[Haskell-cafe] Re: The problem with Monads...

[Rafael Gustavo da Cunha Pereira Pinto]

Wadler's examples are way better than the ones in the documentation. But
without his explanations of what he is doing, the examples alone are pretty
worthless.

I agree that all_about_monads could be a better source for the
documentation, at least for completeness. But even there I would consider
choosing simpler examples, like Wadler's expression evaluator.




On Tue, Jan 13, 2009 at 21:07, Benedikt Huber <benjovi at gmx.net> wrote:

> Rafael Gustavo da Cunha Pereira Pinto schrieb:
>
>> Yes, I've read it twice, and it is a nice explanation that "yes, the
>> reader monad is an application and is a monad". How do I use it? Why not the
>> function itself? How would the plumbing work in a real world example?
>>
> Hi Rafael,
>
> First of all, I agree that the documentation for mtl should be improved.
> Especially Control.Monad.RWS and Control.Monad.List really need some more
> information.
>
> The documentation for the Reader and Identity monad are quite detailled
> though. They seem to be "inspired" by
> http://www.haskell.org/all_about_monads/, which also has documentation on
> Writer and Cont.
>
> Control.Monad.List is a good example for the lack of documentation:
> There is a single sentence at the beginning
> > "The List monad."
> and then one declaration
> > newtype ListT m a = ListT { runListT :: m [a] }
> while the important information is hidden as one of many instance
> declarations:
> > Monad m => Functor (ListT m)
> > Monad m => MonadPlus (ListT m)
>
> Btw, the quality of the examples in Control.Monad.Reader is debutable. From
> Example 1:
>
> > -- The Reader monad, which implements this complicated check.
> > calc_isCountCorrect :: Reader Bindings Bool
> > calc_isCountCorrect = do
> >    count <- asks (lookupVar "count")
> >    bindings <- ask
> >    return (count == (Map.size bindings))
>
> I think it is wrong (or weird at least) to call the function a 'Reader
> monad'; the name 'calc_isCountCorrect' is horrible too (because of the calc_
> prefix),  Finally, implementing
>
> > isCountCorrect :: Bindings -> Bool
> > isCountCorrect bs = (bs Map.! "count") == Map.size bs
>
> using the Reader monad will convince everybody _not_ to use it.
>
> A suggestion: If license permits it, short versions of the articles on
> all_about_monads would make a great documentation for mtl (except for RWS
> and List, which are missing).
>
> benedikt
>
>
>
>> BTW, the article is really great as an brief introduction to monad
>> transformers. For the whole concept of monads, my all time favorite is "The
>> Haskell Programmer's Guide to the IO Monad" by Stefan Klinger.
>>
>> Chapters 14 to 19 of "Real World Haskell" also have a good introduction on
>> the usage of the monads, but it lacks other monads, like the RWS or the
>> Continuation...
>>
>> See, that is my point. The mathematical concept of monads is very
>> palatable. The idea that monads are either patterns or structures to hide
>> computations in sequence is also very easy to see. But how do we use them?
>> Why should I use a Writer monad when I can use a (a,w) tuple?
>>
>>
>>
>> On Tue, Jan 13, 2009 at 13:51, Jonathan Cast <jonathanccast at fastmail.fm<mailto:
>> jonathanccast at fastmail.fm>> wrote:
>>
>>    On Tue, 2009-01-13 at 12:56 -0200, Rafael Gustavo da Cunha Pereira
>> Pinto
>>    wrote:
>>     >
>>     > Last night I was thinking on what makes monads so hard to take, and
>>     > came to a conclusion: the lack of a guided tour on the implemented
>>     > monads.
>>
>>    ...
>>
>>     > Inspired by the paper "Functional Programming with Overloading and
>>     > Higher-Order Polymorphism",
>>     >         Mark P Jones
>>     > (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html<http://web.cecs.pdx.edu/%7Empj/pubs/springschool.html>
>>    <http://web.cecs.pdx.edu/%7Empj/pubs/springschool.html>)
>>     >           Advanced School of Functional Programming, 1995.
>>     >
>>     > SO WHAT?
>>
>>    So have you read Jones' paper?  Or do you have a *concrete* explanation
>>    of how it differs from your desired `guided tour'?
>>
>>    jcc
>>
>>
>>
>>
>>
>> --
>> Rafael Gustavo da Cunha Pereira Pinto
>> Electronic Engineer, MSc.
>>
>>
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>>
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>
>


-- 
Rafael Gustavo da Cunha Pereira Pinto
Electronic Engineer, MSc.
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