Re: Followup question on defining methods dynamically
- From: Kenneth McDonald <kenneth.m.mcdonald@xxxxxxxxxxxxx>
- Date: Sat, 4 Aug 2007 08:05:45 +0900
Do modules not execute their own code (such as the 'p "Testing"') below in their own namespace, then? I'm doing something that's not at all object-oriented, and simply want to ensure that a bunch of methods don't leak out into the global namespace. One module will define those methods, and other modules (not classes, unless I really have to hack it that way) will use the first module, eg.
module Test
p "Testing"
eval("def foo\np 'OK'\nend")
end
module Run
include Test
foo
end
The above doesn't work, for the reasons you gave, but I still don't understand the semantics of why not, I'm afraid. No doubt my Python way of thinking. In Python, the equivalent code would create two modules, Test and Run, which are effectively just hashmaps mapping names to objects. The eval would define a 'foo' object in Test, so there would exist a function Test.foo. The include statement in Run (equivalent to Python's "from Test import *" I believe) would cause all names in Test to be imported into Run, resulting in a 'foo' entry in the Run hashmap; this would be Run.foo. Finally, code run within a module always has access to the module's namespace (hashmap) for variable lookup, so the call on 'foo' within Run would find 'Run.foo' and execute it.
Now, since that interpretation is obviously incorrect in Ruby, could someone provide details as to what is actually going on? If possible, I prefer explanations of the 'behind-the-scenes' stuff--how variables are looked up, namespaces are defined, etc., as understanding the background mechanisms will then make everything else pretty easy.
Thanks,
Ken
dblack@xxxxxxxxxxx wrote:
Hi --
On Sat, 4 Aug 2007, Kenneth McDonald wrote:
Simple example that doesn't work:
----------------
module Test
p "Testing"
eval("def foo\np 'OK'\nend")
foo
end
----------------
'foo' does not get defined in a place where it can be found by the call on foo, and I get an error. However, when I execute the two lines
eval("def foo\np 'OK'\nend")
foo
in IRB, everything works. I'm guessing that this is a namespace problem, but can't see why it would be. Then again, I'm accustomed to the Python namespace model, and still don't understand all of the ways in which the Ruby model is different.
Your first example is essentially doing this:
module Test
def foo
p "OK"
end
foo
end
When you define an instance method inside a module, you're defining it
for the future use of objects with that module in their ancestry:
class C
include Test
end
C.new.foo # OK
David
.
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