Define conditions on a method's parameter

Question

I have a method taking only one parameter:

def my_method(number)

end

How can I raise an error if the method is called with a number < 2? And generally, how can I define conditions on a method's parameter?

For example, I want to have an error when calling:

my_method(1)

You can't put such constraints directly in the parameter definition. You can do a check in the first line of the method's body: `raise :whatever if number < 2`. — ndnenkov, Jan 06 '16 at 16:08

score 6 · Accepted Answer · edited May 23 '17 at 11:44

You can add guard in the beginning of function and raise an exception if arguments are not valid. For example:

def my_method(number)
    fail ArgumentError, "Input should be greater than or equal to 2" if number < 2

    # rest of code follows
    # ...
end

# Sample run
begin
  my_method(1)
rescue => e
    puts e.message
end
#=> Input should be greater than or equal to 2

You could define custom exception class if you don't want to use the ArgumentError

If you are building something like a framework, then, you can make use of meta-programming techniques to intercept method invocations and apply some validations. Refer Executing code for every method call in a Ruby module. You may have to come up with some kind of DSL to express those validations - a typical example of validation DSL is Active Record Validations in Rails.

In summary, for day-to-day use cases, simple raise (or fail) and rescue should suffice. The meta-programming and DSL based validations are needed only if you are building a general purpose framework.

You don't have to call `new`, just pass the exception class and the message, i.e. `raise ArgumentError, "Input should be ..."` (or `fail` according to the Ruby style guide) — Stefan, Jan 06 '16 at 16:25
@Stefan Thanks a lot. I updated the code. Appreciate the inputs you keep leaving behind on various posts. — Wand Maker, Jan 06 '16 at 16:30

score 1 · Answer 2 · answered Jan 06 '16 at 16:08

1

You'll have to check the condition and raise it inside method body. There's no builtin option like you want.

answered Jan 06 '16 at 16:08

Babar Al-Amin

3,939
1
16
20

score 1 · Answer 3 · answered Jan 06 '16 at 18:06

1

You could do this:

def my_method arg, dummy = (raise ArgumentError, "arg < 2" if arg < 2) 
  puts "arg=#{arg}"
end
my_method 3
  # arg=3
my_method 1
  # ArgumentError: arg < 2

answered Jan 06 '16 at 18:06

Cary Swoveland

106,649
6
63
100

No... Cary... You know this is just evil. (: – ndnenkov Jan 06 '16 at 18:09
1

@ndn, in my experience, evil answers are often overlooked. – Cary Swoveland Jan 06 '16 at 18:11

Define conditions on a method's parameter

3 Answers3