Difference between revisions of "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 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!
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All you really need to know, is that the <code>(>>=)</code> operator either returns <code>Nothing</code> if it is passed <code>Nothing</code> on its left-hand side; or if its left-hand side is a <code>Just …</code> it strips off the <code>Just</code>, and passes the contents into the function supplied on its right-hand side. Simple!
   
Some simple exercises:
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===Some simple exercises===
   
 
What would the following snippets resolve to?
 
What would the following snippets resolve to?
   
<haskell>
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* <haskell>
 
Just 0 >>= (\ x -> if (x == 0) then fail "zero" else Just (x + 1) )
 
Just 0 >>= (\ x -> if (x == 0) then fail "zero" else Just (x + 1) )
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</haskell>
  +
  +
* <haskell>
 
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>

Latest revision as of 21:49, 5 April 2021

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.