# Simple monad examples

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<haskell>Just 6</haskell> | <haskell>Just 6</haskell> | ||

− | All you really need to know, is that the >>= operator either returns "Nothing" if it is passed "Nothing" on | + | 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: | Some simple exercises: |

## Latest revision as of 19:54, 30 April 2012

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.