On Sat, 31 Jul 2010 17:30:54 -0400, Brandon S Allbery KF8NH
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On 7/31/10 16:58 , wren ng thornton wrote:
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
Given all the messes seq makes ("hey, go behind the compiler's back and touch this arbitrary value of arbitrary type"), I generally consider it to be unsafeSeq :)
I would deeply in favor of renaming seq to unsafeSeq, and introduce a type class to reintroduce seq in a disciplined way. -- Nicolas Pouillard http://nicolaspouillard.fr