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comp.lang.ruby

A question on singleton methods

mikeleonard

3/8/2008 11:33:00 PM

Hello,

I have a hash full of objects, like so:

a_hash = { :a => "alpha", :b => "beta" }

I don't I need to define a singleton method for one of the items in
the class. I thought I could do:

def a_hash.fetch(:a).to_s
"ALPHA"
end

But all this gets me is a big fat error:

t.rb:2: syntax error, unexpected tSYMBEG, expecting ')'
def a_hash.fetch(:a).to_s
^

This is just a minimal example. In real life, the objects in the hash
have been generated programatically, so I don't have variable
references for them -- just the keys.

Is it possible that there is a way around this error? If not, is there
another way to do what I'm trying to do? I'm pretty new to this, so
apologies in advance if it is an obvious mistake.

Thank you,

Mike Leonard
10 Answers

Arlen Cuss

3/8/2008 11:47:00 PM

0

[Note: parts of this message were removed to make it a legal post.]

Hi Mike,

On Sun, Mar 9, 2008 at 10:34 AM, mike leonard <mikeleonard@gmail.com> wrote:

> def a_hash.fetch(:a).to_s
> "ALPHA"
> end
>

The problem here is that you can't involve a method call/dispatch (or two)
in the method definition. "a_hash.fetch(:a).to_s" has to make at least one
method call (a_hash.fetch(:a)), and then one defines the method on the
resulting object. Method definitions don't work this way (they need to be
resolvable in the current context, I believe). But there's a way to get
there:

>> a ={:a => "q"}
=> {:a=>"q"}
>> b = a[:a]
=> "q"
>> a[:a].object_id
=> -606856898
>> b.object_id
=> -606856898
>> def b.some_method
>> puts "Hi."
>> end
=> nil
>> b.some_method
Hi.
=> nil
>> a[:a].some_method
Hi.
=> nil
>>

By the by, I'm not sure redefining a String's #to_s will have any effect. It
sounds strange - perhaps you're better off building a class for those
objects and defining to_s on that?

Cheers,
Arlen

Sean O'Halpin

3/8/2008 11:55:00 PM

0

On Sat, Mar 8, 2008 at 11:34 PM, mike leonard <mikeleonard@gmail.com> wrote:
> Hello,
>
> I have a hash full of objects, like so:
>
> a_hash = { :a => "alpha", :b => "beta" }
>
> I don't I need to define a singleton method for one of the items in
> the class. I thought I could do:
>
> def a_hash.fetch(:a).to_s
> "ALPHA"
> end
>
> But all this gets me is a big fat error:
>
> t.rb:2: syntax error, unexpected tSYMBEG, expecting ')'
> def a_hash.fetch(:a).to_s
> ^
>
> This is just a minimal example. In real life, the objects in the hash
> have been generated programatically, so I don't have variable
> references for them -- just the keys.
>
> Is it possible that there is a way around this error? If not, is there
> another way to do what I'm trying to do? I'm pretty new to this, so
> apologies in advance if it is an obvious mistake.
>
> Thank you,
>
> Mike Leonard
>
>
You could try this:

a_hash = { :a => "alpha", :b => "beta" }

def (a_hash[:a]).to_s
"ALPHA"
end

a_hash.each do |key, value|
puts "#{key} = #{value.to_s}"
end
__END__
a = ALPHA
b = beta

Regards,
Sean

Daniel Brumbaugh Keeney

3/9/2008

0

On Sat, Mar 8, 2008 at 5:47 PM, Arlen Cuss <celtic@sairyx.org> wrote:
> On Sun, Mar 9, 2008 at 10:34 AM, mike leonard <mikeleonard@gmail.com> wrote:
>
> > def a_hash.fetch(:a).to_s
> > "ALPHA"
> > end

Congratulations Sean O'Halpin, you beat me to send button.

> By the by, I'm not sure redefining a String's #to_s will have any effect. It
> sounds strange - perhaps you're better off building a class for those
> objects and defining to_s on that?

I have no idea what the possible purpose of redefining String#to_s is.
However, I think this brings to mind an interesting idea. As part of
Ruby's duck typing initiative, many methods call to_str on objects
they expect to be strings. Having String#to_str return something other
than self could produce some extremely confusing results.

