Re: [Haskell-cafe] Typing problems with basic arithmetic - help!
For some reason the following code is producing an error message from ghci that the the patterns are non-exhaustive. Does anyone have any idea why that could be, given that the patterns are, at least in my meaning, provably exhaustive? choosenonuniqueset n (a:r) | (length r) > (n-1) = [ (sort (a:x)) | x <- (choosenonuniqueset (n-1) r)] `union` [ (sort (a:x)) | x <- (choosenonuniqueset n r)] | (length r) == (n-1) = [a:r] | (length r) < (n-1) = [] Error message is: *Main> :reload Compiling Main ( birthday.hs, interpreted ) Ok, modules loaded: Main. *Main> choosenonuniqueset 2 [1..5] *** Exception: birthday.hs:(22,0)-(27,29): Non-exhaustive patterns in function choosenonuniqueset *Main> choosenonuniqueset 5 [1..5] [[1,2,3,4,5]] *Main>
In case I confused anyone, this is a separate issue. Thanks for your attention and help. On Sat, Sep 24, 2005 at 01:20:22AM +0100, Marcin Tustin wrote:
For some reason the following code is producing an error message from ghci that the the patterns are non-exhaustive. Does anyone have any idea why that could be, given that the patterns are, at least in my meaning, provably exhaustive?
choosenonuniqueset n (a:r) | (length r) > (n-1) = [ (sort (a:x)) | x <- (choosenonuniqueset (n-1) r)] `union` [ (sort (a:x)) | x <- (choosenonuniqueset n r)] | (length r) == (n-1) = [a:r] | (length r) < (n-1) = []
Error message is:
*Main> :reload Compiling Main ( birthday.hs, interpreted ) Ok, modules loaded: Main. *Main> choosenonuniqueset 2 [1..5] *** Exception: birthday.hs:(22,0)-(27,29): Non-exhaustive patterns in function choosenonuniqueset
*Main> choosenonuniqueset 5 [1..5] [[1,2,3,4,5]] *Main> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Well...your recursion will fail if (a:r) is matched against the empty set. That will trigger your Exception. So does your code avoid this ? No, it does not. cus 2 [1,2,3,4,5] recurses to cus 1 [2,3,4,5] to cus 0 [3,4,5] to cus (-1) [4,5] to cus (-2) [5] to cus (-3) [] ---Exception You need a n<=0 guard case AND an empty list base case chooseuniqueset _ [] = [] choosenonuniqueset n (a:r) | n<=0 = [] -- This needs to be the first guard Also, you're sorting is poor. The first case sorts too much and the second case does no sorting. Marcin Tustin wrote:
For some reason the following code is producing an error message from ghci that the the patterns are non-exhaustive. Does anyone have any idea why that could be, given that the patterns are, at least in my meaning, provably exhaustive?
choosenonuniqueset n (a:r) | (length r) > (n-1) = [ (sort (a:x)) | x <- (choosenonuniqueset (n-1) r)] `union` [ (sort (a:x)) | x <- (choosenonuniqueset n r)] | (length r) == (n-1) = [a:r] | (length r) < (n-1) = []
Error message is:
*Main> :reload Compiling Main ( birthday.hs, interpreted ) Ok, modules loaded: Main. *Main> choosenonuniqueset 2 [1..5] *** Exception: birthday.hs:(22,0)-(27,29): Non-exhaustive patterns in function choosenonuniqueset
*Main> choosenonuniqueset 5 [1..5] [[1,2,3,4,5]] *Main> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
On Sat, 24 Sep 2005, Marcin Tustin wrote:
For some reason the following code is producing an error message from ghci that the the patterns are non-exhaustive. Does anyone have any idea why that could be, given that the patterns are, at least in my meaning, provably exhaustive?
choosenonuniqueset n (a:r) | (length r) > (n-1) = [ (sort (a:x)) | x <- (choosenonuniqueset (n-1) r)] `union` [ (sort (a:x)) | x <- (choosenonuniqueset n r)] | (length r) == (n-1) = [a:r] | (length r) < (n-1) = []
You need to compare only once. Better write: choosenonuniqueset n (a:r) = case compare (length r) (n-1) of GT -> ... EQ -> [a:r] LT -> []
participants (3)
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ChrisK -
Henning Thielemann -
Marcin Tustin