Composing functions in Ruby
Posted by Tom Moertel Fri, 07 Apr 2006 15:55:00 GMT
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
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 }
=> #<Proc:0x00002aaaaaad7810@(irb):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.)
readers
Thanks for this, Tom. I hope to see more Haskell constructs translated to Ruby in the future!
Hi there!
I think you’d like to see something I did based on the problem described here:
http://eustaquiorangel.com/blog/show/460
It’s a Brazilian Portuguese post but the code speaks for itself. :-)
Best regards,