[Xmonad] Re: Xmonad no longer compile since latest darcs pull

[Andrea Rossato]

On Fri, Aug 17, 2007 at 10:38:12AM +0200, Kai Grossjohann wrote:
> The part that I'm having trouble with is monads and the associated
> operators (liftm comes to mind, it seems to be somehow related).  I read
> Hal Daume's tutorial.  Do you have advice what else I can read to grok?
> 
> (And I'm having trouble to understand the relationship between "<-" and
> "=".  Apparently, "<-" somehow implies that "statements" are "executed"
> in some order.  But Haskell tries really hard to hide the fact that
> there are statements and that there is such as thing as execution.  As
> Haskell is based on lambda calculus, I am not surprised -- there are no
> statements and there is no order of execution in lambda calculus.  So
> Haskell tries so hard to hide them that I can't find them...)


Hi Kay,
may I point you to this?
http://www.haskell.org/haskellwiki/The_Monadic_Way

I wrote it when I was trying to grasp monads myself, with the hope to
provide newcomers with a helpful guide through stuff like <-, >> and
>>=.

A monad is just a way to nest anonymous functions so that bound
variables (<-) can be later used by other (nested) anonymous
functions. And by nesting functions you can simulate an order of
execution. The complexity of this operation is hidden by the "do"
notation, where every new line is indeed an anonymous function.

The advise I can give you is to try to re-implement monads (and later
arrows, and so on), by reading the papers that discovered them.

For monads I suggest this paper:
P. Wadler, Monads for functional programming.
http://homepages.inf.ed.ac.uk/wadler/papers/marktoberdorf/baastad.pdf

This is how I came to understand monads.

Hope this helps
Andrea