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comp.lang.ruby

Lazy function definition pattern in Ruby?

Sam Kong

8/16/2007 5:59:00 PM

Hi,

Yesterday, I read a blog about lazy function definition pattern in
JavaScript at http://peter.michaux.ca/ar... .
It was interesting and insightful.

<snip>
Write a function foo that returns a Date object that holds the time
that foo was first called.

var foo = function() {
var t = new Date();
foo = function() {
return t;
};
return foo();
};
</snip>

In ruby, one would write the following way or something like that if
he wants to cache the first value.

def foo
@t or (@t = Time.new)
end

But the writer wants to remove the conditional part because it's run
every time the function is called.
JavaScript allows functions to be redefined very easily.
I think ruby allows it but not very easily.

I came up with this idea.

class Lazy
def method_missing *args
if args[0] == :foo
@t = Time.new
class << self
def foo
@t
end
end
return foo
end
end
end

But I believe that ruby gurus will have better ideas.
What would be the lazy function definition pattern in ruby?
And do you think it's useful?

Thanks in advance.

Sam
10 Answers

David A. Black

8/16/2007 8:15:00 PM

0

Wolfgang Nádasi-donner

8/16/2007 8:18:00 PM

0

Sam Kong wrote:
> Hi,
>
> Yesterday, I read a blog about lazy function definition pattern in
> JavaScript at http://peter.michaux.ca/ar... .
> It was interesting and insightful.
>
> <snip>
> Write a function foo that returns a Date object that holds the time
> that foo was first called.
>
> var foo = function() {
> var t = new Date();
> foo = function() {
> return t;
> };
> return foo();
> };
> </snip>
>
> In ruby, one would write the following way or something like that if
> he wants to cache the first value.
>
> def foo
> @t or (@t = Time.new)
> end
>
> But the writer wants to remove the conditional part because it's run
> every time the function is called.
> JavaScript allows functions to be redefined very easily.
> I think ruby allows it but not very easily.
>
> I came up with this idea.
>
> class Lazy
> def method_missing *args
> if args[0] == :foo
> @t = Time.new
> class << self
> def foo
> @t
> end
> end
> return foo
> end
> end
> end
>
> But I believe that ruby gurus will have better ideas.
> What would be the lazy function definition pattern in ruby?
> And do you think it's useful?
>
> Thanks in advance.
>
> Sam

You can define a method inside a method directly.

class Bar
def foo
@t = Time.new
def foo
@t
end
@t
end
end

x=Bar.new
p x.foo # => Thu Aug 16 22:17:17 +0200 2007
sleep 5
p x.foo # => Thu Aug 16 22:17:17 +0200 2007

Wolfgang Nádasi-Donner
--
Posted via http://www.ruby-....

Chris Carter

8/16/2007 8:21:00 PM

0

On 8/16/07, dblack@rubypal.com <dblack@rubypal.com> wrote:
> You could do this (and I think it's similar to the "once" technique
> [pattern?] that's used in the Date library and talked about in the
> Pickaxe):
>
> def my_time
> t = Time.now
> (class << self; self; end).class_eval do
> define_method(:my_time) { t }
> end
> t
> end

Why not just use a closure, and do it the way you would in JS?

foo = proc { x=Time.now; foo = proc { x } }; foo.call

Then you just access it with foo[]/foo.call

--
Chris Carter
concentrationstudios.com
brynmawrcs.com

Sam Kong

8/16/2007 8:58:00 PM

0

On Aug 16, 1:20 pm, "Chris Carter" <cdcar...@gmail.com> wrote:
> On 8/16/07, dbl...@rubypal.com <dbl...@rubypal.com> wrote:
>
> > You could do this (and I think it's similar to the "once" technique
> > [pattern?] that's used in the Date library and talked about in the
> > Pickaxe):
>
> > def my_time
> > t = Time.now
> > (class << self; self; end).class_eval do
> > define_method(:my_time) { t }
> > end
> > t
> > end
>
> Why not just use a closure, and do it the way you would in JS?
>
> foo = proc { x=Time.now; foo = proc { x } }; foo.call
>
> Then you just access it with foo[]/foo.call
>
> --
> Chris Carter
> concentrationstudios.com
> brynmawrcs.com

I also thought of it.
But foo.call isn't so good.
I think JavaScript's syntax is better than ruby's about returning a
function.

Sam

Sam Kong

8/16/2007 9:01:00 PM

0

On Aug 16, 1:18 pm, "Wolfgang N?dasi-Donner" <ed.oda...@wonado.de>
wrote:
> Sam Kong wrote:
> > Hi,
>
> > Yesterday, I read a blog about lazy function definition pattern in
> > JavaScript athttp://peter.michaux.ca/ar....
> > It was interesting and insightful.
>
> > <snip>
> > Write a function foo that returns a Date object that holds the time
> > that foo was first called.
>
> > var foo = function() {
> > var t = new Date();
> > foo = function() {
> > return t;
> > };
> > return foo();
> > };
> > </snip>
>
> > In ruby, one would write the following way or something like that if
> > he wants to cache the first value.
>
> > def foo
> > @t or (@t = Time.new)
> > end
>
> > But the writer wants to remove the conditional part because it's run
> > every time the function is called.
> > JavaScript allows functions to be redefined very easily.
> > I think ruby allows it but not very easily.
>
> > I came up with this idea.
>
> > class Lazy
> > def method_missing *args
> > if args[0] == :foo
> > @t = Time.new
> > class << self
> > def foo
> > @t
> > end
> > end
> > return foo
> > end
> > end
> > end
>
> > But I believe that ruby gurus will have better ideas.
> > What would be the lazy function definition pattern in ruby?
> > And do you think it's useful?
>
> > Thanks in advance.
>
> > Sam
>
> You can define a method inside a method directly.
>
> class Bar
> def foo
> @t = Time.new
> def foo
> @t
> end
> @t
> end
> end
>
> x=Bar.new
> p x.foo # => Thu Aug 16 22:17:17 +0200 2007
> sleep 5
> p x.foo # => Thu Aug 16 22:17:17 +0200 2007
>
> Wolfgang N?dasi-Donner
> --
> Posted viahttp://www.ruby-....

I didn't know defining the same method in a method works.
It's very similar to the one David suggested above.
But the syntax is easier and more intuitive.
It's almost same as JavaScript's pattern.
It's good to know.^^

Thanks.

David A. Black

8/16/2007 10:29:00 PM

0

Logan Capaldo

8/16/2007 10:47:00 PM

0

On 8/16/07, dblack@rubypal.com <dblack@rubypal.com> wrote:
> A possible problem with that code is that it only works for one
> instance of Bar:

That's easily fixable, just s/@/@@/

David A. Black

8/16/2007 11:04:00 PM

0

Logan Capaldo

8/16/2007 11:08:00 PM

0

On 8/16/07, dblack@rubypal.com <dblack@rubypal.com> wrote:
> Hi --
>
> On Fri, 17 Aug 2007, Logan Capaldo wrote:
>
> > On 8/16/07, dblack@rubypal.com <dblack@rubypal.com> wrote:
> >> A possible problem with that code is that it only works for one
> >> instance of Bar:
> >
> > That's easily fixable, just s/@/@@/
>
> As long as you don't want to do the same thing in a subclass....
Everything is a tradeoff.

David A. Black

8/16/2007 11:38:00 PM

0