Are you implying that Java cannot be used to make useful software?
No, of course not. Are people without feet useless?
What does it mean to say that Java does not have feet?
It means that it lacks a capability for which it also lacks a need (shoes). This capability is that of expressing purity via the type system. Monads - and by extension, monad transformers - are a tool designed to assist the expression of imperative algorithms in a pure context.
The reason I phrase it this way is because people often make the mistake of assuming Haskell is lacking expressive power due to its purity. This is untrue. Haskell is perfectly capable of expressing messy, mutable, imperative algorithms just like any other language. Other languages, on the other hand, struggle to express many of the pure algorithms and data structures used in Haskell; they simply lack the ability to enforce purity within the language and are thus reduced to purity by convention.
Does lack of ability to enforce purity equal lack of ability to express the algorithm/data structure? Or does it just equal lack of ability to prove the purity?
You can only express some kinds of purity if you can enforce it at the type level. An otherwise pure data structure that contains mutable elements is a violation of purity; a contradiction. If you cannot express a data structure which guarantees that its elements (and its elements' fields/elements) are pure, you cannot express a pure data structure (also known as a value).
