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comp.lang.ruby

Can a TkVariable store a symbol?

Alex DeCaria

4/7/2007 7:19:00 PM

If I put a symbol into a TkVariable, it returns a string. The following
code
illustrates this:

require 'tk'
a = TkVariable.new
a.value = :hi
puts a.value == :hi
puts a.value == "hi"

This results in

"false"
"true"

My question is, "Is this what's supposed to happen, or is this a bug?"

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

Wim Vander Schelden

4/7/2007 11:16:00 PM

0

On Sun, 8 Apr 2007 04:18:59 +0900
Alex DeCaria <alex.decaria@millersville.edu> wrote:

> If I put a symbol into a TkVariable, it returns a string. The following
> code
> illustrates this:
>
> require 'tk'
> a = TkVariable.new
> a.value = :hi
> puts a.value == :hi
> puts a.value == "hi"
>
> This results in
>
> "false"
> "true"
>
> My question is, "Is this what's supposed to happen, or is this a bug?"
>

I think this is supposed to happen, as

:hi == 'hi'

also returns false.

Wim

Alex DeCaria

4/8/2007 1:31:00 AM

0

Wim Vander Schelden wrote:
> On Sun, 8 Apr 2007 04:18:59 +0900
> Alex DeCaria <alex.decaria@millersville.edu> wrote:
>
>> This results in
>>
>> "false"
>> "true"
>>
>> My question is, "Is this what's supposed to happen, or is this a bug?"
>>
>
> I think this is supposed to happen, as
>
> :hi == 'hi'
>
> also returns false.
>
> Wim

Wim,

Thanks for responding. The problem is that if you put a symbol into a
non-TkVariable, such as

b = :hi

then it stays as a symbol, and

b == :hi returns "true", while b == "hi" is "false".

But if b is a TkVariable, then the opposite occurs. This seems
inconsistent. I would expect that when I assign a symbol to a variable
that it won't change to a string on me arbitrarilly. But, there may be
a good reason why it does this, and hopefully someone can explain why.

Alex

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

Morton Goldberg

4/8/2007 2:02:00 AM

0

On Apr 7, 2007, at 3:18 PM, Alex DeCaria wrote:

> If I put a symbol into a TkVariable, it returns a string. The
> following
> code
> illustrates this:
>
> require 'tk'
> a = TkVariable.new
> a.value = :hi
> puts a.value == :hi
> puts a.value == "hi"
>
> This results in
>
> "false"
> "true"
>
> My question is, "Is this what's supposed to happen, or is this a bug?"

It's not a bug. A TkVariable object doesn't maintain a @value
instance variable. So when you call TkVariable#value= and pass it a
symbol, the symbol will have to be coerced into something that the
underlying tcl/tk interpreter understands, which is a string. When
you call TkVariable#value, the string is retrieved from the tcl/tk
interpreter, but your TkVariable object has no memory of how the
value originated at this point.

To see how TkVariable handles different kinds of values, you can
evaluate code similar to the following:

<code>
require "tk"
v = TkVariable.new(:foo)
v.value # => "foo"
v = TkVariable.new([1, 2, 3])
v.value # => "1 2 3"
v = TkVariable.new(:foo => "red", :bar => "green")
v.value # => {"foo"=>"red", "bar"=>"green"}
</code>

Note that hashes are handled specially. Personally, I have not
encountered a situation which needed a TkVariable object that was
built on a hash, but I'm sure such situations exist. Indeed, I would
be interested if someone would post something instructive about such
situations.

Regards, Morton

Alex DeCaria

4/8/2007 11:27:00 PM

0

Thanks Morton - That's what I needed to know.

