Re: [Haskell-cafe] warning - Euler problem spoiler enclosed

The problem is lexOrder s@[_] = s where you just give back what you receive, i.e. [Char]. But you claim to give back [[Char]]. Try [s] on the right-hand side. On 05/04/2011 02:41 PM, Barbara Shirtcliff wrote:

On May 4, 2011, at 7:21 AM, Ivan Lazar Miljenovic wrote:

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On 4 May 2011 13:13, Barbara Shirtcliff wrote:

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Hi,

In the following solution to problem 24, why is nub ignored? I.e. if you do lexOrder of "0012," you get twice as many permutations as with "012," even though I have used nub.

[snip]

lexOrder :: [Char] -> [[Char]] lexOrder s | length s == 1 = [s] | length s == 2 = z : [reverse z] | otherwise = concat $ map (\n -> h n) [0..((length s) - 1)] where z = sort $ nub s -- why is the nub ignored here? h :: Int -> [String] h n = map (z!!n :) $ lexOrder $ filter (\c -> lexI c z /= n) z As a guess, I think it's from the usage of length on the right-hand size.

Also, note that "lexOrder s@[_] = [s]" is nicer than "lexOrder s | length s == 1 = [s]". I agree that that initial version was a little clumsy, but your suggestion doesn't really seem to work:

lexOrder :: [Char] -> [[Char]] lexOrder s@[_] = s lexOrder s = concat $ map (\n -> h n) [0..((length z) - 1)] where z = sort $ nub s h :: Int -> [String] h n = map (z!!n :) $ lexOrder $ filter (\c -> lexI c z /= n) z

Euler.hs:8:18: Couldn't match expected type `[Char]' with actual type `Char' Expected type: [[Char]] Actual type: [Char] In the expression: s In an equation for `lexOrder': lexOrder s@[_] = s

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