[Haskell-cafe] Diagnose stack space overflow

[John Lato]

> From: Logo Logo <saraslogo at gmail.com>
>
> Hi,
>
> For the following error:
>
> Stack space overflow: current size 8388608 bytes.
> Use `+RTS -Ksize -RTS' to increase it.
>
> I want to find out the culprit function and rewrite it tail-recursively. Is
> there a way to find out which function is causing this error other
> than reviewing the code manually?
>

I'd like to point out that a stack-space overflow in Haskell isn't quite the
same thing as in other functional languages.  In particular, it's possible
for tail-recursive functions to overflow the stack because of laziness.

Consider this tail-recursive sum function:

> trSum :: [Int] -> Int
> trSum l = go 0 l
>  where
>   go acc [] = acc
>   go acc (x:xs) = go (acc+x) xs

It's tail-recursive.  But if you enter this in ghci and run it, you'll find
that it uses increasing stack space, and will likely cause a stack overflow
for large enough inputs.  The problem is that the accumulator 'acc' isn't
strict and builds up a thunk of the form:

0+n1+n2+...+nn

The solution is to add strictness.  For this example, a '!' on the
accumulator will do.  GHC will sometimes spot cases where extra strictness
is helpful (it'll figure this one out when compiled with -O), but it often
needs help.

I'd recommend Edward Yang's series of blog posts about debugging, space
leaks, and the Haskell heap.  One useful article is
http://blog.ezyang.com/2011/05/anatomy-of-a-thunk-leak/ , but you may want
to start at the beginning of the heap series.

John L.
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