[Haskell-beginners] list monad question
Matthias Guedemann
matthias.guedemann at ovgu.de
Fri Oct 30 07:44:41 EDT 2009
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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