In Ruby, why does nil[1]=1 evaluate to nil?

Question

For example:

nil[1]     #=> NoMethodError
nil[1]=1   #=> nil

It's not just syntax, as it happens with variables too:

a = nil
a[1]       #=> NoMethodError
a[1]=1     #=> nil

Oddly:

nil.method(:[]=)   #=> NameError
[].method(:[]=)    #=> #<Method...>

Ruby 2.3.0p0

Milkwhite

Can't reproduce on Ruby 2.2.4. Maybe a new feature or a bug in your version? – Tallage

Weird... Maybe it's a bug. – Insignia

I can only reproduce this in Ruby 2.3.0 and I'm going to presume it's a bug. It might be worth checking the bug tracker for this issue. – Dyche

Probably it is fixed already. Cannot be reproduced on ruby 2.3.0p71 (2016-03-30 revision 54426). – Constructivism

I believe the fix was reported here ("unexpected safe call"). larsch, how did you happen to stumble onto that? Everyone, raises your glasses! It's not everyday that a suspected bug turns out to actually be a bug. – Uranic

This is why we avoid the latest version – Breakout

I simple had an instance variable @idx = Hash.new that I was trying to use as @index[key] = value. The unittest was failing in strange ways because the assignment didn't fail on the nil value. – Milkwhite

@CarySwoveland Do you want to post that info as an answer so I can mark this question as a duplicate? – Jacquelinjacqueline

Thanks, @Jordan, but I'd prefer to leave it as a comment. – Uranic

Answer 1

Some random findings: [only in Ruby 2.3.0p0]

The method doesn't seem to exist:

nil.method(:[]=)      #=> NameError: undefined method `[]='
nil.respond_to?(:[]=) #=> false

And you can't invoke it using send:

nil.send(:[]=)        #=> NoMethodError: undefined method `[]='

Ruby evaluates neither the right hand side, nor the argument, i.e.

nil[foo]=bar

doesn't raise a NameError, although foo and bar are undefined.

The expression seems to be equivalent to nil:

$ ruby --dump=insns -e 'nil[foo]=bar'
== disasm: #<ISeq:<main>@-e>============================================
0000 trace            1                                               (   1)
0002 putnil
0003 leave

$ ruby --dump=insns -e 'nil'
== disasm: #<ISeq:<main>@-e>============================================
0000 trace            1                                               (   1)
0002 putnil
0003 leave

Proudman