[Haskell-cafe] Stupid newbie question

Brian Hulley brianh at metamilk.com
Sat Jan 6 06:54:59 EST 2007

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 

    nth 0 (x:_) = Some x
    nth i (_:xs) = of i < 0 then Empty else (nth $! i-1) xs
    nth _ [] = Empty

where a general rule of thumb is always to replace (fun intexp) by (fun $! 
intexp) whenever intexp is just a trivial arithmetic expression that doesn't 
involve traversing (ie forcing of) any other data structure.


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