A left-exclusive Range class

[Cyrus Hall]

The current Range class in ruby is quite useful, however, I was recently
trying to capture a simple algorithm and ended up needing to express
left-exclusion. This is the one feature ruby currently does not support
in its Range class.

I did some digging into 1.8.4's Range implementation and it appears
that the changes needed to support left-exclusion would be minimal.
Rather, the problem seems syntactical. Currently '..' and '...' are
used to represent right-inclusion and -exclusion, respectively. This
syntax clearly can not be extended to left-exclusion. There appears to
have been a related debate about this on ruby-talk back in Oct. 2004,
with the conclusion being, to paraphrase the entire discussion, "this is
hard, lets forget about it for now."

Has this been discussed since 2004? Would people be interested in
finding a way to get left-exclusion into the Range class? This may be
something that has been addressed in all the language work going into
1.9/2.0, and if so, any pointers would be greatly appreciated.

Ciao,
Cyrus Hall

 

[Brian Guthrie]

The current Range class in ruby is quite useful, however, I was recently
trying to capture a simple algorithm and ended up needing to express
left-exclusion. This is the one feature ruby currently does not support
in its Range class.

I did some digging into 1.8.4's Range implementation and it appears
that the changes needed to support left-exclusion would be minimal.
Rather, the problem seems syntactical. Currently '..' and '...' are
used to represent right-inclusion and -exclusion, respectively. This
syntax clearly can not be extended to left-exclusion. There appears to
have been a related debate about this on ruby-talk back in Oct. 2004,
with the conclusion being, to paraphrase the entire discussion, "this is
hard, lets forget about it for now."

Has this been discussed since 2004? Would people be interested in
finding a way to get left-exclusion into the Range class? This may be
something that has been addressed in all the language work going into
1.9/2.0, and if so, any pointers would be greatly appreciated.

Ciao,
Cyrus Hall

You could try just appropriating a different piece of syntax and using
it for your own evil purposes, e.g. <<

class Fixnum
def <<(number)
(self + 1)..number
end
end

for i in 1<<10
# i starts at 2
end

Brian Guthrie

 

[Brian Guthrie]

You could try just appropriating a different piece of syntax and using
it for your own evil purposes, e.g. <<

class Fixnum
def <<(number)
(self + 1)..number
end
end

for i in 1<<10
# i starts at 2
end

Brian Guthrie

Of course by doing that you've totally just broken the existing
Fixnum#<<. Bummer, should've checked first. Ah, well.

Brian Guthrie

 

[Mike Fletcher]

Doesn't preserve the fact that it started as a left exclusive range, but
how about:

class Range
alias rig_initialize :initialize
def initialize( s, e, exclusive=false )
case exclusive
when false, true
orig_initialize( s, e, exclusive )
when :left
orig_initialize( s.succ, e )
when :right
orig_initialize( s, e, true )
when :both
orig_initialize( s.succ, e, true )
end
end
end

Then you can do Range.new( 1, 5, :left ) or Range.new( 1, 5, :right ) or
use the .. / ... operators as well.