[Haskell-cafe] Reader monad

[David Leimbach]

On Wed, Dec 29, 2010 at 1:48 PM, Michael Lazarev
<lazarev.michael at gmail.com>wrote:

> 2010/12/29 michael rice <nowgate at yahoo.com>
> > I had an "Aha!" moment and it all makes sense now. Just as the State
> monad can hold a generator (which can change) and pass it down a calculation
> chain, a Reader monad can hold an environment (which doesn't change) and
> pass it down a calculation chain. I was wondering how I could include a
> (global) house betting limit in that craps application I've been playing
> with (without passing it as a parameter) and it sounds like the Reader monad
> would be an ideal candidate. Correct? It also sounds like a job for monad
> transforms.
>
> That is right. You need transformers if you want to have one value as
> settings to read, other value as a state, and so on, and they must be
> simultaneously accessible in some function. Or, for example, if you
> want to build a sequence of IO actions by functions that share the
> same environment.
>
> After you said that you had an "Aha!" moment, I remembered how I had
> something alike not very long ago. That was surprising event when I
> ended up with some transformer stack written myself although just
> several minutes ago I would consider this to be some dark wizardry.
>
> When I was dealing with monads for the first time, I tried reading
> source code and explanations. Soon I found that pure unapplied theory
> was making such a dismal, depressing feeling on me that I was not able
> to continue.
>
> But some time after I was writing an application in Haskell. It was
> real, and contrary to my previous theoretical studies the process was
> much fun. I had good FP background at that time, and had no problem
> writing everything in functional style. Since everything that can be
> done with monads can also be done without them, I didn't use any monad
> except IO (in main function :) ). Not that I especially avoided them,
> I just didn't think about them.
>
> And then I noticed that I constantly pass one same parameter to a lot
> of functions. And then -- since I remembered the boring theory -- bam!
> -- I introduced Reader into the code. And it worked. Which tempted me
> to interweave Reader and Writer, and so on, and twenty minutes later I
> had monstrosity that I only saw before in others' code: ReaderT
> WriterT State .... and so on :)
>
>
Reader Writer State is commonly needed in big applications so transformers
provides one for us:

http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/Control-Monad-Trans-RWS-Lazy.html

Pretty cool stuff if you ask me.  I often wondered about the correct
stacking order of Monad transformers, or how often it mattered.

Dave



> So, good luck with your application!
>
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