# Composing functions in Ruby

Posted on
Tags:

One of the things I miss when coding in Ruby is inexpensive function composition. In Haskell, for example, I can compose functions using the dot (.) operator:

inc        = (+1)
twice      = (*2)
twiceOfInc = twice . inc

Because of Ruby’s open classes, however, I can easily add the feature to the language. In the code below, I introduce Proc.compose and overload the star (**) operator for the purpose:

# func_composition.rb
class Proc
def self.compose(f, g)
lambda { |*args| f[g[*args]] }
end
def *(g)
Proc.compose(self, g)
end
end

And that’s all it takes:

\$ irb --simple-prompt -r func_composition.rb

>> inc = lambda { |x| x + 1 }

>> twice = lambda { |x| x * 2 }
=> #<Proc:0x00002aaaaabd2d18@(irb):2>

>> inc[1]
=> 2

>> twice[2]
=> 4

>> twice_of_inc = twice * inc
=> #<Proc:0x00002aaaaab32458@./func_composition.rb:3>

>> twice_of_inc[1]
=> 4

>> twice_of_inc[2]
=> 6

Now, isn’t that refreshing?

Update: Vincent Foley pointed out on comp.lang.ruby that Ruby Facets has a nearly identical implementation that also uses the star operator for composition. (Its version of compose, however, is an instance method whereas my version is a class method.)