Implementing if in Ruby

It was with great interest that I watched J. Pablo Fernández’s presentation at the London Ruby User Group tonight entitled “What Ruby Can’t Do”. Concerned of being myopic about the language that happens to provide my income, I am keen to see what we can unabashedly steal from other languages (see Joseph Wilk’s excellent “Testing Outside of the Ruby World” for a similar sentiment). Pablo offered two alternatives to Ruby – Smalltalk and Lisp – and then demonstrated their relative power by implementing his own if condition in both.

While the Lisp example demonstrated homoiconicity and the power of macros, the Smalltalk example was (barring syntactic oddities) much closer to home for Rubyists.

To summarise: Smalltalk eschews keywords for control structures such as if in favour of message sends (to use the Smalltalk parlance for method calls). This means that conditional logic is just another method on an object (not entirely dissimilar to Ruby’s infamous 5.times { ... } example):

somePredicate ifTrue:  [ Transcript show: 'I am true'.  ]
              ifFalse: [ Transcript show: 'I am false'. ].

The implementation of these messages is simple: define a method that takes two arguments, one block to call when true (e.g. trueBlock) and one block to call when false (e.g. falseBlock). Objects that can be considered “truthy” evaluate the first block and those that are “falsey” evaluate the second:

ifTrue: trueBlock ifFalse: falseBlock
  "Implementation for truthy objects."
  ^ trueBlock value

ifTrue: trueBlock ifFalse: falseBlock
  "Implementation for falsey objects."
  ^ falseBlock value

Pablo bemoaned that such a construction cannot be done in Ruby easily without chaining Proc objects and Tom Stuart summarised the problem on Twitter: “Spoiler alert: Smalltalk lets you pass multiple blocks to a method naturally, Ruby doesn’t.” However, I tweeted that this might not be true from Ruby 1.9 onwards.

To mimic the Smalltalk example, the following syntax is actually valid Ruby:

some_predicate.if ->{ "It's true!" }, else: ->{ "It's false!" }

While it might at first seem odd, it’s actually equivalent to the following:

some_predicate.if(lambda { "It's true!" },
    { :else => lambda { "It's false!" } })

To implement this functionality, we can take advantage of Ruby’s notoriously open classes:

class BasicObject
  def if(if_true, options = {})
    if_true.call
  end
end

module Falsey
  def if(if_true, options = {})
    if_false = options.fetch(:else, ->{})

    if_false.call
  end
end

class NilClass
  include Falsey
end

class FalseClass
  include Falsey
end

Note that I default the if_false case to a no-op to avoid using if internally (and therefore sidestep some furore).

(Those of you wishing to draw ire from your colleagues are welcome to download and use the above code as a gem.)

Practically speaking, there is likely to be a performance penalty in avoiding Ruby’s intrinsic optimisations for the if keyword. However, it implements functionality in typical Ruby that was previously reserved as a “special” language feature and, at the same time, demonstrates achieving conditional logic purely via polymorphism.

Update: Yehuda Katz beat me to this by two years in his 2009 post “Emulating Smalltalk’s Conditionals in Ruby”. He uses separate, chainable if_true and if_false methods rather than multiple blocks to mirror Smalltalk’s own ifTrue and ifFalse messages giving you code like so:

some_predicate
  .if_true { puts "true" }
  .if_false { puts "false" }