How do Ruby programmers do type checking?

ruby oop duck-typing
pythoniku picture pythoniku · Mar 22, 2012 · Viewed 17.4k times · Source

Since there is no type in ruby, how do Ruby programmers make sure a function receives correct arguments? Right now, I am repeating if object.kind_of/instance_of statements to check and raise runtime errors everywhere, which is ugly. There must be a better way of doing this.

Answer

sawa picture sawa · Mar 22, 2012

My personal way, which I am not sure if it a recommended way in general, is to type-check and do other validations once an error occurs. I put the type check routine in a rescue block. This way, I can avoid performance loss when correct arguments are given, but still give back the correct error message when an error occurs.

def foo arg1, arg2, arg3
  ...
  main_routine
  ...
rescue
  ## check for type and other validations
  raise "Expecting an array: #{arg1.inspect}" unless arg1.kind_of?(Array)
  raise "The first argument must be of length 2: #{arg1.inspect}" unless arg1.length == 2
  raise "Expecting a string: #{arg2.inspect}" unless arg2.kind_of?(String)
  raise "The second argument must not be empty" if arg2.empty?
  ...
  raise "This is `foo''s bug. Something unexpected happened: #{$!.message}"
end

Suppose in the main_routine, you use the method each on arg1 assuming that arg1 is an array. If it turns out that it is something else, to which each is not defined, then the bare error message will be something like method each not defined on ..., which, from the perspective of the user of the method foo, might be not helpful. In that case, the original error message will be replaced by the message Expecting an array: ..., which is much more helpful.

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