A let-bound term is usually shared within its scope. In particular, a top-level term in a module is shared in the entire program. However, you have to be careful about the type of the term. If the term is a function, then sharing means that just the lambda abstraction is shared, so the function isn't memoized. An overloaded term is internally represented as a function, and therefore sharing is rather meaningless for an overloaded term as well.
So if you have a monomorphic list of numbers, then it's going to be shared. By default, a list such as fib
as you've given will be monomorphic, because of the "monomorphism restriction" (actually here's a case where it's useful). However, these days it's in fashion to disable the monomorphism restriction, so in any case I recommend giving an explicit type signature such as
fib :: [Integer]
to be sure and make it clear to everyone that you're expecting this to be a monomorphic list.