From: Thomas Sawyer Date: 2011-11-07T00:41:38+09:00 Subject: [ruby-core:40782] [ruby-trunk - Feature #5534] Redefine Range class and introduce RelativeNumeric and RelativeRange Issue #5534 has been updated by Thomas Sawyer. "This way it would be clear what a range is --- just an infinite set of a certain form, with methods to work with it." How is that a Range at all? A Range is only "infinite" if one of the sentinels is infinite. "Mathematically, ranges (2..1) et (3..0) are equal --- both empty, but Ruby remembers their "bounds" and treats them differently." How is a Range to work without remembering the bounds? If anything I wish Ruby would understand "counting down" ranges. Currently `(3..1).to_a` returns `[]`. "x == y must be true if and only if x.include?(z) == y.include?(z) is true for every possible object z." For better or worse, Range serves a double (perhaps triple) purpose, e.g. it can serve as interval or it can serve as a way to define a sequential set. So the meaning of #== might not be considered that same in these cases. So Range was given a general definition that works for who it is designed --based on the range bounds. To do as you suggest would, I thing require separating Range into Interval and Sequence classes, so to speak, where you definition of #== is more fitting a Sequence, albeit one might still argument that the order of sequence should be significant too. ---------------------------------------- Feature #5534: Redefine Range class and introduce RelativeNumeric and RelativeRange http://redmine.ruby-lang.org/issues/5534 Author: Alexey Muranov Status: Open Priority: Normal Assignee: Category: core Target version: I started by commenting on Feature #4541, but ended up with proposing a new feature myself. I suggest to redefine the behavior of Range class so that all empty ranges be equal: (2..1) == (1..-1) and (2..1) == (1...1) and (2..1) == ('z'..'a') # => true In other fords, ranges `r1` and `r2` should be equal if and only if `r1.include?` and `r2.include?` give identical results for all inputs. (Why is it not `includes?` by the way?) Thus Range would simply be a way to store certain infinite sets. This change will result in not being able to slice an array `a` from beginning and from the end simultaneously with `a[1..-2]`. To resolve this, i propose to introduce `RelativeNumeric` and `RelativeRange` classes. Each `RelativeNumeric` would be a `Numeric` with an "anchor", which is an arbitrary symbol. For example: 3.from(:bottom) # would return a "relative" 3 with "anchor" :bottom One can define shortcuts `#from_bottom` for `#from(:bottom)` and `#from_top` for `#from_top`. A `RelativeRange` is a range with relative bounds. If bounds of a relative range r are relative to the same anchor and the range is seen to be empty, it should be equal to *the* empty relative range with this anchor. For example: (3.from(:center)..2.from(:center)) == (0.from(:center)...0.from(:center)) # => true Now, to do what is currently done by `a[1..-2]`, one can redefine `Array#slice` to use instead: a[1.from_bottom..(-1).from_top] What do you think? -- http://redmine.ruby-lang.org