Am Freitag 23 Oktober 2009 18:30:53 schrieb Philip Scott:
Hello again,
then, barring memory pressure forcing it out, it will be computed only once (each list element will be computed only once, when it's first needed).
Thanks Daniel, that was what I was after. Is there any way of investigating these things without using the profiler? E.g. is there any way to stick a debug print statement inside a function without moving over to sideeffects and IO Monads etc.. I know printing is a side effect, but it would be nice to say 'I can has itsy sneeky side effect plz Haskell, just for little testing while'
Cheers,
Philip
import Debug.Trace infixl 0 `debug` debug = flip trace dfib :: Int -> Integer dfib = let fib 0 = 0 fib 1 = 1 fib n = dfib (n-2) + dfib (n-1) `debug` "eval fib " ++ show n in (map fib [0 .. ] !!) Ok, modules loaded: MFib. *MFib> dfib 4 eval fib 4 eval fib 2 eval fib 3 3 *MFib> dfib 7 eval fib 7 eval fib 5 eval fib 6 13 *MFib> dfib 15 eval fib 15 eval fib 13 eval fib 11 eval fib 9 eval fib 8 eval fib 10 eval fib 12 eval fib 14 610 *MFib> The trick with debug = flip trace makes commenting out the debug-code easier: fun x = trace ("fun " ++ show x) $ body x ~> fun x = {- trace ("fun " ++ show x) $ -} body x vs. fun x = body x `debug` "fun " ++ show x ~> fun x = body x -- `debug` "fun " ++ show x But beware, including the argument in the trace message can lead to recalculation of values which would be cached without it, it's a hairy issue.