[Haskell-cafe] Re: A guess on stack-overflows - thunks build-up and tail recursion

[Matthew Brecknell]

GüŸnther Schmidt wrote:
> the point I wanted to stress though is that the stack overflow does 
> actually not occur doing the recursive algorithm, just a build-up of thunks.

You can also observe this with suitable "trace" statements. For example:

> import Debug.Trace
> import System.Environment
> 
> my_foldl f z [] = trace "o" z
> my_foldl f z (x:xs) = trace "^" (my_foldl f (f z x) xs)
> 
> my_plus x y = trace "+" (x + y)
> 
> main = do
>   [n] <- getArgs
>   print (my_foldl my_plus 0 [1..read n])

Compile and execute with suitable options, including a reduced maximum
stack size. This works for me:

$ ghc --make stack.hs
$ ./stack 20 +RTS -K100

Also try the strict version:

> my_foldl' f z [] = trace "o" z
> my_foldl' f z (x:xs) = trace "^" (z' `seq` my_foldl' f z' xs)
>   where z' = f z x