[Haskell-cafe] Generalized list-comprehension

[andrew cooke]

is this what you want?  not sure if i've understood, and not sure it's
efficient.

andrew

grow :: Int -> [[Int]] -> [[Int]]
grow n (x:xs) = x:(grow n (xs ++ [x++[i] | i <- [1..n]]))

seqn :: Int -> [[Int]]
seqn n = grow n [[i] | i <- [1..n]]

main :: IO ()
main = do print $ take 100 (seqn 3)



Ron de Bruijn said:
> Hi there,
>
> I have written this little function:
>
> f :: (Num a, Enum a) => a -> [[a]]
> f n =  [[a]|a<-fu n] ++ [a:[b]|a<-fu n,b<-fu n] ++
> [a:b:[c]|a<-fu n,b<-fu n,c<-fu n]
> fu n = [1..n]
>
> This is an example of the function in action:
>
> *Mod> f 4
> [[1],[2],[3],[4],[1,1],[1,2],[1,3],[1,4],[2,1],[2,2],[2,3],[2,4],[3,1],[3,2],[3,
> 3],[3,4],[4,1],[4,2],[4,3],[4,4],[1,1,1],[1,1,2],[1,1,3],[1,1,4],[1,2,1],[1,2,2]
> ,[1,2,3],[1,2,4],[1,3,1],[1,3,2],[1,3,3],[1,3,4],[1,4,1],[1,4,2],[1,4,3],[1,4,4]
> ,[2,1,1],[2,1,2],[2,1,3],[2,1,4],[2,2,1],[2,2,2],[2,2,3],[2,2,4],[2,3,1],[2,3,2]
> ,[2,3,3],[2,3,4],[2,4,1],[2,4,2],[2,4,3],[2,4,4],[3,1,1],[3,1,2],[3,1,3],[3,1,4]
> ,[3,2,1],[3,2,2],[3,2,3],[3,2,4],[3,3,1],[3,3,2],[3,3,3],[3,3,4],[3,4,1],[3,4,2]
> ,[3,4,3],[3,4,4],[4,1,1],[4,1,2],[4,1,3],[4,1,4],[4,2,1],[4,2,2],[4,2,3],[4,2,4]
> ,[4,3,1],[4,3,2],[4,3,3],[4,3,4],[4,4,1],[4,4,2],[4,4,3],[4,4,4]]
> *Mod>
>
> This is ofcourse nice and all, but I need a function
> that does this same trick, but then doesn't only
> generate listelements
> of the maximum hard-coded length of three, but to
> length m, where m is an extra parameter.
> It's important that the elements are precisely in this
> order.
>
> I tried rewriting the listcomprehension to map, concat
> and filter, but that only complicated things.
>
> There is a possibility where I can write a function
> that gives the next of a list. So the next of [1,1,1]
> would be [1,1,2], but that would be somewhat
> inefficient when the list becomes large.
> Then I just call this function with (replicate n (head
> $ fu n)) and (last $ fu n).
> Then I just apply this function next to the rest,
> until I reach a state where all elements of the list
> equal the last value of the inputlist, so I have the
> required list of length n.
> Then to simply get the complete list as in function f.
> I need to map(\x->otherFunction x) [1..], but this
> "next"-function is almost identical to what the
> built-in listcomprehension does.
> I just don't like my solution.
>
> Then I can probably use Template Haskell(which I have
> never used), but that seems overkill.
>
> So does anyone has a better solution?
>
> Greets Ron
>
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-- 
personal web site: http://www.acooke.org/andrew
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