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Comment by kingkilr | Hacker News Reader

Far more interesting (IMO), would be a constrains based library based on predicates (iterable, hashable, arithmatic-able?)

In ruby you can ensure this by calling responds_to?(:method).

So for example to see if some object is iterable, you just check if it responds to each.

   obj.responds_to?(:each)

That doesn't guarantee that "each" is actually the right "each".