[Haskell-cafe] Explaining monads
Kim-Ee Yeoh
a.biurvOir4 at asuhan.com
Thu Aug 16 12:55:11 EDT 2007
Paul Hudak wrote:
>
> [I]n the one-of-many ways that I view monads, a monad is just a high-order
> function that abstracts away function composition. In particular, if I
> have an action f, and an action g, I can think of them as recipes, because
> I can combine them via f >>= g. It's only after I combine all of my
> actions together that I apply the result to my input (via "run").
>
> Well, that's just like function composition. In particular, if I have a
> function f, and a function g, I can think of them as recipes, because I
> can combine them via f . g.
>
First a nitpick:
At the beginning of the thread I had pointed out how co/monadic
co/bind generalizes function /application/, i.e. flip ($).
Generalized function /composition/ on the other hand has a different
type. For instance, in the case of monads it's
\begin{code}
(<.>) :: Monad m => (a -> m b) -> (c -> m a) -> (c -> m b)
(<.>) f g c = (g c) >>= f
\end{code}
Function (<.>) is flip (>>>) in the instance of Monad m => Arrow
(Kleisli m). (For a moment there I couldn't find it in the haskell
libs. Would be nice if Hoogle could read instances such as these. I
must not be using Hoogle right, I couldn't even locate (>>=) by
type signature.)
Returning to the main point, like you and Dan P, I also have my
reservations explaining a monadified value as a recipe, although
for arguably different reasons. It's too chef-centric for me, to
continue with the original analogy. I have an allergic reaction to
"run" functions, at least what they connote. I claim that in
genuinely monadic programming, you never think about "run". Code
that's genuinely monadic works for all monads. Code like that is
what you want to write.
Or put another way, you just program like before except lifting
stuff into an arbitrary monad. You then write different monads
to retrofit the various add-on computations you want performed.
I thought this is well-known for over 15 years?
--
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