Re: irb: require returns false?

Alle Wednesday 31 December 2008, Mike Carbs ha scritto:
> hey everyone (ruby n00b here),
>
> I am trying to compile ruby 1.8.6-p287. I built it using:
>
> ./configure --prefix=/usr/local/ruby --enable-pthread --enable-shared
>
> I was playing with how support for things like iconv, gdbm, openssl, etc
> worked and did:
>
> irb(main):001:0> require 'gdbm'
> LoadError: no such file to load -- gdbm
>   from (irb):1:in `require'
>   from (irb):1
>
> so I installed the gdbm devel libs, and did recompiled. Now I get:
>
> irb(main):004:0> require 'gdbm'
> => false
>
> When I use Redhats stock 1.8.5 rpm, I get:
>
> irb(main):001:0> require 'gdbm'
> => true
>
> So my question is... what does "false" mean? The first time I tried, it
> complained about not being able to load the file. After installing the
> devel package, it now just says false. Does this mean that it properly
> loading it but just not enabling it somehow?
>
> Can someone shed some light on this for me?
>
> Thanks!

require returns false if the given file had already been loaded. For example:

require 'singleton'
=> true
require 'singleton'
=> false

The second time, the 'singleton' feature has already been loaded (by the first 
require), so there's no need to load that again and require tells you that by 
returning false. You can see which features have already been loaded by 
inspecting the global variable $". For my irb session, this is the result:

p $"
=> ["enumerator.so", "rubygems/rubygems_version.rb", "rubygems/defaults.rb", 
"thread.so", "thread.rb", "rbconfig.rb", "rubygems/exceptions.rb", 
"rubygems/requirement.rb", "rubygems/version.rb", "rubygems/dependency.rb", 
"rubygems/gem_path_searcher.rb", "rubygems/user_interaction.rb", 
"rubygems/platform.rb", "rubygems/specification.rb", 
"rubygems/source_index.rb", "rubygems/builder.rb", "stringio.so", 
"yaml/error.rb", "syck.so", "yaml/ypath.rb", "yaml/basenode.rb", 
"yaml/syck.rb", "yaml/tag.rb", "yaml/stream.rb", "yaml/constants.rb", 
"rational.rb", "date/format.rb", "date.rb", "yaml/rubytypes.rb", 
"yaml/types.rb", "yaml.rb", "rubygems/config_file.rb", 
"rubygems/custom_require.rb", "rubygems.rb", "auto_gem.rb", "e2mmap.rb", 
"irb/init.rb", "irb/workspace.rb", "irb/context.rb", "irb/extend-command.rb", 
"irb/output-method.rb", "irb/notifier.rb", "irb/slex.rb", "irb/ruby-token.rb", 
"irb/ruby-lex.rb", "readline.so", "irb/input-method.rb", "irb/locale.rb", 
"irb.rb", "irb/completion.rb", "irb/ext/save-history.rb", "singleton.rb"]

Calling require with one of these features will return false, because they've 
already been loaded.

What is important is to understand that the return value of require never 
means that an error has occurred. If an error occurs, you'll get an exception, 
as it happened to you when calling require 'gdbm' the first time. 

I hope this helps

Stefano