Re: [Haskell-beginners] Re: list monad question

30 Oct 2009

      Hello Ertugrul,

this looks interesting. I read Brent Yorgeys Typeclassopedia and also the work
you cite in your tutorial. 
So I am trying to learn about Applicative, Monoids, Monads etc. by using
them. Your description of the library functions comes handy, as reading the
source directly does not really help if you're still struggling with
understanding the concepts. And on the other hand, due to the abstract nature of
monads, a lot of functionality should be "hidden" in the library (just like my
rewrite of replicateM)

thank you very much, will read it on the weekend

regards
Matthias

Excerpts from Ertugrul Soeylemez's message of Fr Okt 30 15:01:29 +0100 2009:
...
Hello Matthias,
you may want to have a look at section 11 of my monads tutorial [1],
which contains monadic library functions like replicateM together with
examples and detailed explanations.
[1] http://ertes.de/articles/monads.html#section-11
Greets,
Ertugrul.
Matthias Guedemann  wrote:
...
Hi,
a friend of mine wanted to write function (in Perl) that creates all tuples of
length 3 of the elements of a given list,
e.g. [(0,0,0),(0,0,1),(0,0,2),...,(5,5,5)] for the list [0..5]. Trying to get
better at Haskell, I wrote a small function using the list monad for this (tuples
replaced with lists)
all3 ls = do
  a <- ls
  b <- ls
  c <- ls
  return [a,b,c]
Now I want to make it capable to create all combinations of length n instead of
fixed length 3 (that's why list instead of tuple), but I don't really see how.
As the do notation translates to
ls >>= \a ->  etc.
I thought it should be possible to have some sort of "foldr (>>=)" over a list
of length n, but I can't figure out how to collect the variable number of
results in a list for the "return".
Any hints for that?
best regards
Matthias
-- 
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Dipl. Inf. Matthias Guedemann              / __\/ _\  /__\
Computer Systems in Engineering           / /   \ \  /_\
Otto-von-Guericke Universitaet Magdeburg / /___ _\ \//__
Tel.: 0391 / 67-19359                    \____/ \__/\__/
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