# Local constant **URL:** \ **Category:** ruby-talk\ **Created:** [26 July 2008 14:28 UTC](https://rubytalk.org/t/local-constant/47945 "2008-07-26T14:28:55Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![aidy1](https://avatars.discourse-cdn.com/v4/letter/a/ecccb3/32.png) [@aidy1](https://rubytalk.org/u/aidy1)\ **Post date:** [26 July 2008 14:28 UTC](https://rubytalk.org/t/local-constant/47945/1 "2008-07-26T14:28:55Z")
Hi, Could anyone tell me why Ruby does not allow a local constant? FIREFOX\_CLASS = "[CLASS:MozillaUIWindowClass]" Thank You Aidy ---
**Author:** ![David\_A\_Black1](https://avatars.discourse-cdn.com/v4/letter/d/bcef8e/32.png) [@David\_A\_Black1](https://rubytalk.org/u/David_A_Black1)\ **Post date:** [26 July 2008 14:32 UTC](https://rubytalk.org/t/local-constant/47945/2 "2008-07-26T14:32:06Z")
Hi -- > **···** > > On Sat, 26 Jul 2008, aidy wrote: > > > Hi, > > > > Could anyone tell me why Ruby does not allow a local constant? > > > > FIREFOX\_CLASS = "[CLASS:MozillaUIWindowClass]" > > I'm afraid I don't follow. That statement will execute perfectly well. > What exactly are you trying to do? > > David > > -- > Rails training from David A. Black and Ruby Power and Light: >      Intro to Ruby on Rails July 21-24 Edison, NJ >   \* Advancing With Rails August 18-21 Edison, NJ >   \* Co-taught by D.A. Black and Erik Kastner > See [http://www.rubypal.com](http://www.rubypal.com) for details and updates! ---
**Author:** ![aidy1](https://avatars.discourse-cdn.com/v4/letter/a/ecccb3/32.png) [@aidy1](https://rubytalk.org/u/aidy1)\ **Post date:** [26 July 2008 15:09 UTC](https://rubytalk.org/t/local-constant/47945/3 "2008-07-26T15:09:02Z")
Hi David Thanks for getting back > \> Hi, > > \> Could anyone tell me why Ruby does not allow a local constant? > > \> FIREFOX\_CLASS = "[CLASS:MozillaUIWindowClass]" > > I'm afraid I don't follow. That statement will execute perfectly well. > What exactly are you trying to do? I am a little puzzled irb(main):001:0\> def aidy irb(main):002:1\> FIREFOX\_CLASS = "[CLASS:MozillaUIWindowClass]" irb(main):003:1\> end SyntaxError: compile error (irb):2: dynamic constant assignment FIREFOX\_CLASS = "[CLASS:MozillaUIWindowClass]"                 ^ Thanks Aidy > **···** > > On Jul 26, 3:32 pm, "David A. Black" \ wrote: > > > On Sat, 26 Jul 2008, aidy wrote: ---
**Author:** ![Phlip1](https://avatars.discourse-cdn.com/v4/letter/p/df788c/32.png) [@Phlip1](https://rubytalk.org/u/Phlip1)\ **Post date:** [26 July 2008 15:18 UTC](https://rubytalk.org/t/local-constant/47945/4 "2008-07-26T15:18:56Z")
aidy wrote: > irb(main):001:0\> def aidy > irb(main):002:1\> FIREFOX\_CLASS = "[CLASS:MozillaUIWindowClass]" > irb(main):003:1\> end > SyntaxError: compile error > (irb):2: dynamic constant assignment > FIREFOX\_CLASS = "[CLASS:MozillaUIWindowClass]" The purpose of a constant is to configure two or more methods with the same invariable value. Within one method, there's no difference between assigning a putative local constant, and simply assigning a variable - then not varying it. Ruby expects to assign constants only once. If you called aidy() twice, the constant would re-assign, and it therefor would not be constant. Put these rules together, and Ruby can only enforce its constant concept if you can't write them locally. > **···** > > -- >    Phlip ---
**Author:** ![aidy1](https://avatars.discourse-cdn.com/v4/letter/a/ecccb3/32.png) [@aidy1](https://rubytalk.org/u/aidy1)\ **Post date:** [26 July 2008 15:53 UTC](https://rubytalk.org/t/local-constant/47945/5 "2008-07-26T15:53:57Z")
Excellant explanation. Aidy > **···** > > On Jul 26, 4:19 pm, Phlip \ wrote: > > > Ruby expects to assign constants only once. If you called aidy() twice, the > > constant would re-assign, and it therefor would not be constant. ---
