Simple monad examples

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This page is designed to show some simple examples of using monads, specifically using Maybe.

I personally found that I reached monad-enlightenment once I contrived this simple example while playing around to see the "guts" of a monadic expression:

Just 5 >>= (\ x -> if (x == 0) then fail "zero" else Just (x + 1) )

Which results in:

Just 6

All you really need to know, is that the (>>=) operator either returns Nothing if it is passed Nothing on its left-hand side; or if its left-hand side is a Just … it strips off the Just, and passes the contents into the function supplied on its right-hand side. Simple!

Some simple exercises

What would the following snippets resolve to?

  • Just 0 >>= (\ x -> if (x == 0) then fail "zero" else Just (x + 1) )
  • Nothing >>= (\ x -> if (x == 0) then fail "zero" else Just (x + 1) )

More examples can be found in the reference guide A tour of the Haskell Monad functions, by Henk-Jan van Tuyl.