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One of the features of Ruby’s Kernel module (which is imported into Object, so it’s available in every class) is the binding. The binding allows you to keep a pointer to the scope at a particular location in your application and then later evaluate code from that location.
For example (and this came up as the result of a question on the ruby talk mailing list), one of the things you can do is obtain a list of the local variables of a method:
| | | copy code | | ? |
| 1 | def my_vars(a) |
| 2 | b=a |
| 3 | c=b+a |
| 4 | puts local_variables |
| 5 | end |
returns the three variables, a, b, and c. That’s fine, we’re still within the scope of the method. But how about outside of our method? This is where #binding and the Binding class come into play:
| | | copy code | | ? |
| 1 | def outside(b) |
| 2 | puts eval("local_variables", b) |
| 3 | end |
| 4 | |
| 5 | def my_vars(a) |
| 6 | b=a |
| 7 | c=b+a |
| 8 | outside(binding) |
| 9 | end |
One thing to note: in order to work with the binding, we have to #eval code within its scope. Thus the call to #eval with the method we are invoking and the binding. This leads to the question from the mailing list:
we already have #has_block? to see if a block was passed. So how about
a #has_arguments? to query if _any_ arguments have been passed — Intransition
So how would we go about writing a #has_arguments? — here is a first stab:
| | | copy code | | ? |
| 01 | class Object |
| 02 | def has_arguments?(b) |
| 03 | vars = eval("local_variables",b) |
| 04 | return false if vars.length == 0 |
| 05 | vars.each do |v| |
| 06 | return false if eval("#{v}.nil?",b) |
| 07 | end |
| 08 | return true |
| 09 | end |
| 10 | end |
| 11 | |
| 12 | class Foo |
| 13 | def initialize(first=nil) |
| 14 | if (has_arguments? binding) |
| 15 | puts "We're defined" |
| 16 | else |
| 17 | puts "Not!" |
| 18 | end |
| 19 | end |
| 20 | end |
The only real drawback here is that we’ve got to pass in the binding itself. Let’s try again:
| | | copy code | | ? |
| 01 | class Object |
| 02 | def has_arguments?(&b) |
| 03 | vars = eval("local_variables",b.binding) |
| 04 | return false if vars.length == 0 |
| 05 | vars.each do |v| |
| 06 | return false if eval("#{v}.nil?",b) |
| 07 | end |
| 08 | return true |
| 09 | end |
| 10 | end |
| 11 | |
| 12 | class Foo |
| 13 | def initialize(first=nil) |
| 14 | if (has_arguments? {}) # note the empty block |
| 15 | puts "We're defined" |
| 16 | else |
| 17 | puts "Not!" |
| 18 | end |
| 19 | end |
| 20 | end |
In this case, instead of explicitly calling #binding, we’re passing in an empty block — the block has it’s own scope, but it happens to contain the scope of then enclosing method. It’s still not perfect, however:
- You need to pass in an empty block.
- It fails if you have local variables other than those passed in as arguments. (Pointed out by Joel VanderWerf)
Ok, it’s got some serious flaws (Charles Nutter posted a different way of solving the problem, which does work for all cases, and doesn’t use #binding). But it does give an idea of what you can do with #binding.
The final example doesn’t work on Ruby 1.9.1, line 6 can be changed to:
return false if eval(“#{v}.nil?”,b.binding)
and it will work with both Ruby 1.8.6 and 1.9.1.