**Author:** ![Forum](https://avatars.discourse-cdn.com/v4/letter/f/43a26b/32.png) [@Forum](https://rubytalk.org/u/Forum)\ **Post date:** [26 July 2008 16:07 UTC](https://rubytalk.org/t/local-constant/47945/6 "2008-07-26T16:07:24Z")
But not quite correct, constants \*can\* be reassigned all you get is a warning. Nevertheless I think it is good semantics to forbid constant assignments in methods. If for one reason or another you need such a beast you can still do it of course. def a x; Object.const\_set "A", x end a 42 p A a 44 # get a warning p A HTH Robert > **···** > > On Sat, Jul 26, 2008 at 5:53 PM, aidy \ wrote: > > > On Jul 26, 4:19 pm, Phlip \ wrote: > > > > > Ruby expects to assign constants only once. If you called aidy() twice, the > > > constant would re-assign, and it therefor would not be constant. > > > > Excellant explanation. > > -- > [http://ruby-smalltalk.blogspot.com/](http://ruby-smalltalk.blogspot.com/) > > There's no one thing that's true. It's all true. > -- > Ernest Hemingway ---
**Author:** ![Phlip1](https://avatars.discourse-cdn.com/v4/letter/p/df788c/32.png) [@Phlip1](https://rubytalk.org/u/Phlip1)\ **Post date:** [26 July 2008 16:18 UTC](https://rubytalk.org/t/local-constant/47945/7 "2008-07-26T16:18:57Z")
Robert Dober wrote: > > > Ruby expects to assign constants only once. If you called aidy() twice, the > > > constant would re-assign, and it therefor would not be constant. > > > > Excellant explanation. > > But not quite correct, constants \*can\* be reassigned all you get is a warning. Read my verbiage again. And... constants are only labels. Unless you .freeze their targets, you can .replace them, etc. etc... One almost misses C++, where they fill the language up with risks and loopholes, all to allow most 'const' things to optimize at compile time without global knowledge... > **···** > > -- >    Phlip ---
**Author:** ![Marc\_Heiler](https://avatars.discourse-cdn.com/v4/letter/m/e8c25b/32.png) [@Marc\_Heiler](https://rubytalk.org/u/Marc_Heiler)\ **Post date:** [26 July 2008 19:54 UTC](https://rubytalk.org/t/local-constant/47945/8 "2008-07-26T19:54:38Z")
> Read my verbiage again. Not trying to nitpick but I agree with Robert Dober about the formulation: > > If you called aidy() twice, the constant would re-assign, > > and it therefor would not be constant. > constants \*can\* be reassigned Especially since the error will happen no matter if the constant already existed, or did not yet exist. Ruby allows one to reassign constants freely, while only raising a warning. One can even go further and misuse constants as hash/array storage places and put stuff in or get it out again easily. Ruby seems to dislike this only if it is done within a method, by raising the "dynamic constant assignment" error. > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Forum](https://avatars.discourse-cdn.com/v4/letter/f/43a26b/32.png) [@Forum](https://rubytalk.org/u/Forum)\ **Post date:** [27 July 2008 11:25 UTC](https://rubytalk.org/t/local-constant/47945/9 "2008-07-27T11:25:17Z")
Sorry if I misunderstood, I have misinterpreted your sentence "it would not be a constant anymore". Well it is not a constant. That was what I meant with not quite correct, I should have used "I do not completely agree" maybe. R. > **···** > > On Sat, Jul 26, 2008 at 6:18 PM, Phlip \ wrote: > > > Robert Dober wrote: > > > > > > > Ruby expects to assign constants only once. If you called aidy() twice, > > > > > the > > > > > constant would re-assign, and it therefor would not be constant. > > > > > > > > Excellant explanation. > > > > > > But not quite correct, constants \*can\* be reassigned all you get is a > > > warning. > > > > Read my verbiage again. > > -- > [http://ruby-smalltalk.blogspot.com/](http://ruby-smalltalk.blogspot.com/) > > There's no one thing that's true. It's all true. > -- > Ernest Hemingway ---
**Author:** ![Phlip1](https://avatars.discourse-cdn.com/v4/letter/p/df788c/32.png) [@Phlip1](https://rubytalk.org/u/Phlip1)\ **Post date:** [27 July 2008 05:08 UTC](https://rubytalk.org/t/local-constant/47945/10 "2008-07-27T05:08:53Z")
Marc Heiler wrote: > > Read my verbiage again. > > Not trying to nitpick but I agree with Robert Dober about the formulation: > > > > If you called aidy() twice, the constant would re-assign, and it therefor would not be constant. The quote depends on magically suspending the Ruby error message. Hence it does not say out-of-the-box Ruby cannot reassign. (We get reassignments, with their warnings, all the time in Rails, partly due to its Magic Loader facility...) > **···** > > -- >    Phlip ---
