Question

I was wondering if or how it is possible to map a function to a value of a hash.

For example:
—-Start Class————

def foo(var)
    return var + 2
end

hash_var = { func => foo() }

—-End Class————–

so that I could later call

Class::hash_var["func"][10]

or

Class::hash_var["func"](10)

and that would return 12?

Answer 1

Functions/methods are one of the few things in Ruby that are not objects, so you can’t use them as keys or values in hashes. The closest thing to a function that is an object would be a proc. So you are best off using these…

The other answers pretty much listed all possible ways of how to put a proc into a hash as value, but I’ll summarize it nonetheless 😉

hash = {}

hash['variant1'] = Proc.new {|var| var + 2}
hash['variant2'] = proc     {|var| var + 2}
hash['variant3'] = lambda   {|var| var + 2}

def func(var)
  var + 2
end

hash['variant4'] = method(:func) # the *method* method returns a proc
                                 # describing the method's body 

there are also different ways to evaluate procs:

hash['variant1'].call(2) # => 4
hash['variant1'][2]      # => 4
hash['variant1'].(2)     # => 4