Skip to content
This repository
branch: master
Fetching contributors…

Octocat-spinner-32-eaf2f5

Cannot retrieve contributors at this time

file 324 lines (203 sloc) 20.235 kb

Anaphora in Ruby, 2012 Edition

The following is an update to a post I wrote in September, 2009. I was spurred to update it by the release of a new anaphoric library for Ruby called Ampex. It seems that if people have a problem that doesn't get solved, they will keep re-inventing solutions for it.

In natural language, an anaphor is an expression which refers back in the conversation. The most common anaphor in English is probably "it," as in "Get the wrench and put it on the table." Anaphora are a great convenience in everyday language--imagine trying to get along without them--but they don't appear much in programming languages. For the most part, this is good. Anaphoric expressions are often genuinely ambiguous, and present-day programming languages are not designed to handle ambiguity. --Paul Graham, On Lisp

Old School Global Variable Anaphora

Anaphora have actually been baked into Ruby from its earliest days. Thanks to its Perl heritage, a number of global variables act like anaphora. For example, $& is a global variable containing the last successful regular expression match, or nil if the last attempt to match failed. So instead of writing something like:

if match_data = /reg(inald)?/.match(full_name) then puts match_data[0] end

You can use $& as an anaphor and avoid creating another explicit temporary variable, just like the anaphor in a conditional:

if /reg(inald)?/.match(full_name) then puts $& end 

These 'anaphoric' global variables have a couple of advantages. Since they are tied to the use of things like regular expression matching rather than a specific syntactic construct like an if expression, they are more flexible and can be used in more ways. Their behaviour is very well defined.

The disadvantage is that there is a complete hodge-podge of them. Some are read only, some read-write, and none have descriptive names. They look like line noise to the typical programmer, and as a result many people (myself included) simply don't use them outside of writing extremely short shell scripts in Ruby.

Anaphors like the underscore or a special variable called "it" have the advantage of providing a smaller surface area for understanding. Consider Lisp's anaphoric macro where "it" refers to the value of the test expression and nothing more (we ignore the special cases and other ways Ruby expresses conditionals). Compare:

if /reg(inald)?/.match(full_name) then puts $& end

To:

if /reg(inald)?/.match(full_name) then puts it[0] end

To my eyes, "it" is easier to understand because it is a very general, well-understood anaphor. "It" always matches the test expression. We don't have to worry about whether $& is the result of a match or all the text to the left of a match or the command line parameters or what-have-you. Of course, "it" isn't an anaphor in Ruby. It is (forgive the expression) in other languages like Groovy.

Could anaphors be added to Ruby where none previously existed? Yes. Sort of.

New School Block Anaphora

Methodphitamine

A block anaphor is a meta-variable that can be used in a Ruby block to refer to its only parameter. Consider the popular Symbol#to_proc. Symbol#to_proc is the standard way to abbreviate blocks that consist of a single method invocation, typically without parameters. For example if you want the first name of a collection of people records, you might use Person.all(...).map(&:first_name).

Some languages provide a special meta-variable that can be used in a similar way. if it was a block anaphor in Ruby, you could write Person.all(...).map { it.first_name }. Of course, Ruby doesn't have block anaphora built in, so people kludged workarounds, and Symbol#to_proc was so popular that it became enshrined in the language itself.

Jay Phillips implemented a simple block anaphor called Methodphitamine. it doesn't seem like much of a win when you just want to send a message without parameters. But if you want to do more, such as invoke a method with a parameter, or if you want to chain several methods, you are out of luck. Symbol#to_proc does not allow you to write Person.all(...).map(&:first_name[0..3]). With Methodphitamine you can write:

Person.all(...).map(&it.first_name[0..3])

Likewise with Symbol#to_proc you can't write Person.all(...).map(&:first_name.titlecase). You have to write Person.all(...).map(&:first_name).map(&:titlecase). With Methodphitamine you can write:

Person.all(...).map(&it.first_name.titlecase)

This is easy to read and does what you expect for simple cases. Methodphitamine uses a proxy object to create the illusion of an anaphor, allowing you to invoke method with parameters and to chain more than one method. Here's some code illustrating the technique:

class AnaphorProxy < BlankSlate

  def initialize(proc = lambda { |x| x })
    @proc = proc
  end

  def to_proc
    @proc 
  end

  def method_missing(symbol, *arguments, &block)
    AnaphorProxy.new(
      lambda { |x| self.to_proc.call(x).send(symbol, *arguments, &block) }
    )
  end

end

class Object

  def it
    AnaphorProxy.new
  end

end

(1..10).map(&it * 2 + 1) # => [3, 5, 7, 9, 11, 13, 15, 17, 19, 21]

What happens is that "it" is a method that returns an AnaphorProxy. The default proxy is an object that answers the Identity function in response to #to_proc. Think about how (1..10).map(&it) => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] works: "it" is a method that returns the default AnaphorProxy; using &it calls AnaphorProxy#to_proc and receives lambda { |x| x } in return; #map now applies this to 1..10 and you get [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].

