Haskell-style monad do-notation for Ruby

Example:

class Array
  extend Monad

  def self.unit x
    [x]
  end

  def bind &f
    map(&f).inject([]){ |a,b| a+b }
  end
end

Array.run do
  x <- ['A', 'a']
  y <- ['B', 'b']

  unit(x+y)
end

# => ["AB", "Ab", "aB", "ab"]

Its biggest failing, and I don't see a way out, is that you don't get lexical scope. ParseTree is wonderful, but it can't work miracles.

You can work around it, though, by passing in stuff from the outside as arguments to `run', and specifying those arguments on the block you pass in as well.

For more examples, see the test suite.

By Aanand Prasad (aanand.prasad@gmail.com)