[Haskell-cafe] Functions with side-effects?

Dylan Thurston dthurston at barnard.edu
Mon Dec 26 14:25:19 EST 2005

On Wed, Dec 21, 2005 at 11:43:42AM +0000, Daniel Carrera wrote:
> Hi all,
> I'm a Haskell newbie and I don't really understand how Haskell deals 
> with functions that really must have side-effects. Like a rand() 
> function or getLine().
> I know this has something to do with monads, but I don't really 
> understand monads yet. Is there someone who might explain this in newbie 
> terms? I don't need to understand the whole thing, I don't need a rand() 
> function right this minute. I just want to understand how Haskell 
> separates purely functional code from non-functional code (I understand 
> that a rand() function is inevitably not functional code, right?)

You said you were a mathematician, so let me give you the mathematical
version in case you like category theory:

  A monad is an monoid object in the category of endofunctors.

That is, a monad is an endofunctor 'F' (i.e., a data type depending on
a single parameter, which is furthermore an instance of Functor), with
a natural transformations from 'F (F a)' to 'F a' and from 'a' to
'F a', satisfying the usual rules for identity and associativity.

Exercise: List is a monad.  So is Maybe.

You can think about a monad 'F a' as a computation of an object of
type 'a' with side effects; for instance, the List monad allows the
side-effect of multiple answers (like non-determinism), and the Maybe
monad allows the side-effect of failure.  One particularly useful and
complicated monad is the IO monad, which allows interactions with the
outside world.

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