If you send messages to an AnaphorProxy, you get another AnaphorProxy that "records" the messages you send. So it * 2 + 1 evaluates to an AnaphorProxy that returns lambda { |x| lambda { |x| lambda { |x| x }.call(x) * 2 }.call(x) + 1 }. This is equivalent to lambda { |x| x * 2 + 1} but more expensive to compute and dragging with it some closed over variables.

As you might expect from a hack along these lines, there are all sorts of things to trip us up. (1..10).map(&it * 2 + 1) works, however what would you expect from:

(1..10).map(&1 + it * 2) # no!

This does not work with Methodphitamine, and neither does something like:

Person.all(...).select(&it.first_name == it.last_name) # no!

Also, unexpected things happen if you try to "record" an invocation of #to_proc:

[:foo, :bar, :blitz].map(&it.to_proc.call(some_object)) # no!

We'll have another look at these "gotchas" below.

Ampex: Block anaphora updated

Ampex is a new block anaphora library. Instead of it, Ampex uses X:

["a", "b", "c"].map &(X * 2)
  # => ["aa", "bb", "cc"]

As Conrad Irwin explains in a blog post announcing Ampex:

The ampex library is distributed as a rubygem, so to use it, you can either install it one-off or add it to your Gemfile. We've been using ampex in production for over a year now, and beacuse it's written in pure Ruby, it works on Ruby 1.8.7, Ruby 1.9 and JRuby out of the box.

Technical Gotchas

Using proxy objects (as methodphitimine and ampex do) runs you into that curious problem of trying to implement symmetrical behaviour in object-oriented languages where everything is inherently asymmetrical. Block anaphora implemented by proxy objects only work properly when they're a receiver in a block. You cannot, for example, use methodphitimine or ampex to write:

(1..10).map { 1 + it * 2 }
(1..10).map { 1 + X * 2 }

You also have certain issues with respect to when arguments are evaluated:

i = 1
(1..10).map { &it.frobbish(i += 1) }

i +=1 is only evaluated once, not for each iteration.

Anaphora via AST Rewriting

To "fix" the problems with using a proxy to implement anaphora, you need to parse and rewrite Ruby directly. No sane person would do this just for the convenience of using block anaphora in their code, however Github archeologists report that a now-extinct society of programmers did this very thing:

The abandonware gem rewrite_rails supported it, its, or _ as block anaphora for blocks taking one argument. When writing a block that takes just one parameter, you can use either it or its as a parameter without actually declaring the parameter using { |it| ... }.

Like Methodphitimine and Ampex, you can supply parameters:

User.all(...).each { it.increment(:visits) }

Or chain methods:

Person.all(...).map { its.first_name.titlecase }

Unlike the other gems, it needn't be the receiver:

Person.all(...).each { (name_count[its.first_name] ||= 0) += 1 }

This style of code works best when you would naturally use the word "it" or the possessive "its" if you were reading the code aloud to a colleague. (You can use the underscore, _ instead of it or its for visual compatibility with certain functional programming languages)Stri. rewrte_rails does its magic by parsing the block and rewriting it. So when you write:

(1..10).map { 1 + it * 2 }
Person.all(...).select { its.first_name == its.last_name } # and,
[:foo, :bar, :blitz].map { it.to_proc.call(some_object) }
(1..100).map { (1/_)+1 }

rewrite_rails actually rewrites your code into:

(1..10).map { |it| 1 + it * 2 }
Person.all(...).select { |its| its.first_name == its.last_name } # and,
[:foo, :bar, :blitz].map { |it| it.to_proc.call(some_object) }
(1..100).map { |_| (1/_)+1 }

Needless to say, this is a very heavyweight approach to implementing block anaphora, although the result is semantically much cleaner. It's best considered a proof of concept, a pointer towards what could be done if the people governing the Ruby language want to consider baking Anaphora directly into the interpreter.

Speculative Digression: Anaphors for conditionals

Many people are familiar with the andand gem. Say you want to write some code like this:

big_long_calculation() && big_long_calculation().foo

Most of the time you ought to "cache" the big long calculation in a temporary variable like this:

(it = big_long_calculation()) && it.foo

That's such a common idiom, #andand gives you a much more succinct way to write it:

big_long_calculation().andand.foo

So the idea behind #andand is to express a test for nil and doing something with the result if it is not nil in a very compact way. This is not a new idea. Paul Graham gives this very example when describing the rationale for anaphoric macros:

It's not uncommon in a Lisp program to want to test whether an expression returns a non-nil value, and if so, to do something with the value. If the expression is costly to evaluate, then one must normally do something like this:

(let ((result (big-long-calculation)))
  (if result
      (foo result)))

Wouldn't it be easier if we could just say, as we would in English:

(if (big-long-calculation)
    (foo it))

In natural language, an anaphor is an expression which refers back in the conversation. The most common anaphor in English is probably "it," as in "Get the wrench and put it on the table." Anaphora are a great convenience in everyday language--imagine trying to get along without them--but they don't appear much in programming languages. For the most part, this is good. Anaphoric expressions are often genuinely ambiguous, and present-day programming languages are not designed to handle ambiguity.

