Constants in modules

Ruby
1

Hello,

I need to do something like this (this is a simplified version of some
other code, of course, but I don’t want to get you lost in details
unrelated to the problem):

module M

CONST_A = { ‘k1’ => ‘v1’, ‘k2’ => ‘v2’ }
CONST_B = invert_hash(CONST_A)

def method1
# Do something with CONST_A
puts CONST_A
end

def method2
# Do something with CONST_B
puts CONST_B
end

def invert_hash(h)
inverted = {}
h.each_pair { |k, v| inverted[v] = k }
inverted
end

end

My intention is to be able to calculate CONST_B by invoking a method in
the module itself… but I get this error message:

tests.rb:4: undefined method `invert_hash’ for M:Module (NoMethodError)
from tests.rb:1

I have tried lots of combinations by prefixing the module name, using
::, using module_function, etc., but nothing has worked.

Is there a way to do this? Of course I know there are obvious
workarounds such as defining the method outside the module, but that
would be quite ugly… and there should be a way to use code in my own
module from my own module!!!

Thanks in advance for your help,

Luis.
2

Two things - firstly, Ruby is interpreted, so invert_hash needs to be
declared before you use it. Then, your methods need to be module
methods - just put a “self.” before each one, and it should work.

3

Bryan JJ Buckley wrote:

Two things - firstly, Ruby is interpreted, so invert_hash needs to be
declared before you use it. Then, your methods need to be module
methods - just put a “self.” before each one, and it should work.

Yes, it works!!

I already tried defining the method as a module method by prepending the
module name to the method name, which AFAIK it is totally equivalent to
prepending self.

…But I did not realize that the constant code is executed when reading
the module, so I had to define the method before.

Thank you!

Luis.
4

Luis Crespo wrote:

…But I did not realize that the constant code is executed when reading
the module, so I had to define the method before.

Yes, this is a common gotcha.

Personally I like to have the main program at the top of my code, but
Ruby wants me to put it at the bottom, so it’s seen all the function
definitions before I try to call them. You can get around it like this:

def main

…

end

def function1

…

end

def function2

…

end

main

Defining a “main” function comes naturally to C programmers, but in Ruby
you actually have to call it at the bottom. :wink: