Hi Frodo, If you happen to have a memory consumption graph somewhere on the screen you can see that while testr does not need considerable amounts of memory, testr' does. This can also be seen by ghci’s output. The reason is that with testr we are building something like True && [thunk] which then evalutes to [thunk] which looks at the next element in the list and evaluates to True && [thunk] while with testr', we start with [thunk] && True and becaues && evalutes its first agument first, this thunk is entered and we get ([thunk] && True) && True and so on: (([thunk] && True) && True) && True ((([thunk] && True) && True) && True) && True ... until we reach the end of the list, with an inner True && True. At this point, this chain of thunks is reduced to True as you can see, there is much more memory stuff to do in testr' than in testr. If you replace && by || you’ll see that testr' behaves very similar to the && variant, while testr finishes immediately, no matter how large the parameter, due to the short circuiting mentioned by Daniel. Greetings, Joachim -- Joachim "nomeata" Breitner mail@joachim-breitner.de | nomeata@debian.org | GPG: 0x4743206C xmpp: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/