WIth an anaphoric macro, the anaphor "it" is bound to the result of the if expression's test clause, so you can express "test for nil and do something with the result if it is not nil" in a compact way.

Anaphors for conditionals in Ruby?

Reading about Lisp's anaphoric macros made me wonder whether anaphora for conditionals would work in Ruby. I find (it = big_long_calculation()) && it.foo cluttered and ugly, but perhaps I could live without #andand if I could write things like:

if big_long_calculation(): it.foo end

This is relatively easy to accomplish using rewrite_rails. In the most naïve case, you want to rewrite all of your if statements such that:

if big_long_calculation()
  it.foo
end

Becomes:

if (it = big_long_calculation())
  it.foo
end

You can embellish such a hypothetical rewriter with optimizations such as not assigning it unless there is a variable reference somewhere in the consequent or alternate clauses and so forth, but the basic implementation is straightforward.

The trouble with this idea is that in Ruby, There Is More Than One Way To Do It (for any value of "it"). If we implement anaphora for conditionals, we ought to implement them for all of the ways a Ruby programmer might write a conditional. As discussed, we must support:

if big_long_calculation()
  it.foo
end

Luckily, that's the exact same thing as:

if big_long_calculation(): it.foo end

They both are parsed into the exact same abstract syntax tree expression. Good. Now what about this case:

it.foo if big_long_calculation()

That doesn't read properly. The anaphor should follow the subject, not precede it. If we want our anaphora to read sensibly, we really want to write:

big_long_calculation().foo if it           # or
big_long_calculation().foo unless it.nil?

These read more naturally, but supporting these expressions would invite Yellow Edge Case Cranial Headache or "YECCH." Behind the scenes, Ruby parses both of the following expressions identically:

big_long_calculation().foo unless it.nil? # and
unless it.nil?
  big_long_calculation().foo
end

So you would have to have a rule that if the anaphor appears in the test expression, it refers to something from the consequent expression, not from any preceding test expression. But if you tried that rule, how would you handle this code?

if calculation_that_might_return_a_foobar()
  if it.kind_of?(:Foobar)
    number_of_foobars += 1
  end
end

This doesn't work as expected because the anaphor would refer forward to its consequent expression number_of_foobars += 1 rather than backwards to the enclosing test expression calculation_that_might_return_a_foobar(). You can try to construct some rules for disambiguating things, but you're going to end up asking programmers to memorize the implementation of how things actually work rather than relying on familiarity with how anaphora work in English.

Another problem with supporting big_long_calculation().foo unless it.nil? is that we now need some rules to figure out that the anaphor refers to big_long_calculation() and not to big_long_calculation().foo. Whatever arbitrary rules we pick are going to introduce ambiguity. What shall we do about:

big_long_calculation().foo       unless it.nil?
big_long_calculation().foo.bar   unless it.nil?
big_long_calculation() + 3       unless it.nil?
3 + big_long_calculation()       unless it.nil?
big_long_calculation(3)          unless it.nil?
big_long_calculation(foo())      unless it.nil?
big_long_calculation(foo(bar())) unless it.nil?

In my opinion, if we can't find clean and easy to understand support for writing conditionals as suffixes, we aren't supporting Ruby conditionals. To underscore the difficulty, let's also remember that Ruby programmers idiomatically use operators to express conditional expressions. Given:

big_long_calculation() && big_long_calculation().foo

We want to write:

big_long_calculation() && it.foo

This is near and dear to my heart: The name "andand" comes from this exact formulation. #andand doesn't enhance an if expression, it enhances the double ampersand operator. One can see at a glance that implementing support for big_long_calculation() && it.foo is fraught with perils. What about big_long_calculation() + it.foo? What about big_long_calculation().bar && it.foo?

It seems that it is much harder to support anaphora for conditionals in Ruby than it is to support anaphora for conditionals in Lisp. This isn't surprising. Lisp has an extremely regular lack of syntax, so we don't have to concern ourselves with as many cases as we do in Ruby.

Summing "it" Up

Anaphora allow us to abbreviate code, hiding parameters and temporary variables for certain special cases. This can be a win for readability for short code snippets where the extra verbiage is almost as long as what you're trying to express. That being said, implementing anaphora in Ruby is a hard design problem, in part because There Is More Than One Way To Do It, and trying to provide complete support leads to ambiguities, inconsistencies, and conflicts. And old school anaphora? They are clearly an acquired taste.

More to read


My recent work:

JavaScript AllongéCoffeeScript RistrettoKestrels, Quirky Birds, and Hopeless Egocentricity


(Spot a bug or a spelling mistake? This is a Github repo, fork it and send me a pull request!)

Reg Braithwaite | @raganwald

Something went wrong with that request. Please try again.