Brian Hulley wrote:
Brian Hurt wrote:
nth 0 (x:xs) = Some x
nth i (x:xs) = if i < 0 then Empty else nth (i-1) xs
nth i [] = Empty
[blows stack on large i]
As other people have pointed out, this is due to laziness. I'd write
it like:
nth 0 (x:_) = Some x
nth i (_:xs) = of i < 0 then Empty else (nth $! i-1) xs
^^^ -- oops!
nth _ [] = Empty
I think the above code would be more efficient written as:
nth n xs = if n < 0 then Empty else nth' n xs
where
nth' 0 (x:_) = Some x
nth' i (_:xs) = (nth' $! i - 1) xs
nth' _ _ = Empty
since the first 2 cases deal with every situation where we've got a
non-empty list and an index >= 0 (so no need to keep checking that the index
is >= 0 in the second case - instead the test can be moved outside the
"loop") and there's no need to pattern match against [] in the last case
because we already know this must be an empty list.
(Though in general it's probably a good idea to make each pattern as
independent as possible when writing a function so that if some cases of a
function are later edited there's more chance of the compiler being able to
lead you to errors via the "incomplete patterns" warning - hopefully the
compiler can optimize out any redundant checks)
Brian.
--
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