Allows you check if an object match a class expression. It is typically used to check the type of method paraameters. It is an alternative to using Ruby-3 .rbs files but with a different syntax and only dynamic checks Typically you'll include the Constrain module and use #constrain to check the type of method parameters: include Constrain # f takes a String and an array of Integer objects. Raise a Constrain::Error # if parameters doesn't have the expected types def f(a, b) constrain a, String constrain b, [Integer] end Constrain works with ruby-2 (and maybe ruby-3)