[Haskell-cafe] Stack overflow

[Bertram Felgenhauer]

Krzysztof Skrzętnicki wrote:
> The code for modifying the counter:
> (\ msg -> atomicModifyIORef ioref (\ cnt -> (cntMsg cnt msg,())))

atomicModifyIORef does not force the new value of the IORef.
If the previous contents of the IORef is x, the new contents
will be a thunk,

   (\ cnt -> (cntMsg cnt msg,())) x

You can try forcing the new value, say by adding

   >> readIORef ioref >>= (return $!)

after the atomicModifyIORef.

> The datatype itself is strict. So where is the thunk actually accumulating?

In the IORef.

HTH,

Bertram