How to test for existence of a method

Ruby
1

I’m a little stumped on this one. I looked at several ideas, and none
panned out.

I have a string, which may name a method in a module required by my
program. How can I test programmatically to see if the method actually
exists? I’d like to do something better than just catch an exception
generated when I call a non-existent method.

Is there a way to do this?

Tom

2

Object#respond_to? doesn’t do it for you?

3

Thanks. Does just fine - now that I know about it. Handy!

t.

Chris K. wrote:

exists? I’d like to do something better than just catch an exception

–

Tom C., MS MA, LMHC - Private practice Psychotherapist
Bellingham, Washington, U.S.A: (360) 920-1226
<< [email protected] >> (email)
<< TomCloyd.com >> (website)
<< sleightmind.wordpress.com >> (mental health weblog)
4

2009/4/22 Tom C. [email protected]:

Thanks. Does just fine - now that I know about it. Handy!

t.

Chris K. wrote:

Object#respond_to? doesn’t do it for you?

This is unsafe:

irb(main):001:0> o = Object.new
=> #Object:0x10171bb8
irb(main):002:0> def o.method_missing(s,*a,&b) s == :foo ? “yes” : super
end
=> nil
irb(main):003:0> o.respond_to? :foo
=> false
irb(main):004:0> o.respond_to? “foo”
=> false
irb(main):005:0> o.foo
=> “yes”
irb(main):006:0>

The best approach is to actually invoke the method and deal with
exceptions IMHO (-> duck typing).

Kind regards

robert

5

Jesús Gabriel y Galán wrote:

=> #Object:0x10171bb8

Just a question: isn’t it good practice to override respond_to? when
you use method missing to handle things, so that it “tells the truth”?
Or is it something people don’t usually do?

Jesus.

How would you do this?

t.

–

Tom C., MS MA, LMHC - Private practice Psychotherapist
Bellingham, Washington, U.S.A: (360) 920-1226
<< [email protected] >> (email)
<< TomCloyd.com >> (website)
<< sleightmind.wordpress.com >> (mental health weblog)
6

On Wednesday 22 April 2009 05:43:06 Robert K. wrote:

The best approach is to actually invoke the method and deal with
exceptions IMHO (-> duck typing).

I think that depends on the situation. If respond_to? is known to work,
it’s
still a lot more duck-friendly than kind_of?, and is probably the
easiest
choice, unless it really would be exceptional for it not to have that
method.

But yes, it’s true – overriding respond_to? gets you partway there, but
you
never know how an object is going to respond to a method until you call
it.
After all, from your perspective, there’s really no difference between

class Foo
end

and

class Foo
def foo
raise NoMethodError …
end
end

Once you consider that, it becomes obvious that there may be a situation
where
respond_to? either can’t be reliable, or would be expensive (for
example, if
you’re doing something like DRb).

7

On Wed, Apr 22, 2009 at 7:34 PM, Tom C. [email protected] wrote:

How would you do this?
Well by mirroring the logic of method_missing
two cases were this can be done simply

def method_missing *args, &blk
delegation.send *args, &blk
end

def respond_to? name
delegation.respond_to? name
end

def method_missing *args, &blk
fetch( args.first.to_sym ){ super } # déjà vu :wink:
end

def respond_to? name
has_key name.to_sym
end

==========================

Jesus’ question was shall one do this, I believe that more often than
not one shall indeed do this.
Robert’s warning remains very valuable though, because I believe that
very often one does
not go through the trouble.

Cheers
Robert

8

On Wed, Apr 22, 2009 at 12:43 PM, Robert K.
[email protected] wrote:

irb(main):003:0> o.respond_to? :foo
=> false
irb(main):004:0> o.respond_to? “foo”
=> false
irb(main):005:0> o.foo
=> “yes”
irb(main):006:0>

Just a question: isn’t it good practice to override respond_to? when
you use method missing to handle things, so that it “tells the truth”?
Or is it something people don’t usually do?

Jesus.