From: Simon Strandgaard <neoneye@...>
Date: 2004-05-01T19:57:53+09:00
Subject: Re: Lazy evaluation (evil)

Lionel Thiry <lthiryidontwantspam@skynetnospam.be> wrote:
> Ocaml is a functionnal langage of the ML family. It means it abuses of 
> pattern matching and allows for side effects. As far as I know, lisp and 
> scheme are pure functionnal langage as they don't allow side effects. 
> But perhaps I'm wrong. And I'm often wrong. ;)

Standard ml (SML) allows for sideeffects too.  Don't know about Lisp/Scheme.

 
> About Ruby, I must tell I find it hard to see where Ruby is functionnal 
> langage. Coding recursivity in ruby is most of the time so uneasy. I 
> just wonder, if someone can explain, he'll be welcome.

I have just made a regexp engine where the scanner uses recursion.
Fetch the tar.gz file and look at 'source/scanner_nodes.rb'
http://raa.ruby-lang.org/list.rhtml?name=regexp


For instance the following piece of code are responsible for matching 
a charclass.  @succ points to the next node in the AST.
@set is an Array, e.g.  ['a'..'z', '_', '0'..'9']

See the third line from the bottom, how recursion is working.


class Inside < Base
  def initialize(succ, set)
    @succ = succ
    @set = set
    super()
  end
  attr_reader :succ, :set
  def ==(other)
    (self.class == other.class) and
    (@set == other.set) and
    (@succ == other.succ)
  end
  def check_not_endofinput(context)
    return if context.input.has_next?
    context.raise_mismatch("(Inside) end of input")
  end
  def is_member_of_set?(symbol)
    @set.each do |i|
      return true if i.kind_of?(Range) and i.include?(symbol)
      return true if i == symbol
    end
    false
  end
  def check_member_of_set(context)
    symbol = context.current
    return if is_member_of_set?(symbol)
    context.raise_mismatch(
      "(Inside) symbol #{symbol.inspect} is not in set #{@set.inspect}")
  end
  def match(context)
    #puts "Inside, set=#{@set.inspect}"
    check_not_endofinput(context)
    check_member_of_set(context)
    context.input_next { @succ.match(context) }
  end
end


--
Simon Strandgaard