I often use the ||= idiom in ruby for late initialization. But was recently looking for an idiom that would allow me to overwrite a variable if the right side is defined without repeating myself.
I have used the following:
var = val if val
but I have to repeat val and that violates DRY. I thought maybe &&= would work, but did not
var &&= val
That only works if var and val are non-nil.
Any ideas out there?
Steve Tuckner
Hi --
I often use the ||= idiom in ruby for late initialization. But was
recently looking for an idiom that would allow me to overwrite a
variable if the right side is defined without repeating myself.I have used the following:
var = val if val
but I have to repeat val and that violates DRY.
$ ruby -e 'a = b if b'
-e:1: undefined local variable or method `b' for main:Object
(NameError)
I thought maybe &&= would work, but did not
var &&= val
That only works if var and val are non-nil.
Any ideas out there?
I'm not sure it's possible, but it might depend on what state you want
to leave things in under what conditions. Even if you do this:
var = val if defined?(val)
var ends up being nil if val is *not* defined -- so that's
indistinguishable from the case where val exists and is nil. If it's
OK for var to be nil, then that would be OK, otherwise not.
David
On Wed, 29 Sep 2004, stevetuckner wrote:
--
David A. Black
dblack@wobblini.net
stevetuckner wrote:
I often use the ||= idiom in ruby for late initialization. But was recently looking for an idiom that would allow me to overwrite a variable if the right side is defined without repeating myself.
I have used the following:
var = val if val
but I have to repeat val and that violates DRY. [...]
Any ideas out there?
require 'binding_of_caller'
def and_set(name, value)
return if value.nil?
Binding.of_caller do |context|
eval("#{name} = ObjectSpace._id2ref(#{value.id})", context)
end
end
and_set(:var, val)
Not sure if you were looking for this kind of solution though. 
Regards,
Florian Gross
binding_of_caller.rb (2.14 KB)
Well, callcc won't rollback vars. I tried:
val = 10
callcc {|c| (var=val) ? () : c.call }
Which might have worked in future Ruby, but alas no. But then what about
simple transactions? Isn't there a lib for something like:
transaction(:var) do
(var = val) ? commit : rollback
end
Just some wild guesses.
T.
On Tuesday 28 September 2004 04:29 pm, stevetuckner wrote:
I often use the ||= idiom in ruby for late initialization. But was
recently looking for an idiom that would allow me to overwrite a
variable if the right side is defined without repeating myself.I have used the following:
var = val if val
but I have to repeat val and that violates DRY. I thought maybe &&=
would work, but did notvar &&= val
That only works if var and val are non-nil.
Any ideas out there?
A sentence disappeared: I meant to say:
Doesn't that not work?
followed by example:
$ ruby -e 'a = b if b'
-e:1: undefined local variable or method `b' for main:Object
(NameError)
David
On Wed, 29 Sep 2004, David A. Black wrote:
--
David A. Black
dblack@wobblini.net
Hi --
> I often use the ||= idiom in ruby for late initialization. But was
> recently looking for an idiom that would allow me to overwrite a
> variable if the right side is defined without repeating myself.
>
> I have used the following:
>
> var = val if val
>
> but I have to repeat val and that violates DRY.$ ruby -e 'a = b if b'
On Wed, Sep 29, 2004 at 05:37:21AM +0900, David A. Black wrote:
On Wed, 29 Sep 2004, stevetuckner wrote:
===
a
-e:1: undefined local variable or method `b' for main:Object
(NameError)
--
Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com
Florian Gross wrote:
require 'binding_of_caller'
def and_set(name, value)
return if value.nil?
Binding.of_caller do |context|
eval("#{name} = ObjectSpace._id2ref(#{value.id})", context)
end
endand_set(:var, val)
Not sure if you were looking for this kind of solution though.
It probably does the trick (though I can't understand the binding_of_caller without more study), but was quite a bit more heavyweight than I was looking for. Thanks for trying... 
Steve Tuckner
David A. Black wrote:
$ ruby -e 'a = b if b'
-e:1: undefined local variable or method `b' for main:Object
(NameError)
Perhaps I should clarify how I was using this. Lets say I have a variable with a default value and I want to overwrite it with something else as long as it is non-nil.
a = "default"
a = foo.value if foo.value
I just wanted to see if there was a simple way to avoid the double foo.value that I was missing. Thanks for all your responses.
Steve Tuckner
Hi --
On Wed, 29 Sep 2004, Mauricio Fernández wrote:
On Wed, Sep 29, 2004 at 05:37:21AM +0900, David A. Black wrote:
> Hi --
>
> On Wed, 29 Sep 2004, stevetuckner wrote:
>
> > I often use the ||= idiom in ruby for late initialization. But was
> > recently looking for an idiom that would allow me to overwrite a
> > variable if the right side is defined without repeating myself.
> >
> > I have used the following:
> >
> > var = val if val
> >
> > but I have to repeat val and that violates DRY.
>
> $ ruby -e 'a = b if b'
===
a
Ummmm, no. Look at Steve's code again.
David
--
David A. Black
dblack@wobblini.net
Steve Tuckner wrote:
a = "default"
a = foo.value if foo.valueI just wanted to see if there was a simple way to avoid the double
foo.value that I was missing. Thanks for all your responses.
Ah, well _that's_ easy:
a = foo.value || "default"
Gavin
I was reading up this thread (I've been in meetings & working for
the past 8 hrs or so, and thus neglecting my duties as a netizen)
wondering if anyone had suggested this yet. Will I be the one to post
it? Will I?
*sigh*
I will not that (in the original poster's terms), this is
var = val || var
which still repeats (but the computationally cheap var instead of the
presumably costly val). There may be an idiom that avoids even that,
but I'll be darned if I know what it is.
-- MarkusQ
P.S. In Icon (IIRC) you can write either
var \= val
or
var /= val
one of which effectively does
var ||= val
and the other
var = val || var
but the semantics of icon are sufficiently different from ruby to make
this only an approximate analogy.
On Tue, 2004-09-28 at 21:30, Gavin Sinclair wrote:
Steve Tuckner wrote:
> a = "default"
> a = foo.value if foo.value
>
> I just wanted to see if there was a simple way to avoid the double
> foo.value that I was missing. Thanks for all your responses.Ah, well _that's_ easy:
a = foo.value || "default"
Gavin
I will not that (in the original poster's terms), this is
var = val || var
or
a = foo.value || a
which more clearly shows the value (!) of this construct. Smart.
Thinking about my own code, I tend to write
var = val unless val.nil?
which is even more verbose, but seems a bit clearer, and works for the case
where val=false. However I end up assigning to a temporary variable 'val' in
the case of a complex expression. Maybe I'll adopt your pattern instead,
although I think it would leave some people scratching their heads at first.
There is the more general case of
if some.complex.expression
... do something with some.complex.expression
end
which I typically write as
if (val = some.complex.expression)
... do something with val
end
This does suggest some other solutions:
a = tmp if tmp = foo.value ## fails - first tmp is treated as method
if tmp = foo.value; a = tmp; end
(tmp = foo.value) && (a = tmp)
Icky though. It might be nice if the value of the condition evaluated in an
'if', or the LHS of && or ?:, were somehow available to the body/RHS. I
can't think how it would work nicely:
foo.value && a = $# #yuk
Regards,
Brian.