Effective Haskell

Ordering and Combining IO Actions

In the last section, you learned how we could use a function like andThen :: IO a -> (a -> IO b) -> IO b and newIO :: a -> IO a to work with IO in a pure lazy language. Now you’ll have the opportunity to see how functions like this are implemented, and if you follow along with the example you can write them yourself:

andThen :: IO a -> (a -> IO b) -> IO b
andThen = (>>=)

newIO :: a -> IO a
newIO = return

That might have been a little bit anticlimactic. It turns out that our implementations are actually just providing new names to the existing >>= operator (pronounced bind) and the return function.

If you look at the types of (>>=) and return in ghci you’ll see that they are actually more general ...