**Author:** ![David\_A\_Black1](https://avatars.discourse-cdn.com/v4/letter/d/bcef8e/32.png) [@David\_A\_Black1](https://rubytalk.org/u/David_A_Black1)\ **Post date:** [27 July 2008 11:06 UTC](https://rubytalk.org/t/local-constant/47945/11 "2008-07-27T11:06:28Z")
Hi -- > **···** > > On Sun, 27 Jul 2008, Phlip wrote: > > > Marc Heiler wrote: > > > > > > Read my verbiage again. > > > > > > Not trying to nitpick but I agree with Robert Dober about the formulation: > > > > > > > > If you called aidy() twice, the constant would re-assign, and it therefor would not be constant. > > > > The quote depends on magically suspending the Ruby error message. Hence it does not say out-of-the-box Ruby cannot reassign. > > > > (We get reassignments, with their warnings, all the time in Rails, partly due to its Magic Loader facility...) > > The magic loading shouldn't lead to those errors. It only does the > magic loading for unresolved constant references, and once it loads > the necessary file, any further references to the constant won't be > unresolved. > > Maybe you've got some rogue "load" commands somewhere. > > David > > -- > Rails training from David A. Black and Ruby Power and Light: >   \* Advancing With Rails August 18-21 Edison, NJ >   \* Co-taught by D.A. Black and Erik Kastner > See [http://www.rubypal.com](http://www.rubypal.com) for details and updates! ---
**Author:** ![Phlip1](https://avatars.discourse-cdn.com/v4/letter/p/df788c/32.png) [@Phlip1](https://rubytalk.org/u/Phlip1)\ **Post date:** [27 July 2008 14:14 UTC](https://rubytalk.org/t/local-constant/47945/12 "2008-07-27T14:14:46Z")
David A. Black wrote: > The magic loading shouldn't lead to those errors. It only does the > magic loading for unresolved constant references, and once it loads > the necessary file, any further references to the constant won't be > unresolved. > > Maybe you've got some rogue "load" commands somewhere. See where I said 'partly due'? There are three kinds of loading:    require 'foo' # must be on the $: path    require 'path/to/foo'    Foo.new # magic loading (via ActiveSupport?) If the system requires the same file twice thru different paths, it will load the same file twice. The (primitive) require only avoids reloading a file if it sees the exact same path. The magic loader adds a third system to load - one where the path is invisible. Hence it increases the odds of accidentally reloading a file. (This is not a request for newb advice, BTW!) > **···** > > -- >    Phlip ---
**Author:** ![David\_A\_Black1](https://avatars.discourse-cdn.com/v4/letter/d/bcef8e/32.png) [@David\_A\_Black1](https://rubytalk.org/u/David_A_Black1)\ **Post date:** [27 July 2008 17:29 UTC](https://rubytalk.org/t/local-constant/47945/13 "2008-07-27T17:29:20Z")
Hi -- > David A. Black wrote: > > > The magic loading shouldn't lead to those errors. It only does the > > magic loading for unresolved constant references, and once it loads > > the necessary file, any further references to the constant won't be > > unresolved. > > > > Maybe you've got some rogue "load" commands somewhere. > > See where I said 'partly due'? > > There are three kinds of loading: > > require 'foo' # must be on the $: path > > require 'path/to/foo' > > Foo.new # magic loading (via ActiveSupport?) There's also "load", which will (re)load unconditionally. That's why I was thinking you might have some of those around. > If the system requires the same file twice thru different paths, it will load the same file twice. The (primitive) require only avoids reloading a file if it sees the exact same path. So it does. Interesting. > The magic loader adds a third system to load - one where the path is invisible. Hence it increases the odds of accidentally reloading a file. I guess I try to do one or the other: either an explicit require, or using the magic loader, but not both, so ideally the warning won't happen. Then again, it's only a warning, so not too dire if it does happen. > (This is not a request for newb advice, BTW!) I'm afraid I don't understand. David > **···** > > On Sun, 27 Jul 2008, Phlip wrote: > > -- > Rails training from David A. Black and Ruby Power and Light: >   \* Advancing With Rails August 18-21 Edison, NJ >   \* Co-taught by D.A. Black and Erik Kastner > See [http://www.rubypal.com](http://www.rubypal.com) for details and updates!