method_missing

The Method gem you've been waiting for.

The method_missing gem brings the functional tools you need to Ruby: method composition, sequencing, and repeating. These are useful for when you must combine methods, apply different methods to the same argument, or "grow" a method.

I'll explain, but first:

Installing

gem install method_missing

Usage: Composing

This is the classic. In algebra class you learned that f(g(x)) can also be written (f . g)(x). This is handy because now you have this (f . g) object that you can pass to integrals and whatnot.

So in Ruby, using the method_missing gem:

def escape_everything(text)
  everything_escaper.call(text)
end

def everything_escaper
  method(:html_escape) * method(:escape_javascript) * method(:json_escape)
end

And more algebraically:

(f * g).call(x) == f.call(g.call(x))

Usage: Sequencing

This doesn't come up as often but when it does, oh boy, does it ever! This is useful for when you have a bunch of methods to apply to the same argument. For example, using the method_missing gem:

def valid_options?(options_checker, options)
  options_checker.at_most_one(
    (method(:sort_mod_time) / method(:sort_access_time)).call(options)) &&
  options_checker.at_most_one(
    (method(:sort_file_size) / method(:sort_mod_time)).call(options))
 end

Again, more algebraically:

(f / g / h).call(x) == [f.call(x), g.call(x), h.call(x)]

Usage: Repeating

This one comes up the least in most people's day-to-day life. It's the concept of applying a method to its output, repeatedly. Here's a contrived example, using the method_missing gem:

def church_encoding(n)
  if n.zero?
    lambda{|f,x| x}
  else
    lambda{|f,x| (f ^ n).call(x)}
  end
end

Algebraically:

(f ^ 3).call(4) == f.call(f.call(f.call(4)))

Usage: Combining

These are objects which can be combined! Here's an example, also contrived, that turns an object into a number, n, then adds four to n and multiplies n by six, then sums those results, using primitive methods that add one to a number and multiply a number by two:

def interesting_math(o)
  four_and_six.call(o).sum
end

def four_and_six
  ((add1 ^ 4) / (mul2 ^ 3)) * method(:to_i)
end

To show this algebraically:

(((add1 ^ 4) / (mul2 ^ 3)) * method(:to_i)).call(o) == [
  add1.call(add1.call(add1.call(add1.call(o.to_i)))),
  mul2.call(mul2.call(mul2.call(o.to_i)))
]