That's true, but rubys library design is based on the idea that you should not tie down the type of a variable in the syntax, because if you do that, polymorphism becomes a lot more difficult (c.f. Java's interfaces, or C++'s multiple-inheritance).

Take this simple function:

def print_all(array)
   array.each do |el|
       puts el
   end
end
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The name of the parameter is "array", but because there is no type indication, this will work just as well with a File (print each line) a Set (print every element of the set), a Struct or some other object that happens to have an each() method that works like this. According to the idiom, an each method should "walk through" a collection - as long as you use idiomatic design, your code will remain relatively easy to interpret.

I like this strategy, but as you noted, it does have some drawbacks if you want to figure out what a complex piece of code does exactly, or when the idiom is broken.