[Haskell-cafe] Empty Input list

[Chris Wong]

On Tue, Mar 13, 2012 at 12:24 PM, Chris Smith <cdsmith at gmail.com> wrote:
> On Mon, Mar 12, 2012 at 3:14 PM, Kevin Clees <k.clees at web.de> wrote:
>> Now my function looks like this:
>>
>> tmp:: [(Int, Int)] -> Int -> (Int, Int)
>> tmp [] y = (0,0)
>> tmp xs y = xs !! (y-1)
>
> Just a warning that this will still crash if the list is non-empty by
> the index exceeds the length.  That's because your function is no
> longer recursive, so you only catch the case where the top-level list
> is empty.  The drop function doesn't crash when dropping too many
> elements though, so you can do this and get a non-recursive function
> that's still total:
>
> tmp :: [(Int,Int)] -> Int -> (Int, Int)
> tmp xs y = case drop (y-1) xs of
>    []         -> (0,0)
>    Just (x:_) -> x

That last line should be

    (x:_) -> x

without the "Just". Hopefully that'll save a bit of confusion.

Chris

> --
> Chris Smith
>
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