Daniel Carrera
I have finished the tutorial at http://ertes.de/articles/monads.html and my understanding of monads has increased greatly. I still need to cement some concepts in my mind. What exactly is the difference between a computation and a function? Monads revolve around computations, so I'd like to understand computations better.
What I refer to as a 'computation' in the article is actually just a value of type 'Monad m => m a'. I have chosen that term, because you can apply it to any monad I've seen. As mentioned in section 5, you can think of 'Just 3' as being a computation, which results in 3. But it's important that this is not a function, but just an independent value. You can think of a function of type 'a -> b' as a parametric value -- a value of type 'b' depending on some value of type 'a'. That makes a function of type 'Monad m => a -> m b' a parametric computation. A computation, where something is missing, like with an open slot, where you need to plug a cable in first. By the way, this is where (>>=) comes into play. If you have a computation, which needs a value, but that value comes as the result of another computation, you can use the binding operator. f :: Monad m => a -> m b The 'f' function is a parametric computation of type 'm b', which depends on a value of type 'a'. Now if c :: Monad m => m a and 'm' and 'a' in 'f' are the same as 'm' and 'a' in 'c', then c >>= f takes 'c', puts its result (of type 'a') into 'f', resulting in a computation of type 'm b'. Example: You have a computation 'myComp', which outputs a string to stdout prefixed with "+++ ": myComp :: String -> IO () myComp str = putStrLn ("+++ " ++ str) If that string is available directly, just pass it to 'myComp', which results in a computation. If that string is not available directly, but comes as the result of another computation 'getLine', you use (>>=): getLine >>= myComp Greets, Ertugrul. -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://blog.ertes.de/