[Haskell-cafe] Empty Input list

Chris Smith cdsmith at gmail.com
Tue Mar 13 00:24:28 CET 2012

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

Chris Smith

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