Daniel Brumbaugh Keeney

Jeff Swope

3/9/2008 12:22:00 AM

0

[Note: parts of this message were removed to make it a legal post.]

unsubscribe

On Sat, Mar 8, 2008 at 6:34 PM, mike leonard <mikeleonard@gmail.com> wrote:

> Hello,
>
> I have a hash full of objects, like so:
>
> a_hash = { :a => "alpha", :b => "beta" }
>
> I don't I need to define a singleton method for one of the items in
> the class. I thought I could do:
>
> def a_hash.fetch(:a).to_s
> "ALPHA"
> end
>
> But all this gets me is a big fat error:
>
> t.rb:2: syntax error, unexpected tSYMBEG, expecting ')'
> def a_hash.fetch(:a).to_s
> ^
>
> This is just a minimal example. In real life, the objects in the hash
> have been generated programatically, so I don't have variable
> references for them -- just the keys.
>
> Is it possible that there is a way around this error? If not, is there
> another way to do what I'm trying to do? I'm pretty new to this, so
> apologies in advance if it is an obvious mistake.
>
> Thank you,
>
> Mike Leonard
>
>

mikeleonard

3/9/2008 3:26:00 AM

0

> By the by, I'm not sure redefining a String's #to_s will have any effect. It
> sounds strange - perhaps you're better off building a class for those
> objects and defining to_s on that?

Agreed. I'm really wanting to redefine #to_s for a String; it was just
the first method that I thought of when I was typing up a minimal
example of what I was trying to do. The important bit was the thing
with the singleton method. Many thanks for your help.

--Mike

mikeleonard

3/9/2008 3:27:00 AM

0

On Mar 8, 9:26 pm, mike leonard <mikeleon...@gmail.com> wrote:
> > By the by, I'm not sure redefining a String's #to_s will have any effect.. It
> > sounds strange - perhaps you're better off building a class for those
> > objects and defining to_s on that?
>
> Agreed. I'm really wanting to redefine #to_s for a String; it was just
> the first method that I thought of when I was typing up a minimal
> example of what I was trying to do. The important bit was the thing
> with the singleton method. Many thanks for your help.
>
> --Mike

Sorry, that should read I'm *not* really wanting to redefine #to_s for
a String.

7stud --

3/9/2008 4:40:00 AM

0

mike leonard wrote:
> Hello,
>
> I have a hash full of objects, like so:
>
> a_hash = { :a => "alpha", :b => "beta" }
>
> I don't I need to define a singleton method for one of the items in
> the class. I thought I could do:
>
> def a_hash.fetch(:a).to_s
> "ALPHA"
> end
>

a_hash = { :a => "alpha", :b => "beta" }

val = a_hash.fetch(:a)
puts val.class #String

$ri String#to_s

------------------------------------------------------------ String#to_s
str.to_s => str
str.to_str => str
------------------------------------------------------------------------
Returns the receiver.


And you don't define methods like this:

def "sayhi"
puts 'hi'
end

You define them like this:

def sayhi
puts 'hi'
end

A method name is not a string.


--
Posted via http://www.ruby-....

Arlen Cuss

3/9/2008 5:00:00 AM

0

[Note: parts of this message were removed to make it a legal post.]

Hi,

On Sun, Mar 9, 2008 at 3:39 PM, 7stud -- <bbxx789_05ss@yahoo.com> wrote:

> A method name is not a string.
>

I think you're a bit off the mark. He's redefining an existing method with a
new singleton one.

>> h = {:a => 1}
=> {:a=>1}
>> puts h.class
=> Hash

$ ri Hash#length
------------------------------------------------------------ Hash#length
hsh.length => fixnum
hsh.size => fixnum
------------------------------------------------------------------------


Yet:

>> def h.length
>> puts "This works."
>> end
=> nil
>>

A method name is not a fixnum, either. He's not trying to define a method
called 'alpha', it was changing the definition of to_s.

Arlen

Sebastian Hungerecker

3/9/2008 9:30:00 AM

0

mike leonard wrote:
> =A0 =A0 def a_hash.fetch(:a).to_s
> =A0 =A0 =A0 =A0 "ALPHA"
> =A0 =A0 end

This doesn't work because ruby thinks you want to define a_hash.fetch and i=
s=20
then confused by the symbol in the parameter list. This however will work:

def (a_hash.fetch(:a)).to_s
"ALPHA"
end


HTH,
Sebastian
=2D-=20
Jabber: sepp2k@jabber.org
ICQ: 205544826

mikeleonard

3/9/2008 1:43:00 PM

0

On Mar 9, 4:29 am, Sebastian Hungerecker <sep...@googlemail.com>
wrote:
> mike leonard wrote:
> >     def a_hash.fetch(:a).to_s
> >         "ALPHA"
> >     end
>
> This doesn't work because ruby thinks you want to define a_hash.fetch and is
> then confused by the symbol in the parameter list. This however will work:
>
> def (a_hash.fetch(:a)).to_s
>   "ALPHA"
> end
>
> HTH,
> Sebastian
> --
> Jabber: sep...@jabber.org
> ICQ: 205544826

That got it. Thanks to everyone who chimed in to clear that up for me.