From: MikkelFJ <mikkelj-anti-spam@...1.dknet.dk>
Date: 2001-11-30T07:15:27+09:00
Subject: [ruby-talk:26976] first class functions in Ruby

In the thread on language design, I mentioned a wish for functions as first
class values.

I hope someone knowing Ruby deeper than I will follow up on this.

I'd like to be able to write a function that I can pass as argument to a
Ruby message or to another function. A function is a new concept in Ruby
context.

There is a difference between messages and functions. The difference being
that messages has a receiver (the object the member function is a member
of).
A function is a pure mathematical abstraction that does not have any
implicit state like a receiver.

First a way to simulate a function, then a less combersome syntax

<snip>
class DoubleFunction
  def evaluate arg
    arg + arg
  end
end

#example of use

def aMessage f
  f.evaluate(42) + f.evaluate(27)
end

def main
  aMessage MyFunction.new
end
</snip>

improved syntax:

<snip>
fun squareFunction arg
  arg + arg
end
def aMessage f
  f(42) + f(27)
end
def main
  aMessage squareFunction
end
</snip>

"fun" is a new keyword that defines a function that is a receiverless
message. It can be treated like any other object.

It would also be nice to be able to pass a message ("def") around as a
value. It may also make the "fun" keyword superflous. An example would be as
above, except that a) "fun" is replaced by "def", b) the object instance the
"def" is available and can by modified by the method the usual way:

<snip>
def squareFunction arg
  arg + arg
end
def accumulate arg
  if @count
    @count = @count + arg
  else
    @count = arg
  end
end
class Processor
  def initilize n
    @weight = n
  end
  def process f
    f(weight)
  end
end
def main
  p = Process.new(4)
  p.process(squareFunction)
  p.process(accumulate)
  aMessage squareFunction
end
</snip>

The above example does not use the "fun" function, instead it uses two
messages ("def"), in which one of the messages uses its ownership context,
the other basically behaves like a function.
Notice that the Process class is a bit tedious to write. It is a class with
a single function. The last example repeated with a function with partial
evaluation instead of a Process class. The "fun" keyword has been
reintroduced:

<snip>
fun squareFunction arg
  arg + arg
end
def accumulate arg
  if @count
    @count = @count + arg
  else
    @count = arg
  end
end
fun process weight f
    f(weight)
end

def main
  p4 = process(4)
  p4(squareFunction)
  process(4, squareFunction) # same as above
  p4(accumulate)
  aMessage squareFunction
end
</snip>

In the above example, "p4" is a partial evaluation of the process function.
What happens is that the applied parameter 4 is saved in an internal
"weight" variable, and a new function is  created with one parameter less.
This is pretty much standard in functional programming, but it should be
pretty straightforward to integrate in the ruby enviroment. In ML this can
be pretty heavy stuff to implement, but due to Ruby's dynamic nature, it
should be easy, and equally powerful.
Microsoft added the concept of delegates to java, dubbing it J++ and spawned
a major lawsuit from Sun. But the concept was great and made it much easier
to create eventhandlers.


MikkelFJ