[Haskell-cafe] Re: Explaining monads
Benjamin Franksen
benjamin.franksen at bessy.de
Mon Aug 13 16:29:37 EDT 2007
Brian Brunswick wrote:
> One thing that I keep seeing people say (not you), is that
monads /sequence/
> side effects. This is wrong, or at
> least a limited picture.
>
> /All/ of the above structures are about combining compatible things things
> together in a row.
> /None/ of them force any particular order of evaluation - that all comes
> from the particular instance. So its
> only a particular feature of IO that it sequences the side effects. Others
> don't - we can have a lazy State
> monad that just builds up big thunks.
I am a bit astonished.
Let's take the simplest example: Maybe. The effect in question is the
premature abortion of a computation (when Nothing is returned). And of
course Maybe sequences these effects, that's what you use it for: the
_first_ action to be encountered that returns Nothing aborts the
computation. Clearly sequencing goes on here.
Similar with the Error Monad (i.e. Either Err, for some Err type).
I won't talk about List Monad because I always had difficulty understanding
the List Monad.
What about State? The effect is reading/writing the state. Again, the State
Monad takes care that these effects get sequenced, and again that's what
you expect it to do for you.
And so on...
This is -- of course -- not a proof, so maybe there /are/ Monads that don't
sequence (their) effects. I'd be most interested to see an example, if
there is one, to bring myself nearer to the -- unattainable -- goal of full
enlightenment wrt Monads.
Cheers
Ben
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