# Can a TkVariable store a symbol? **URL:** \ **Category:** Ruby\ **Created:** [April 7, 2007, 9:18pm UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376 "2007-04-07T21:18:57Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 7, 2007, 9:18pm UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/1 "2007-04-07T21:18:57Z")
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?” ---
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 8, 2007, 1:16am UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/2 "2007-04-08T01:16:40Z")
On Sun, 8 Apr 2007 04:18:59 +0900 Alex DeCaria [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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 ---
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 8, 2007, 3:30am UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/3 "2007-04-08T03:30:31Z")
Wim Vander S. wrote: > On Sun, 8 Apr 2007 04:18:59 +0900 > Alex DeCaria [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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 ---
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 8, 2007, 4:02am UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/4 "2007-04-08T04:02:48Z")
On Apr 7, 2007, at 3:18 PM, Alex DeCaria wrote: > 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: ` 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"} ` 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 ---
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 9, 2007, 1:26am UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/5 "2007-04-09T01:26:32Z")
Thanks Morton - That’s what I needed to know. -Alex ---
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 9, 2007, 4:25am UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/6 "2007-04-09T04:25:47Z")
From: Alex DeCaria [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Subject: Re: Can a TkVariable store a symbol? Date: Mon, 9 Apr 2007 08:26:33 +0900 Message-ID: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > 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 =\> #\ 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) =\> #\ 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) =\> #\ 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\> ---
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 9, 2007, 8:59am UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/7 "2007-04-09T08:59:08Z")
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 ---
**Author:** ![alexd](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/96bed5/32.png) [@alexd](https://www.ruby-forum.com/u/alexd)\ **Post date:** [April 9, 2007, 8:39am UTC](https://www.ruby-forum.com/t/can-a-tkvariable-store-a-symbol/91376/8 "2007-04-09T08:39:06Z")
On Apr 8, 2007, at 10:24 PM, Hidetoshi NAGAI wrote: > * * * > > ## =\> :hi irb(main):011:0\> a.numeric =\> :string irb(main):020:0\> a.list =\> 0 irb(main):031:0\> a.value = true =\> “0” $ irb irb(main):006:0\> a + ‘foo’ =\> “3zzz” irb(main):012:0\> Thank you for posting this. I learned several new things about the capabilities of TkVariable from it. Regards, Morton