (foo), I don’t specify the block at all, yielding to the implicit block. In the second case, I specify a block parameter, print its object_id, and then send it on to the baz method. In the third case, I specify the block, print out its object_id and yield to the block.

In Ruby’s semantics, these three uses are identical. In the first case, yield calls an implicit Proc object. In the second case, it takes an explicit Proc, then sends it on to the next method. In the last case, it takes an explicit Proc, but yields to the implicit copy of the same object.

So what’s the & thing for?

In Ruby, in addition to normal arguments, methods can receive a Proc in a special slot. All methods can receive such an argument, and Procs passed in that slot are silently ignored if not yielded to:

def foo
  puts "HELLO"
end
 
foo { something_crazy } #=> "HELLO"

On the other hand, if you want a method to receive a Proc in that slot, and thus be able to yield to it, you specify that by prefixing it with an &:

def foo
  yield
end
 
my_proc = Proc.new { puts "HELLO" }
foo(&my_proc)

Here, you’re telling the foo method that my_proc is not a normal argument; it should be placed into the proc slot and made available to yield.

Additionally, if you want access to the Proc object, you can give it a name:

def foo(&block)
  puts block.object_id
  yield
end
 
foo { puts "HELLO" } #=> "2148084320\nHELLO"

This simply means that you want access to the implicit Proc in a variable named block.

Because, in most cases, you’re passing in a block (using do/end or {}), and calling it using yield, Ruby provides some syntax sugar to make that simple case more pleasing. That does not, however, mean that there is a special block construct in Ruby’s semantics, nor does it mean that the & is casting the a block to a Proc.

You can tell that blocks are not being semantically wrapped and unwrapped because blocks passed along via & share the same object_id across methods.

Mental Models

For the following code there are two possible mental models.

def foo(&block)
  puts block.object_id
  yield
end
 
b = Proc.new { puts "OMG" }
puts b.object_id
foo(&b) #=> 2148084040\n2148084040\nOMG

In the first, the &b unwraps the Proc object, and the &block recasts it into a Proc. However, it somehow also wraps it back into the same wrapper that it came from into the first place.

In the second, the &b puts the b Proc into the block slot in foo‘s argument list, and the &block gives the implicit Proc a name. There is no need to explain why the Proc has the same object_id; it is the same Object!

These two mental models are perfectly valid (the first actually reflects Ruby’s internal implementation). I claim that those who want to use the first mental model have the heavy burden of introducing the new concept to the Ruby language of a non-object block, and that as a result, it should be generally rejected.

Metaclasses

Similarly, in Ruby’s internal implementation, an Object does not get a metaclass until you ask for one.

obj = Object.new
# internally, obj does not have a metaclass here
 
obj.to_s
# internally, Ruby skips right up to Object when searching for #to_s, since
# it knows that no metaclass exists
 
def obj.hello
  puts "HELLO"
end
# now, Ruby internally rewrites obj's class pointer to point to a new internal
# metaclass which has the hello method on it
 
obj.to_s
# Now, Ruby searches obj's metaclass before jumping up to Object
 
obj.to_s
# Now, Ruby skips the search because it's already cached the method
# lookup

All of the comments in the above snippet are correct, but semantically, none of them are important. In all cases, Ruby is semantically looking for methods in obj‘s metaclass, and when it doesn’t find any, it searches higher up. In order to improve performance, Ruby skips creating a metaclass if no methods or modules are added to an object’s metaclass, but that doesn’t change Ruby’s semantics. It’s just an optimization.

By thinking in terms of Ruby’s implementation, instead of Ruby’s semantics, you are forced to think about a mutable class pointer and consider the possibility that an object has no metaclass.

Mental Models

Again, there are two possible mental models. In the first, Ruby objects have a class pointer, which they manipulate to point to new metaclass objects which are created only when methods or modules are added to an object. Additionally, Ruby objects have a method cache, which they use to store method lookups. When a method is looked up twice, in this mental model, some classes are skipped because Ruby already knows that they don’t have the method.

In the second mental model, all Ruby objects have a metaclass, and method lookup always goes through the metaclass and up the superclass chain until a method is found.

As before, I claim that those who want to impose the first mental model on Ruby programmers have the heavy burden of introducing the new concepts of “class pointer” and “method cache”, which are not Ruby objects and have no visible implications on Ruby semantics.

Regular Expression Matches

In Ruby, certain regular expression operations create implicit local variables that reflect parts of the match:

def foo(str)
  str =~ /e(.)l/
  p $~
  p $`
  p $'
  p $1
  p $2
end
 
foo("hello") #=> #<MatchData "ell" 1:"l">\n"h"\n"o"\n"l"\nnil

This behavior is mostly inherited from Perl, and Matz has said a few times that he would not support Perl’s backrefs if he had it to do over again. However, the provide another opportune example of implicit objects in Ruby.

Mental Models

In this case, there are three possible mental models.

In the first mental model, if you don’t use any $ local variables, they don’t exist. When you use a specific one, it springs into existence. For instance, when using $1, Ruby looks at some internal representation of the last match and retrieves the last capture. If you use $~, Ruby creates a MatchData object out of it.

In the second mental model, when you call a method that uses regular expressions, Ruby walks back up the stack frames, and inserts the $ local variables on it when it finds the original caller. If you later use the variables, they are already there. Ruby must be a little bit clever, because the most recent frame on the stack (which might include C frames, Java frames, or internal Ruby frames in Rubinius) is not always the calling frame.

