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 <jonathancc...@fastmail.fm
<mailto:jonathancc...@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>)
> 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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