But in Haskell your "do, see reaction" loop is the compiler. Most of the interesting bit of it is incrementally defining types and functions for your program and verifying that it's still legal. That way, when the compiler responds with something very confusing looking you know what caused it and that can help a lot with understanding what it's saying. And when it still doesn't help, you can ask on one of the various forums and gain some important new understanding that I don't think I would ever associate or understand from a book reading.