Brandon S Allbery KF8NH wrote:
michael rice wrote:
Are you saying:
[ head x ] -> [ *thunk* ] and length [ *thunk* ] -> 1, independent of what *thunk* is, even head [], i.e., *thunk* never needs be evaluated?
Exactly. (I was being cagey because the first response was cagey, possibly suspecting a homework question although it seems like an odd time for it.)
length not only does not look inside of the thunk, it *can't* look inside it; all it knows is that it has a list, it specifically does *not* know what that list can hold. So the only thing it can do is count the number of "unknown somethings" in the list.
Not entirely true: stupidlyStrictLength :: [a] -> Integer stupidlyStrictLength [] = 0 stupidlyStrictLength (x:xs) = x `seq` 1 + stupidlyStrictLength xs Though, of course, if we actually wanted this function we should use an accumulator in order to avoid stack overflow when evaluating the (1+(1+...0)) thunk at the end. -- Live well, ~wren