Is my method defined?

[Fredrik]

I have a method by the name methodA. I want to access this method like
this

a = 'methodA'
eval(a)

But how do I know if the variable a actually holds the name of a
defined method? An exception is raised if I try to run eval(a) with an
incorrect method name, but I need to know this before I call eval(a).
How do I do that?

I found this solution:

def method?(arg)
begin
method(a)
rescue
nil
end
end

which does work, but why is this function "method?" not already in the
Ruby language then?

 

[Stefano Crocco]

Alle Saturday 13 September 2008, Fredrik ha scritto:

I have a method by the name methodA. I want to access this method like
this

a = 'methodA'
eval(a)

But how do I know if the variable a actually holds the name of a
defined method? An exception is raised if I try to run eval(a) with an
incorrect method name, but I need to know this before I call eval(a).
How do I do that?

I found this solution:

def method?(arg)
begin
method(a)
rescue
nil
end
end

which does work, but why is this function "method?" not already in the
Ruby language then?

In my opinion, for two reasons:
1) the name is misleading. Methods ending in ? usually are method which only
answer a Yes/No question, without taking any action. The method you propose,
instead, perform an action and doesn't give an answer to a question. If I were
a user seeing a method called 'method?', I'd think it's a synonym for
respond_to?, or similar to it.
2) This functionality isn't needed very often, and it's very easy to write
your own method if you need it, as you have done.

Stefano

 

[Sebastian Hungerecker]

I have a method by the name methodA. I want to access this method like
this

a = 'methodA'
eval(a)

You should use send if a only contains a methodname.

But how do I know if the variable a actually holds the name of a
defined method?

respond_to? method_name

HTH,
Sebastian

 

[Fredrik]

You should use send if a only contains a methodname.


respond_to? method_name

HTH,
Sebastian

I got it! respond_to? is what I was looking for. Thanks!
I will open my Ruby book and look into that send thing...

/Fredrik

 

[Sebastian Hungerecker]

I will open my Ruby book and look into that send thing...

send is quite simple actually:
send("foo") is the same as foo
send("foo", bar) is the same as foo(bar)
object.send("foo", bar) is the same as object.foo(bar)
That's basically it.

HTH,
Sebastian

 

[David A. Black]

Hi --

send is quite simple actually:
send("foo") is the same as foo
send("foo", bar) is the same as foo(bar)
object.send("foo", bar) is the same as object.foo(bar)
That's basically it.

Almost
NoMethodError: private method `x' called for #<C:0x3c95ec>


David

--
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails January 12-15 Fort Lauderdale, FL
Advancing with Rails January 19-22 Fort Lauderdale, FL *
* Co-taught with Patrick Ewing!
See http://www.rubypal.com for details and updates!

 

[Fredrik]

send is quite simple actually:
send("foo") is the same as foo
send("foo", bar) is the same as foo(bar)
object.send("foo", bar) is the same as object.foo(bar)
That's basically it.

HTH,
Sebastian

So how is send better than eval then?
send("foo")
eval("foo")
Same, right?

 

[Fredrik]

You should use send if a only contains a methodname.


respond_to? method_name

HTH,
Sebastian

Now I am utterly confused. Can anyone explain THIS behaviour :

### File : foo.rb ###
def requiredfoo
"I'm here!"
end
#####################

irb> require 'foo.rb'
irb> respond_to? 'requiredfoo'
=> false

irb> def foo ; "I'm here!" ; end
irb> respond_to? 'foo'
=> true

That doesn't make sense at all, right?

/Fredrik

 

[Sebastian Hungerecker]

So how is send better than eval then?

It's more specific. And you can call the method on an object other than self
without string manipulation.

send("foo")
eval("foo")
Same, right?

Yes, but
send("system('rm -rf /')") # Error
eval("system('rm -rf /')") # Works

 

[Sebastian Hungerecker]

irb> require 'foo.rb'
irb> respond_to? 'requiredfoo'
=> false

irb> def foo ; "I'm here!" ; end
irb> respond_to? 'foo'
=> true

respond_to? returns false for private methods. If you define a method outside
of a class/module, it's a private instance method of Object by default, except
when you define it in irb in which case it will be public for whatever reason.

HTH,
Sebastian