# Simple monad examples

### From HaskellWiki

(Difference between revisions)

Twobitsprite (Talk | contribs) |
m (Fixed grammar (it's -> its)) |
||

(4 intermediate revisions by 3 users not shown) | |||

Line 1: | Line 1: | ||

− | This page is designed to show some simple examples of using monads. |
+ | This page is designed to show some simple examples of using [[monad]]s, 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: |
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: |
||

Line 11: | Line 11: | ||

<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 it's left-hand side; or if it's left-hand side is a "Just ..." it strips off the just, and passes the contents into the function supplied on it's right-hand side. Simple! |
+ | 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: |
||

Line 21: | Line 21: | ||

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

</haskell> |
</haskell> |
||

+ | |||

+ | ---- |
||

+ | More examples can be found in the reference guide [http://members.chello.nl/hjgtuyl/tourdemonad.html A tour of the Haskell Monad functions], by Henk-Jan van Tuyl. |
||

+ | ---- |
||

+ | [[Category:Monad]] |

## 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.