Re: [Haskell-cafe] Re: (possibly) a list comprehensions question

19 Nov 2009


      You're right. I do that sometimes, but I must make a habit of "hoogling".

2009/11/19 yairchu@gmail.com 
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This is what happens all the time, the thing I am trying to implement is
already in the library!
You can easily find those library functions using Hoogle.
In this case Hoogle gives the "sequence" function as its second result
for the query "[[a]] -> [[a]]":
Data.List transpose :: [[a]] -> [[a]]
Prelude sequence :: Monad m => [m a] -> m [a]
See: http://haskell.org/hoogle/?hoogle=%5B%5Ba%5D%5D+-%3E+%5B%5Ba%5D%5D
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Thanks for the both quick answers.
This is what happens all the time, the thing I am trying to implement is
already in the library!
Neil,
I clearly understood the way you implemented. Actually I had a similar
implementation with some problems, one of which is the base-case you
mentioned.
Eugene,
I'll definitely have a look at the implementation of sequence.
Cheers!
2009/11/19 Neil Brown 
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Ozgur Akgun wrote:
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Anyway, just forget the fact that these funstions do not do a check on
...
...
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length of the input list for a moment. My question is, how can I
generalize
this function to accept a list of lists of arbitrary length, and
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required result.
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Hi,
...
The concise solution is the list monad, as already posted.  If that
confuses you, here is a version using list comprehensions (well,
mostly):
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allPossibilities :: [[a]] -> [[a]]
allPossibilities [] = [[]]
allPossibilities (l:ls) = [ x : xs | x <- l, xs <- allPossibilities ls]
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The second line prefixes all possibilities from the later lists with
every
element from the the first list.  Note that the base-case is crucial;
if it
is the empty list [], that "xs <- allPossibilities ls" will not find
any
elements, and thus the list comprehension becomes the empty list, and
On Nov 19, 3:58 pm, Ozgur Akgun  wrote:
the
produce the
the
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whole thing falls apart.  Thus the base case must be the list
containing the
empty list, so that you have one possibility arising at the end upon
which
to prefix the items.  Hope that makes sense...
...
Thanks,
...
Neil.
--
Ozgur Akgun
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-- 
Ozgur Akgun