Hello, In a recent thread, it has been asserted that defining type class is something you seldom need when programming in Haskell. There is one thing that as non-professional Haskell programmer I found type-classes useful for: Testing. This is probably very OO and is pretty much influenced by what I read in RWH but I find useful to define TC that abstract away from low-level interactions with other code, possibly IO related, in such a way that I can have one implementation for testing and one implementation for real work that is wired in caller context. This is what is called "mockist" TDD in some circles: Write code that expresses what it needs in its own terms, then implement "glue" to the code that provide the concrete behaviour. For example, while designing some program (a game...) I defined a type class thus:
class (Monad io) => CommandIO io where readCommand :: io Command writeResult :: CommandResult -> io ()
Then I defined in a module Commands.IO :
instance CommandIO IO where readCommand = do input <- getLine ... writeResult r = putStrLn $ show r
and for the purpose of testing I defined in some test module:
instance CommandIO (S.State ([Command],[CommandResult])) where readCommand = do ((c:cs),rs) <- S.get .... writeResult r = do (cs,rs) <- S.get ...
Is this badly designed code that tries to mimic OO in a functional setting? If the answer is yes, how could I achieve same result (eg. testing the code that does command REPL) without defining type classes? Regards, Arnaud