Ruby Facets: Symbol.to_proc, Class.to_proc

One pretty well-know idiom in Ruby, and Facets, is Symbol.to_proc. It lets you turn these:

[1, 2, 3].map { |num| num.next }  #=> [2, 3, 4]

%w[alpha beta gamma].map { |word| word.upcase }
#=> ["ALPHA", "BETA", "GAMMA"]

…into these:

[1, 2, 3].map(&:next)
%w[alpha beta gamma].map(&:upcase)

It’s a nice little trick, though it’s not to everyone’s taste. If you’re already comfortable with Symbol.to_proc, you can skip down to the Class.to_proc section. But if you’re not, it’s worth a minute of your attention to learn it. Read on…

How it’s done

When a method takes a block, you can call yield, to run the block.

def with_a_block(a_param)
    yield
end
with_a_block('param') {
    puts 'in the block'
}

Or, you can name the block as the last parameter to the method, and put an ampersand in front of it. The ampersand makes ruby convert the block to a procedure, by calling to_proc on it. (So any object with a to_proc method can work this way, if you want.) This example works just like the last one:

def named_block(a_param, &blk)
    blk.call
end
named_block('my_param') {
    puts 'in the named block'
}

Symbol’s to_proc method creates a procedure that takes one argument, and sends the symbol to it. Sending a symbol to an object is the same as calling a method on it: object.send(:method) works the same as object.method. In the earlier upcase example, each word is passed to a procedure that calls upcase on it, giving us a list of uppercased strings.

&:upcase
# becomes...
lambda { |obj|
    obj.send(:upcase)
}
# or...
lambda { |obj|
    obj.upcase
}

Class.to_proc

So Symbol.to_proc creates a function that takes an argument, and calls that method on it. Class.to_proc creates a function that passes its argument to its constructor, yielding an instance of itself. This is a welcome addition to the to_proc family.

require 'facets'

class Person
    def initialize(name)
        @name = name
    end
end
names = %w[mickey minney goofy]
characters = names.map(&Person)

puts characters.inspect

&Person
# becomes...
lambda { |obj|
    Person.new(obj)
}

Why it’s nice

  • It’s fewer characters — it semantically compresses your code.
  • It lets you think, makes you think, on a higher level. You think about operations on your data, rather than handling one item at a time. It raises your level of thinking.
  • It works with first-class functions, which are worth understanding. They give you new ways to elegantly solve some problems (well, new to some audiences). They’re not fringe anymore — they’ve been in C# since v2.0.
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Hartford Ruby Brigade starts with a tour of Ruby Facets

As Rob Bazinet has said, the Hartford Ruby Brigade is having its first meeting on March 24. You can get all the details from his post. Come join us! There’s even a book raffle.

I’ll be giving the first presentation, a tour of Ruby Facets. Facets is a pretty huge library (even after they moved some really neat parts into their own projects), and it’s crazy to think we could cover it all in one night. I’ll quickly touch on the simple features, just to let you know they’re there, and I’ll spend more time with some of the interesting parts. If you’re stuck, it’s a good chance Facets has what you need; the trick is knowing it’s there, and where to look — I want to point out enough of Facets to help you with that.

I’ll also start a Tour of Facets series here, starting with this post. I’m aiming for two to four posts a month, and will cover everything in the presentation, and then some. So, on with the tour…

compare_on and equate_on

Remember the first time you saw attr_reader and attr_writer? These tiny helpers got me excited about ruby, not just because they meant less typing and DRY-er code, but because they meant I could make helpers to generate methods, too, if only I could think of a reason to do it.

Facets has a great example of why you’d want to do that: compare_on and equate_on.

Most ruby programmers know you can make your objects sortable by defining <=>, the spaceship method, on them. Typically, you wind up delegating to some attribute:

class Person
   attr_reader :fname, :lname
   def initialize(fname, lname)
      @fname, @lname = fname, lname
   end
   def <=>(person)
      @lname.<=>(person.lname)
   end
end

Facets adds compare_on, which generates the spaceship method for you, based on that attribute. Not only that, but you can compare_on multiple fields, and it handles the hairy logic for you automatically:

require 'facets/compare_on'

class Person
   attr_reader :fname, :lname
   def initialize(fname, lname)
      @fname, @lname = fname, lname
   end
   compare_on :lname, :fname
end

people = []
people.push Person.new('Adam', 'Smith')
people.push Person.new('John', 'Adams')

people.sort #=> John Adams, Adam Smith

Correctly implementing the spaceship operator isn’t too hard, but object equality gets tricky in any language. Facets helps you here by implementing ruby’s main three equality methods for you: ==, eql?, and hash.

require 'facets/compare_on'

class Person
   attr_reader :fname, :lname
   def initialize(fname, lname)
      @fname, @lname = fname, lname
   end
   equate_on :lname, :fname
end

a_pres = Person.new('John', 'Adams')
another_pres = Person.new('John', 'Adams')
[a_pres].include?(another_pres) #=> true

Again, you can equate on multiple attributes (fname and lname), and it handles all the details for you. Hope to see you at the Hartford Ruby Brigade!