Question

What’s the science behind the fact that the to_i method in Ruby’s NilClass instances returns zero? Returning nil or raising an exception would not be more logical?

Answer 1

NilClass defines #to_i for the same reason it defines a #to_a that returns []. It’s giving you something of the right type but an empty sort of value.

This is actually quite useful. For example:

<%= big.long.expr.nil? ? "" : big.long.expr %>

becomes:

<%= big.long.expr %>

Much nicer! (Erb is calling #to_s which, for nil, is “”.) And:

if how.now.brown.cow && how.now.brown.cow[0]
  how.now.brown.cow[0]
else
  0
end

becomes:

how.now.brown.cow.to_a[0].to_i

The short conversions exist when only a representation is needed. The long conversions are the ones that the Ruby core methods call and they require something very close. Use them if you want a type check.

That is:

thing.to_int # only works when almost Integer already. NilClass throws NoMethodError

thing.to_i # this works for anything that cares to define a conversion