I'm not sure what to say beyond what has been said.

Lots of people are able to deal with duck typing without particularly 
finding it harmful. If you are starting a Ruby project that is going to 
require multiple people working with your code, then they are going to 
be used to this style of coding anyway; they won't have a problem with 
it. They know about documentation. They know about unit tests. These are 
things that you are probably doing in your compile-time-type-checked 
languages anyway; you aren't losing a bunch of time with that.

Do you lose some safety? Sure. Is it within an acceptable margin for the 
benefit in agility? Many people think so.

I think the overall point people have been trying to make, is please 
don't go trying to check all of your types all of the time, via is_a? or 
kind_of? checks all over the place (i.e. don't write Ruby like C#). If 
it bugs you that much, well, the other languages with type checking 
exist for a reason!

Again, it's really a matter of trying it. If you find yourself getting 
bitten by it, don't use it. And yes, weigh this before starting some 
sort of gigantic expensive project.
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