From: Robert Klemme <shortcutter@...>
Date: 2007-10-12T20:56:13+09:00
Subject: Re: Array +=

2007/10/12, mortee <mortee.lists@kavemalna.hu>:
> Simon Mullis wrote:
> >>> (a ||= []) << [1, 2, 3, 4]
> > => [[1, 2, 3, 4]]
> >
> > But
> >
> >>> ( a ||= [] ) += [ 1, 2, 3, 4 ]
> > SyntaxError: compile error
> > (irb):28: syntax error, unexpected tOP_ASGN, expecting $end
> > ( a ||= [] ) += [ 1, 2, 3, 4 ]
> >                ^
> >         from (irb):28
> >
> > Why?
> >
> > I guess it's a precedence issue. '<<' is a method in the Array class.
> > '+=' is not.

No, it's not a precedence issue.  Btw, you are comparing apples and
oranges here: += and << are not equivalent (see also further below):

irb(main):001:0> a=%w{foo bar}
=> ["foo", "bar"]
irb(main):002:0> b=a.dup
=> ["foo", "bar"]
irb(main):003:0> c=a.dup
=> ["foo", "bar"]
irb(main):004:0> b << a
=> ["foo", "bar", ["foo", "bar"]]
irb(main):005:0> c += a
=> ["foo", "bar", "foo", "bar"]

<< adds the whole Array as one object while += "appends" the Array.
You rather want Array#concat.

> The difference is that << is a method, += is an assignment. (a ||= [])
> returns an object, on which you can call a method, but you can't assign
> to. You can only assign to the a variable itself, which (a ||= []) is
> not. I guess.

Exactly: the expression (a||=[]) is not an lvalue, i.e. cannot
assigned to.  But you can do

( a ||= [] ).concat [ 1, 2, 3, 4 ]

Simon, please also keep in mind that += and << have different
semantics. += will create a new Array while << appends to the current
one.

Kind regards

robert