Feature #8190

Truly parallel assignment

Added by Lavir the Whiolet over 2 years ago. Updated over 2 years ago.

[ruby-core:<unknown>]
Status:Rejected
Priority:Normal
Assignee:Yukihiro Matsumoto

Description

=begin
i = 0
s = [1,2,3]
i, s[i] =s[i], 7
p s
#=> [1, 7, 3]

s is expected to be [7, 2, 3].
=end

History

#1 Updated by Tom Wardrop over 2 years ago

=begin
Hmm, yeah that request seems reasonable. Right now it seems to be assigning left-to-right. So (({i})) is assigned (({s[i]})), making (({i})) equal to (({1})), only then is (({s[i]})) evaluated and assigned (({7})) as if it's a separate expression on a new line.

Could be a difficult change to make at least in terms of backwards compatibility, but the suggested behaviour is less surprising than the current behavior in my opinion.
=end

#2 Updated by Marc-Andre Lafortune over 2 years ago

  • Category set to core
  • Assignee set to Yukihiro Matsumoto

My first reflex would be to say "don't write code like this".

Do you have an actual use case?

FWIW, CoffeeScript handles this case the same way as MRI does.

#3 Updated by Nobuyoshi Nakada over 2 years ago

The evaluation obeys the left-to-right order.
It doesn't seem like a bug to me.

#4 Updated by Hans Mackowiak over 2 years ago

=begin

((%Lavir_the_Whiolet when you change this you will break code like%))

(({a,b = b,a}))

wich is a feature of ruby

#5 Updated by Anonymous over 2 years ago

I think it's good that Lavir is bringing this up, although I share MA's
feeling. I don't see how his proposal it would break "a, b = b, a". On the
contrary, "a, b = b, a" would only break if the assignment indeed worked
sequentially looks questioningly @ wardrop & nobu. I do not know whether
this slightly counterintuitive behavior is a problem of that requires
change in the code behavior, or just better documentation to remove the
feeling of surprise, but I do not regard design by jury the best approach
in this case.

#6 Updated by Yukihiro Matsumoto over 2 years ago

  • Category deleted (core)
  • Status changed from Open to Rejected

As Nobu stated the evaluation order goes from left to right.
And if you change this, you will introduce incompatibility.

Matz.

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