From: Gavin Sinclair <gsinclair@...>
Date: 2004-10-12T11:17:00+09:00
Subject: Re: Ruby Directions (was Re: quality of error messages)

On Tuesday, October 12, 2004, 11:32:31 AM, David wrote:

> With all the recent talk about ranges (much, but not all, of which
> I've read), I've been trying to put my finger on why I find the idea
> of a mutable Range sort of paradoxical.  I'm far from having any kind
> of sound theoretical grounding or rationale... but anyway:

> I think of a range as kind of a fact.  For example:  (0..10) is "the
> fact of being between 0 and 10, inclusive".  In which case, changing
> *that fact* -- e.g., having "the fact of being between 0 and 10"
> *itself* become "the fact of being between 6 and 12" -- makes no
> sense.  I guess I think of ranges almost more like numbers; it would
> make sense to me if (0..10) were literally the same object everywhere
> it appeared.  

I agree, David, FWIW, but one doesn't even need to go that far in
justifying immutability.  Many things are "value objects" by design
choice.  A Name object, for instance: first, middle, last.  If you're
working with a system that uses Name objects, they're likely to be
immutable by design.  A customer changes their name?  Create a new
object.

Some things are best treated "just like a number".  Ranges are one of
them (they're just two numbers and a flag, after all).  Some say
strings should be immutable as well, and I think it's a justifiable
position.

Gavin