In the last mental model, when you call a method that uses regular expressions, there is an implicit match object available (similar to the implicit Proc object that is available in methods). The $~ variable is mapped to that implicit object, while the $1 variable is the equivalent of $~[1].

Again, this last mental model introduces the least burdensome ideas into Ruby. The first mental model introduces the idea of an internal representation of the last match, while the second mental model (which again, has the upside of being how most implementations actually do it) introduces the concept of stack frames, which are not Ruby objects.

The last mental model uses an actual Ruby object, and does not introduce new concepts. Again, I prefer it.

Conclusion

In a number of places, it is possible to imbue Ruby semantics with mental models that reflect the actual Ruby implementation, or the fact that it’s possible to imagine that a Ruby object only springs into existence when it is asked for.

However, these mental models require that Ruby programmers add non-objects to the semantics of Ruby, and requiring contortions to explain away Ruby’s own efforts to hide these internals from the higher-level constructs of the language. For instance, while Ruby internally wraps and unwraps Procs when passing them to methods, it makes sure that the Proc object attached to a block is always the same, in an effort to hide the internal details from programmers.

As a result, explaining Ruby’s semantics in terms of these internals requires contortions and new constructs that are not natively part of Ruby’s object model, and those explanations should be avoided.

To be clear, I am not arguing that it’s not useful to understand Ruby’s implementation. It can, in fact, be quite useful, just as it’s useful to understand how C’s calling convention works under the covers. However, just as day-to-day programmers in C don’t need to think about the emitted Assembler, day-to-day Ruby programmers don’t need to think about the implementation. And finally, just as C implementations are free to use different calling conventions without breaking existing processor-agnostic C (or the mental model that C programmers use), Ruby implementations are free to change the internal implementation of these constructs without breaking pure-Ruby code or the mental model Ruby programmers use.

Posted in Other | 14 Comments »

Ruby is NOT a Callable Oriented Language (It’s Object Oriented)

February 21st, 2010

I recently ran across a presentation entitled Python vs. Ruby: A Battle to the Death. I didn’t consider it to be a particularly fair battle, and may well reply in more detail in a later post.

However, what struck me as most worthy of explanation was the presenter’s concern about the fact that Procs are not callable via parens.

x = Proc.new { puts "HELLO" }
x() #=> undefined method `x' for #<Object:0x1001bd298><
x.call #=> "HELLO"
x[]    #=> "HELLO"

For those coming from a callable-oriented language, like Python, this seems horribly inconsistent. Why are methods called with (), while Procs are called with [].

But what’s going on here is that Ruby doesn’t have a notion of “callable”, like Python. Instead, it has a pervasive notion of Object. Here, x is an instance of the Proc class. Both call and [] are methods on the Proc class.

Designing for the Common Case: Calling Methods

Coming from a callable-oriented language, this might seem jarring. But Ruby is designed around Objects and the common cases of working with objects.

Calling methods is far more common than wanting to get an instance of Method, so Ruby optimizes the case of calling methods, with a slightly less elegant form to access an instance:

class Greeter
  def say
    puts "Hello world!"
  end
end
 
Greeter.new.say #=> "Hello world!"
 
Greeter.method(:new) #=> #<Method: Class#new>
Greeter.new.method(:say) #=> #<Method: Greeter#say>
 
# This is so that you don't have to say:
 
Greeter.new().say()

Ruby considers the common case of calling methods, and optimizes that case while still making the less common case possible.

Designing for the Common Case: How Blocks Are Really Used

One of the reasons that the Proc.new case throws off Rubyists in debates is that Rubyists literally never call Proc objects using [].

In Ruby, Procs are the object passed to methods when using block syntax. Here is how Procs are actually used:

def hello
  puts "Hello world!"
  yield
  puts "Goodbye cruel world!"
end
 
hello { puts "I am in the world!" }

When examining languages that support passing anonymous functions to functions (like JavaScript), it turns out that the vast majority of such cases involve a single anonymous function. As a result, Matz (inspired by Smalltalk) built in the idea of a block as a core construct in Ruby. In the vast majority of cases, blocks are created using lightweight syntax ({} or do/end) and called using yield.

In some cases, blocks are passed from one method to the next, before they are finally called using yield:

def step1(&block)
  puts "Step 1"
  step2(&block)
end
 
def step2
  puts "Step 2"
  yield
end
 
step1 { puts "Do the action!" } #=> "Step 1\nStep 2\nDo the action!"

As you can see, Ruby builds in the idea of calling a block into the core language. I searched through Rails (a fairly large codebase) for instances of using [] to call a Proc and while we use blocks extremely commonly, we don’t use [] to call them.

I suspect that the reason this comes up is that people who are used to having to define standalone functions, pass them around, and then call them are looking for the analogous constructs in Ruby, but are missing the different paradigm used by Ruby.

Consistent Method Execution

Fundamentally, the issue here comes down to this:

def foo
  proc {}
end
 
foo()

In Ruby, methods are invoked with our without parentheses. All methods return values, which are always Objects. All Objects have methods. So foo() is a method call that returns a Proc object. It’s extremely consistent, with very few axioms. The fact that the axioms aren’t the same as those in a callable-oriented language doesn’t make them “weird”.

Posted in Other | 38 Comments »