Single word for one who imagines positive things which fail to materialize?

What do we call someone who imagines positive things ahead which never come to pass?

Pollyanna may fit the bill. Dictionary.com defines Pollyanna as "an excessively or blindly optimistic person." The adjective form of the word, Pollyannaish, is defined as "unreasonably or illogically optimistic", which suggests that the optimistic results hoped for will most likely fail to materialize.

Dreamer Since by definition the ideas of a dreamer are not realistic ones then they may not come true but dreams are also generally positive. Slightly different from daydreamer as a daydream is sometimes used in close meaning to a fantasy - there is little expectation that it will come true/materialize.

Optimist in contrast to a realist. They may not take all factors into account so this may lead to disappointment (when the good does not come to pass as a realist may have predicted). Similar to idealist.