From: "alexeymuranov (Alexey Muranov)" <redmine@...>
Date: 2012-11-28T23:30:13+09:00
Subject: [ruby-core:50259] [ruby-trunk - Feature #5534] Redefine Range class and introduce RelativeNumeric and RelativeRange


Issue #5534 has been updated by alexeymuranov (Alexey Muranov).


=begin
I propose to cancel the second part of my proposal: about (({RelativeNumeric})) and
(({RelativeRange})).  When i look back at it now, it looks quite crazy and not
particularly useful.

However, i still would like to see the behavior of (({Range})) changed, as in the
first part of the proposal.  In other words, i would like (({Range})) become a ((*lazy
ordered set*)), infinite or finite.

I think i proposed (({RelativeNumeric})) simply because i didn't dare to propose
to deprecate the usage like

  b = a[1..-2]

for an array (({a})) without proposing some similarly looking replacement.
Now i've decided i would prefer this usage be deprecated.

How about a new method (({Array#trim(fixnum, fixnum)})) to write

  b = a.trim(1,1)

instead of

  b = a[1..-2]

? (I can start a new Feature Request if a discussion is necessary.)

It looks strange to have to convert a pair of numbers ((*m*)) and ((*n*)) into a range
(({m..(-1-n}))) to simply ask an array to remove ((*m*)) elements from the beginning and
((*n*)) elements from the end.  If you think about what the range
(({m..(-1-n)})) actually ((*is*)), then the syntax (({a[m..(-1-n)]})) looks plain weird :).
=end

----------------------------------------
Feature #5534: Redefine Range class and introduce RelativeNumeric and RelativeRange
https://bugs.ruby-lang.org/issues/5534#change-34085

Author: alexeymuranov (Alexey Muranov)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: next minor


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?



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