[Haskell-cafe] Typing problems with basic arithmetic - help!
chrisk at MIT.EDU
Fri Sep 23 21:01:24 EDT 2005
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) 
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)]
> [ (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]
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