usually, it is used to define methods, but in function arguments, it serves as syntactic sugar so you don't have to name generic types... but in a return type, it has a meaning that is slightly different, and actually expresses a semantic not even vanilla haskell can represent!
basically, instead of being able to return any type implementing a trait, it states that it can return at least one type that implements a trait.
in haskell terminology, specifying a generic type parameter is "forall a", while returning an "impl" is "exists a".
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