From: Robert Klemme <shortcutter@...>
Date: 2010-02-22T17:58:17+09:00
Subject: Re: Complex type-checking

2010/2/22 Ken Coar <the.rodent.of.unusual.size@gmail.com>:
> I have a number of methods that can take arguments in two different
> forms.  As an example:
>
> method(tuple [, ...])
>
> where 'tuple' can be things like (Point, Point, Integer) or (Numeric,
> Numeric, Numeric, Numeric, Integer) (so either a Point object
> representing the coordinates, or the coordinates directly).
>
> I realise that type-checking in Ruby is anathematic to some, but I want
> to generate some intelligent error messages in some cases.  So please
> let's leave the 'why do you want to do that' bit alone for now. :-)

Sorry, I can't.  Why do you want to do that? :-)

> One meta-thought I had was for the argument vetter to be passed the
> arglist and an array of allowed patterns, like
>
> [
>  %r/^(?:(?:Point\s*){2}\s*Integer)+$/,
>  %r/^(?:(?:Numeric\s*){4}\s*Integer)+$/
> ]
>
> and check each tuple by doing kind_of?() of each element against the
> appropriate words in the pattern.
>
> Which is actually turning out to be a right bugger to code, actually.

Is it?

def pattern_check(args, pattern)
  m = pattern.match(args.map{|x|x.class}.join(' '))

  if m
    m.to_a.each_with_index do |m,i|
      return i if i > 0 && m
    end
  end

  raise ArgumentError, args.inspect
end

def m(*a)
  case pattern_check(a, %r{\A(?:(String String)|(Fixnum))\z})
  when 1
    printf "Two strings: %p\n", a
  when 2
    printf "A number: %p\n", a
  else
    raise ArgumentError, a.inspect
  end
end

Method m does look ugly though.

> So I'm figuring maybe someone could suggest a better method to me?

Provide different methods for different arguments.

Kind regards

robert


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