How to implement Enums in Ruby?

Question

What's the best way to implement the enum idiom in Ruby? I'm looking for something which I can use (almost) like the Java/C# enums.

Answer 1

Two ways. Symbols (:foo notation) or constants (FOO notation).

Symbols are appropriate when you want to enhance readability without littering code with literal strings.

postal_code[:minnesota] = "MN" postal_code[:new_york] = "NY" 

Constants are appropriate when you have an underlying value that is important. Just declare a module to hold your constants and then declare the constants within that.

module Foo   BAR = 1   BAZ = 2   BIZ = 4 end   flags = Foo::BAR | Foo::BAZ # flags = 3 

Added 2021-01-17

If you are passing the enum value around (for example, storing it in a database) and you need to be able to translate the value back into the symbol, there's a mashup of both approaches

COMMODITY_TYPE = {   currency: 1,   investment: 2, }  def commodity_type_string(value)   COMMODITY_TYPE.key(value) end  COMMODITY_TYPE[:currency] 

This approach inspired by andrew-grimm's answer https://stackoverflow.com/a/5332950/13468

I'd also recommend reading through the rest of the answers here since there are a lot of ways to solve this and it really boils down to what it is about the other language's enum that you care about

Answer 2

I'm surprised that no one has offered something like the following (harvested from the RAPI gem):

class Enum    private    def self.enum_attr(name, num)     name = name.to_s      define_method(name + '?') do       @attrs & num != 0     end      define_method(name + '=') do |set|       if set         @attrs |= num       else         @attrs &= ~num       end     end   end    public    def initialize(attrs = 0)     @attrs = attrs   end    def to_i     @attrs   end end 

Which can be used like so:

class FileAttributes < Enum   enum_attr :readonly,       0x0001   enum_attr :hidden,         0x0002   enum_attr :system,         0x0004   enum_attr :directory,      0x0010   enum_attr :archive,        0x0020   enum_attr :in_rom,         0x0040   enum_attr :normal,         0x0080   enum_attr :temporary,      0x0100   enum_attr :sparse,         0x0200   enum_attr :reparse_point,  0x0400   enum_attr :compressed,     0x0800   enum_attr :rom_module,     0x2000 end 

Example:

>> example = FileAttributes.new(3) => #<FileAttributes:0x629d90 @attrs=3> >> example.readonly? => true >> example.hidden? => true >> example.system? => false >> example.system = true => true >> example.system? => true >> example.to_i => 7 

This plays well in database scenarios, or when dealing with C style constants/enums (as is the case when using FFI, which RAPI makes extensive use of).

Also, you don't have to worry about typos causing silent failures, as you would with using a hash-type solution.