From: Brian Candler <B.Candler@...>
Date: 2004-10-06T19:16:29+09:00
Subject: Re: Range behavior (Re: [RCR] New [] Semantics)

On Wed, Oct 06, 2004 at 06:26:25PM +0900, Peter Hickman wrote:
> What are you expecting the to_a method to actually do? Return a list of 
> all the values within the range? Hint: infinite in number.

I think Ranges represent at least two different things in Ruby. I will
classify these as:

(1) Ranges where the lower bound responds to the 'succ' operator

-> generates a set of discrete values
-> can iterate using 'each' and therefore use Enumerable methods
   like collect, grep, map, to_a etc.
-> member?(x) tests whether x == any of the discrete values

(2) Ranges where both bounds respond to the '<=>' operator

-> generates an interval
-> include?(x) tests whether x lies between the bounds

And a Range can behave as both at once, if the bounds satisfy both
conditions at once.

  (1..3.4).each {|s| puts s}   # discrete values 1,2,3
  (1..3.4).include?(3.3)       # interval

However, the first of these two examples shows something of an anomoly; the
values are generated using 'succ', but the *end* of iteration is still
detected using the spaceship operator. So more accurately, the two types of
range are:

(1) Lower bound responds to 'succ' operator to generate elements;
    each element responds to '==' to test membership;
    each element responds to '<=>' to compare to upper bound
      ---> discrete set of values

(2) Lower and upper bounds respond to '<=>' operator
      ---> interval

Now, other uses of ranges have been pointed out:

(3) Ranges where the start and end values represent indexes, and negative
values are offsets from the end of an object

   a = [2,3,5,7,11,13]
   a[2..-2]   #=> [5, 7, 11]

In this case, a range is IMO just a holder for two values; the Range itself
does not have any useful methods apart from Range#first and Range#last

(FWIW, these ranges annoy me: firstly because they're not ranges, and
secondly because I can never remember the difference between a[2..4] versus
a[2,4], and I write a[2,-1] when I should write a[2..-1]. I have to make
test cases in irb every time!)

(4) I'm sure someone mentioned another use, but I can't remember what it is
right now.

(The flip-flop operator looks like a range, but I don't think it actually
creates a Range object)

Regards,

Brian.