-Alex

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

Hidetoshi NAGAI

4/9/2007 2:24:00 AM

0

Morton Goldberg

4/9/2007 6:38:00 AM

0

On Apr 8, 2007, at 10:24 PM, Hidetoshi NAGAI wrote:

> From: Alex DeCaria <alex.decaria@millersville.edu>
> Subject: Re: Can a TkVariable store a symbol?
> Date: Mon, 9 Apr 2007 08:26:33 +0900
> Message-ID: <dd2b9ec33344b0790a73d090f4b02785@ruby-forum.com>
>> Thanks Morton - That's what I needed to know.
>
> TkVariable can convert Tcl's string to Ruby's object automatically.
> If you want to treat a Symbol on a TkVariable,
> you can set 'default_value_type' of the TkVariable object.
> For example,
> -------------------------------------------------------------
> $ irb
> irb(main):001:0> require 'tk'
> => true
> irb(main):002:0> a = TkVariable.new
> => #<TkVariable: v00000>
> irb(main):003:0> a.value = :hi
> => :hi
> irb(main):004:0> a.value
> => "hi"
> irb(main):005:0> a.symbol
> => :hi
> irb(main):006:0> b = TkVariable.new(:hi, :symbol)
> => #<TkVariable: v00001>
> irb(main):007:0> b.value
> => :hi
> irb(main):008:0> b.string
> => "hi"
> irb(main):009:0> a.default_value_type
> => nil
> irb(main):010:0> b.default_value_type
> => :symbol
> irb(main):011:0> a.numeric
> ArgumentError: invalid value for Number: 'hi'
> from /usr/local/lib/ruby/1.8/tk/variable.rb:706:in `number'
> from /usr/local/lib/ruby/1.8/tk/variable.rb:706:in `numeric'
> from (irb):20
> from :0
> irb(main):012:0> a.default_value_type = :symbol
> => :symbol
> irb(main):013:0> a.value
> => :hi
> irb(main):014:0> b.default_value_type = :string
> => :string
> irb(main):015:0> b.value
> => "hi"
> irb(main):016:0> a.value = 3
> => 3
> irb(main):017:0> a.value
> => :"3"
> irb(main):018:0> a.numeric
> => 3
> irb(main):019:0> a.symbol
> => :"3"
> irb(main):020:0> a.list
> => ["3"]
> irb(main):021:0> a.numlist
> => [3]
> irb(main):022:0> a.bool
> => true
> irb(main):023:0> a.value = 1
> => 1
> irb(main):024:0> a.bool
> => true
> irb(main):025:0> a.value = 0
> => 0
> irb(main):026:0> a.bool
> => false
> irb(main):027:0> a.value = 'yes'
> => "yes"
> irb(main):028:0> a.bool
> => true
> irb(main):029:0> a.value = 'no'
> => "no"
> irb(main):030:0> a.bool
> => false
> irb(main):031:0> a.value = true
> => true
> irb(main):032:0> a.bool
> => true
> irb(main):033:0> a.value = false
> => false
> irb(main):034:0> a.bool
> => false
> irb(main):035:0> a.value
> => :"0"
> irb(main):036:0> a.string
> => "0"
> irb(main):037:0> a.value = true
> => true
> irb(main):038:0> a.string
> => "1"
> irb(main):039:0>
> -------------------------------------------------------------
>
> And, following operations are available too.
>
> -------------------------------------------------------------
> $ irb
> irb(main):001:0> require 'tk'
> => true
> irb(main):002:0> a = TkVariable.new(1)
> => #<TkVariable: v00000>
> irb(main):003:0> a.value
> => "1"
> irb(main):004:0> a + 1
> => 2
> irb(main):005:0> a.value
> => "1"
> irb(main):006:0> a + 'foo'
> => "1foo"
> irb(main):007:0> a.value
> => "1"
> irb(main):008:0> a.numeric += 2
> => 3
> irb(main):009:0> a.value
> => "3"
> irb(main):010:0> a.string += 'zzz'
> => "3zzz"
> irb(main):011:0> a.value
> => "3zzz"
> irb(main):012:0>
> -------------------------------------------------------------

Thank you for posting this. I learned several new things about the
capabilities of TkVariable from it.

Regards, Morton

Morton Goldberg

4/9/2007 6:58:00 AM

0

On Apr 8, 2007, at 7:26 PM, Alex DeCaria wrote:

> Thanks Morton - That's what I needed to know.

As you can see from the posting by NAGAI san, there is lot more to
TkVariable than what I had worked out. Most important, of course, is
that I wasn't aware that one _can_ have the behavior that you asked
for if by making use TkVariable#default_value_type=.

Regards, Morton