Haskell is the gold standard for languages that get you to think differently. Forth is another highly unconventional language. Other less exotic languages can

Haskell is the gold standard for languages that get you to think differently. Forth is another highly unconventional language. Other less exotic languages can still be worth taking a look at to see what they do differently. Ada, for instance, is a plain old imperative/OOP language at heart, but it's different enough in the details that it could be worth looking at if you already know C/C++.