Ruby: Getting name of variable

Question

I'm trying to create a class in Ruby that checks for a type and raises an error if it finds something it didn't expect. Here's what I've got so far:

module LibHelper
  class Base
    def self.check_type(variables, types)
      raise "Expected variables to be an array" unless variables.is_a?(Array)
      raise "Expected types to be an array" unless types.is_a?(Array)
      raise "Variable array and type array aren't same length" unless variables.length == types.length
      variables.zip(types).each do |variable, type|
        raise "Expected parameters in variables array to be symbols" unless variable.is_a?(Symbol)
        raise "Expected #{eval(variable.to_s, binding)} to be type: #{type}" unless variable.is_a?(type)
      end
    end

    def self.valid_type?(type)
      valid_types = [String, Fixnum, NilClass, Hash, Symbol]
      raise "Expected type to be String, Fixnum, NilClass, Hash, or Symbol got #{type}" unless valid_types.include?(type)
    end
  end
end

test_var = 'just_a_test_string'
LibHelper::Base.check_type([test_var], [String])

My question is what is the best way to return the name of the variable that wasn't a certain type? I'm trying to do so in this line here:

raise "Expected #{eval(variable.to_s, binding)} to be type: #{type}" unless variable.is_a?(type)

But it seems like binding might not be passed through in the scope? Ideally my return would be 'Expected test_var to be type: String'

Any thoughts or ideas?

Or if you need more info "Expected #{variable.inspect} to be type: #{type}" — Andrew Kozin
– Andrew Kozin, Commented Jul 21, 2015 at 13:40
Those won't give you the name of the variable, rather the string representation of that variable. OP is asking for the name. — Dave Newton
– Dave Newton, Commented Jul 21, 2015 at 13:53

Stefan · Accepted Answer · 2015-07-21 13:51:30Z

3

It's not going to work.

There's no reliable way to retrieve the variable name an object is assigned to. Here's a contrived example:

def check_string(foo)
  bar = foo
  LibHelper::Base.check_type([bar], [String])
end

var = 123
check(var)

What's the correct error message in this case?

#=> Expected var to be type: String
#=> Expected foo to be type: String
#=> Expected bar to be type: String

Furthermore you can easily create objects that are not assigned to a variable:

LibHelper::Base.check_type([123], [String])

A better error message would be:

#=> Expected 123 to be type: String

i.e. just use variable.inspect

edited Jul 21, 2015 at 13:51

answered Jul 21, 2015 at 13:48

Stefan

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4 Comments

Aleksei Matiushkin Over a year ago

Another approach would be to pass symbols among binding into this method, like: LibHelper::Base.check_type(self.send(:binding), [:test_var], [String]).

Stefan Over a year ago

@mudasobwa yeah, but that would limit the method to check local variables within the current scope.

Aleksei Matiushkin Over a year ago

Yes, it depends on what OP actually wanted. Anyway, I was just mentioning another possibility.

CarlyL Over a year ago

Thanks Stefan, it'd be cool if I could put out value and var name, but value will work too.

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