# The subtleties of const\_missing **URL:** \ **Category:** ruby-talk\ **Created:** [16 November 2005 04:36 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513 "2005-11-16T04:36:46Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Nicholas\_Seckar](https://avatars.discourse-cdn.com/v4/letter/n/b77776/32.png) [@Nicholas\_Seckar](https://rubytalk.org/u/Nicholas_Seckar)\ **Post date:** [16 November 2005 04:36 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/1 "2005-11-16T04:36:46Z")
Hello all, I've run into an issue regarding the use of const\_missing. This issue revolves around the fact that these two cases are supposed to behave differently from each other: irb(main):001:0\> B = 10 irb(main):002:0\> module A; B; end =\> 10 irb(main):003:0\> A::B NameError: uninitialized constant A::B          from (irb):3 However it seems impossible to detect which case we are in within the const\_missing method. A more detailed explanation follows: Suppose we are lazy and avoid typing "require 'my\_module.rb'" by defining a const\_missing that performs that require when MyModule is first referenced. Now, suppose we are writing my\_class, and wish to include MyModule. So, we put this code in my\_class.rb: class MyClass    include MyModule    ... end Our const\_missing handler is called, and it loads 'my\_module.rb', finds the now present MyModule constant, and returns it. Everything is good. However, let's say we're slightly misguided and do MyClass::MyModule. Since this is not defined, our const\_missing handler is once again called. 'my\_module.rb' is thus loaded again, (usually to no effect,) and MyModule returned. This is a pretty clear violation of the semantics of ::. Indeed, Ruby will produce a warning regarding our misbehaved const\_missing. That said, there does not seem to be a way to behave correctly; from inside the const\_missing handler there is no way to tell which case the constant is missing in. One way to 'fix' this is to have our const\_missing look in it's parent modules for the missing constant. If we find it there, we know this is a case of A::B. However this only works if the missing constant has been defined in one of the parents. Thus, we can still load ::B using A::B, but afterwards A::B will fail. So, sorry for rambling on. The real question is: Can const\_missing detect if we are in a "A::B" case rather than "module A; B; end" ? Thanks for your time and any replies, Regards, Nicholas Seckar ---
**Author:** ![Ara.T.Howard6](https://avatars.discourse-cdn.com/v4/letter/a/958977/32.png) [@Ara.T.Howard6](https://rubytalk.org/u/Ara.T.Howard6)\ **Post date:** [16 November 2005 04:57 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/2 "2005-11-16T04:57:12Z")
but why do you need to distinguish?    harp:~ \> cat a.rb    class Module      def const\_missing c        puts "const\_missing..."        const\_set c, 42      end    end    module A      p B    end    p A::B    harp:~ \> ruby a.rb    const\_missing...    42 the 'const\_missing' hook will continue to get call only so long as it remain undefined - if your handler merely loads a file but does not actually define any const then of course it will continue to fire. what, exactly, does your handler do? kind regards. -a > **···** > > On Wed, 16 Nov 2005, Nicholas Seckar wrote: > > > Hello all, > > > > I've run into an issue regarding the use of const\_missing. This issue revolves around the fact that these two cases are supposed to behave differently from each other: > > > > irb(main):001:0\> B = 10 > > irb(main):002:0\> module A; B; end > > =\> 10 > > irb(main):003:0\> A::B > > NameError: uninitialized constant A::B > >        from (irb):3 > > > > However it seems impossible to detect which case we are in within the > > const\_missing method. A more detailed explanation follows: > > > > Suppose we are lazy and avoid typing "require 'my\_module.rb'" by defining a > > const\_missing that performs that require when MyModule is first referenced. > > Now, suppose we are writing my\_class, and wish to include MyModule. So, we > > put this code in my\_class.rb: > > > > class MyClass > > include MyModule > > ... > > end > > > > Our const\_missing handler is called, and it loads 'my\_module.rb', finds the > > now present MyModule constant, and returns it. Everything is good. > > > > However, let's say we're slightly misguided and do MyClass::MyModule. Since > > this is not defined, our const\_missing handler is once again called. > > 'my\_module.rb' is thus loaded again, (usually to no effect,) and MyModule > > returned. > > > > This is a pretty clear violation of the semantics of ::. Indeed, Ruby will > > produce a warning regarding our misbehaved const\_missing. That said, there > > does not seem to be a way to behave correctly; from inside the const\_missing > > handler there is no way to tell which case the constant is missing in. > > > > One way to 'fix' this is to have our const\_missing look in it's parent > > modules for the missing constant. If we find it there, we know this is a > > case of A::B. However this only works if the missing constant has been > > defined in one of the parents. Thus, we can still load ::B using A::B, but > > afterwards A::B will fail. > > > > So, sorry for rambling on. The real question is: Can const\_missing detect if > > we are in a "A::B" case rather than "module A; B; end" ? > > # -- > > > ara [dot] t [dot] howard [at] gmail [dot] com > > all happiness comes from the desire for others to be happy. all misery > > comes from the desire for oneself to be happy. > > -- bodhicaryavatara > > =============================================================================== ---
