foo n = if n<0 then [] else n : foo (n-1)
bar n = aux 0 [] where aux i xs = if i>n then xs else aux (i+1) (i:xs)
that foo is more efficient than bar because lazy evaluation of foo just puts the delayed computation in the "cdr" of the list, while lazy evaluation of bar has to keep track of all aux calls (the "closures") which gives much more overhead, maybe even stack overflow? Something like that?
There is absolutely no problem with bar, it will not stack overflow since it _is_ tail-recursive _and_ the comparison i > n force the evaluation of i avoiding the risk of constructing a too big thunk for the first parameter of aux which could bring a stack overflow like in this example : nonStrictLength n [] = n nonStrictLength n (_:xs) = nonStrictLength (n+1) xs (try nonStrictLength 0 [1..10000000] in GHCi to see the stack overflow, GHC strictness analysis would avoid the problem with -O) Though foo is still more interesting than bar since it will work on infinite list, bar is too strict. -- Jedaï