On Thu, Apr 23, 2009 at 9:06 PM, Luke Palmer
However, there is a function "sum" in the prelude, so you can do this much more simply:
sumit :: Int -> Int sumit n = sum [1..n]
:-)
Yeah, but this prelude sum function suffers from the same stack overflow thing (which is a design mistake I think): c:\>ghci GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> sum [0..10^6] *** Exception: stack overflow Prelude> So the easiest way to fix this is by defining a strict sum: import Data.List (foldl') sum' = foldl' (+) 0 Or if you prefer to see how this works internally: sum' xs = aux 0 xs where aux s [] = s aux s (x:xs) = let s' = x+s in s' `seq` aux s' xs An interesting thing I learned from this mailing list is that the stack overflow does not occur because the huge addition expression (0+1+2+3+4+5+6+7+...) is build up in memory, but because the evaluation of this gigantic expression *after* it has been completely build up in memory. One can demonstrate that the addition expression is first build up in memory like this: c:\>ghci GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> sum [0..10^100] <interactive>: out of memory