**Author:** ![Eric\_Hodel1](https://avatars.discourse-cdn.com/v4/letter/e/a9adbd/32.png) [@Eric\_Hodel1](https://rubytalk.org/u/Eric_Hodel1)\ **Post date:** [16 November 2005 05:27 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/3 "2005-11-16T05:27:45Z")
I have found this type of laziness often leads to errors that are difficult to discover because somewhere else a MyModule was defined leaving the file I really want to use unrequired. require 'my\_module' is really not that hard to type. > **···** > > On Nov 15, 2005, at 8:36 PM, Nicholas Seckar wrote: > > > Hello all, > > > > I've run into an issue regarding the use of const\_missing. This issue revolves around the fact that these two cases are supposed to behave differently from each other: > > > > irb(main):001:0\> B = 10 > > irb(main):002:0\> module A; B; end > > =\> 10 > > irb(main):003:0\> A::B > > NameError: uninitialized constant A::B > >         from (irb):3 > > > > However it seems impossible to detect which case we are in within the const\_missing method. A more detailed explanation follows: > > > > Suppose we are lazy and avoid typing "require 'my\_module.rb'" by defining a const\_missing that performs that require when MyModule is first referenced. Now, suppose we are writing my\_class, and wish to include MyModule. So, we put this code in my\_class.rb: > > -- > Eric Hodel - drbrain@segment7.net - [http://segment7.net](http://segment7.net) > FEC2 57F1 D465 EB15 5D6E 7C11 332A 551C 796C 9F04 ---
**Author:** ![Nakada\_Nobuyoshi](https://avatars.discourse-cdn.com/v4/letter/n/848f3c/32.png) [@Nakada\_Nobuyoshi](https://rubytalk.org/u/Nakada_Nobuyoshi)\ **Post date:** [16 November 2005 05:34 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/4 "2005-11-16T05:34:15Z")
Hi, At Wed, 16 Nov 2005 13:36:46 +0900, Nicholas Seckar wrote in [ruby-talk:165996]: > I've run into an issue regarding the use of const\_missing. This issue > revolves around the fact that these two cases are supposed to behave > differently from each other: > > irb(main):001:0\> B = 10 > irb(main):002:0\> module A; B; end > =\> 10 > irb(main):003:0\> A::B > NameError: uninitialized constant A::B >          from (irb):3 > > However it seems impossible to detect which case we are in within the > const\_missing method. A more detailed explanation follows: What about autoload?   $ echo 'B = 10' \> b.rb   $ irb   irb(main):001:0\> autoload :B, "b"   =\> nil   irb(main):002:0\> module A;end   =\> nil   irb(main):003:0\> A::B   NameError: uninitialized constant A::B           from (irb):3   irb(main):004:0\> module A;B;end   =\> 10   irb(main):005:0\> A::B   NameError: uninitialized constant A::B           from (irb):5   irb(main):006:0\> > **···** > > from :0 >           from :0 > > -- > Nobu Nakada ---
**Author:** ![Robert](https://avatars.discourse-cdn.com/v4/letter/r/e95f7d/32.png) [@Robert](https://rubytalk.org/u/Robert)\ **Post date:** [16 November 2005 10:37 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/5 "2005-11-16T10:37:18Z")
Nicholas Seckar wrote: > Hello all, > > I've run into an issue regarding the use of const\_missing. This issue > revolves around the fact that these two cases are supposed to behave > differently from each other: > > irb(main):001:0\> B = 10 > irb(main):002:0\> module A; B; end > =\> 10 > irb(main):003:0\> A::B > NameError: uninitialized constant A::B >          from (irb):3 > > However it seems impossible to detect which case we are in within the > const\_missing method. A more detailed explanation follows: I'm not sure what you're up to. You define B in a completely different scope than you use for lookup. Maybe you can give a more real life example of what you are actually trying to do. Kind regards     robert ---
**Author:** ![Devin\_Mullins](https://avatars.discourse-cdn.com/v4/letter/d/958977/32.png) [@Devin\_Mullins](https://rubytalk.org/u/Devin_Mullins)\ **Post date:** [16 November 2005 14:42 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/6 "2005-11-16T14:42:03Z")
Hi, Rails does this very thing. You should check out their const\_missing and see if it suffers from the self-same problem. Devin Nicholas Seckar wrote: > **···** > > > Hello all, > > > > I've run into an issue regarding the use of const\_missing. This issue revolves around the fact that these two cases are supposed to behave differently from each other: > > > > irb(main):001:0\> B = 10 > > irb(main):002:0\> module A; B; end > > =\> 10 > > irb(main):003:0\> A::B > > NameError: uninitialized constant A::B > >         from (irb):3 > > > > However it seems impossible to detect which case we are in within the const\_missing method. A more detailed explanation follows: > > > > Suppose we are lazy and avoid typing "require 'my\_module.rb'" by defining a const\_missing that performs that require when MyModule is first referenced. Now, suppose we are writing my\_class, and wish to include MyModule. So, we put this code in my\_class.rb: > > > > class MyClass > >   include MyModule > >   ... > > end > > > > Our const\_missing handler is called, and it loads 'my\_module.rb', finds the now present MyModule constant, and returns it. Everything is good. > > > > However, let's say we're slightly misguided and do MyClass::MyModule. Since this is not defined, our const\_missing handler is once again called. 'my\_module.rb' is thus loaded again, (usually to no effect,) and MyModule returned. > > > > This is a pretty clear violation of the semantics of ::. Indeed, Ruby will produce a warning regarding our misbehaved const\_missing. That said, there does not seem to be a way to behave correctly; from inside the const\_missing handler there is no way to tell which case the constant is missing in. > > > > One way to 'fix' this is to have our const\_missing look in it's parent modules for the missing constant. If we find it there, we know this is a case of A::B. However this only works if the missing constant has been defined in one of the parents. Thus, we can still load ::B using A::B, but afterwards A::B will fail. > > > > So, sorry for rambling on. The real question is: Can const\_missing detect if we are in a "A::B" case rather than "module A; B; end" ? > > > > Thanks for your time and any replies, > > Regards, > > Nicholas Seckar ---
**Author:** ![Nicholas\_Seckar](https://avatars.discourse-cdn.com/v4/letter/n/b77776/32.png) [@Nicholas\_Seckar](https://rubytalk.org/u/Nicholas_Seckar)\ **Post date:** [16 November 2005 05:02 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/7 "2005-11-16T05:02:49Z")
The loaded file is expected to define the constant; We assume b.rb contains class or module B (or defines it in any other way). If the constant isn't defined by the loaded file then a NameError is raised as if normal. > **···** > > On 15-Nov-05, at 11:57 PM, Ara.T.Howard wrote: > > > but why do you need to distinguish? > > > > [snip] > > > > the 'const\_missing' hook will continue to get call only so long as it remain > > undefined - if your handler merely loads a file but does not actually define > > any const then of course it will continue to fire. what, exactly, does your > > handler do? ---
**Author:** ![Ara.T.Howard6](https://avatars.discourse-cdn.com/v4/letter/a/958977/32.png) [@Ara.T.Howard6](https://rubytalk.org/u/Ara.T.Howard6)\ **Post date:** [16 November 2005 13:47 UTC](https://rubytalk.org/t/the-subtleties-of-const-missing/22513/8 "2005-11-16T13:47:06Z")
> The loaded file is expected to define the constant; We assume b.rb contains > class or module B (or defines it in any other way). If the constant isn't > defined by the loaded file then a NameError is raised as if normal. i understand that. but in your original message you said > However, let's say we're slightly misguided and do MyClass::MyModule. Since > this is not defined, our const\_missing handler is once again called. > 'my\_module.rb' is thus loaded again, (usually to no effect,) and MyModule > returned. > > This is a pretty clear violation of the semantics of ::. Indeed, Ruby will > produce a warning regarding our misbehaved const\_missing. That said, there > does not seem to be a way to behave correctly; from inside the const\_missing > handler there is no way to tell which case the constant is missing in. here you will see that wether we reference B from inside M, or as M::B the handler is called exactly once.    harp:~ \> cat a.rb    class Module      def const\_missing c        puts "const\_missing called once..."        const\_set c, 42      end    end    module A      p B    end    p A::B    harp:~ \> ruby a.rb    const\_missing called once...    42 here is another example which loads a file    harp:~ \> cat a.rb    class Module      def const\_missing c        puts "const\_missing called once..."        autoload = {          :B =\> 'b.rb'        }        autoload[c] ? require(autoload[c]) : super        const\_set(c, const\_get(c))      end    end    module A      B    end    module A; p B; end    p A::B    harp:~ \> ruby a.rb    const\_missing called once...    42 so what \_\_exactly\_\_ are you trying to do in your handler that does not work? i understand you problem but, as the second example shows, it can easily be solved without distinguishing the two syntax cases because, afaikt, ruby handles them precisely the same. hth. -a > **···** > > # On Wed, 16 Nov 2005, Nicholas Seckar wrote:    A::B -- > > > ara [dot] t [dot] howard [at] gmail [dot] com > > all happiness comes from the desire for others to be happy. all misery > > comes from the desire for oneself to be happy. > > -- bodhicaryavatara > > ===============================================================================