On parallel assignment


#1

The parallel assignment syntax
[a,b] = [c,d] is not allowed

I have to write
a, b = [c,d]

Going deeper
[a, [b, c]] = [1, [2, 3]] is not allowed

I have to write
a, (b, c) = [1, [2, 3]]

I’m sure there is a good reason these constructs are disallowed.
Can somebody enlighten me?

Christer


#2

Christer N. wrote:

The parallel assignment syntax
[a,b] = [c,d] is not allowed

The left side is an array constructor. In other words: take value of a
and value of b and create a two element array. There is no assignment
here. You cannot create an array on the left side of an assignment and
try to assign to it:

[]=[1,2]
SyntaxError: compile error
(irb):38: syntax error
[]=[1,2]
^
from (irb):38
from â?¥:0

Array.new = [1,2,3]
NoMethodError: undefined method `new=’ for Array:Class
from (irb):39
from â?¥:0

(Array.new) = [1,2,3]
SyntaxError: compile error
(irb):40: syntax error
(Array.new) = [1,2,3]
^
from (irb):40
from â?¥:0

I have to write
a, b = [c,d]

I prefer

a,b=1,2
=> [1, 2]

a
=> 1

b
=> 2

Going deeper
[a, [b, c]] = [1, [2, 3]] is not allowed

I have to write
a, (b, c) = [1, [2, 3]]

You can as well do

a,(b,c)=1,[2,3]
=> [1, [2, 3]]

a
=> 1

b
=> 2

c
=> 3

The outermost array is implicit but you must create nested arrays
explicitely.

I’m sure there is a good reason these constructs are disallowed.
Can somebody enlighten me?

Hope so…

Kind regards

robert

#3

Christer N. wrote:

a, (b, c) = [1, [2, 3]]

I’m sure there is a good reason these constructs are disallowed.
Can somebody enlighten me?

Christer

You should check out one of the websites that list upcoming syntax
changes in Ruby 2.0 - parallel assignment is undergoing relatively
significant changes to weed out possible ambiguity when it’s used.

As well as nested parallel assignment is concerned, I’d say avoid - it’s
potentially a very confusing construct, and makes your intent unclear.
Arrays usually imply the storing of values of homogenous type and
purpose - when used in parallel assignment, it means you’re most
probably not using them that way, and should probably use a different
data structure to store the information to make your code easier to
understand.

David V.