How does reverse work in constant space? At the moment I can't imagine it doing so; that's why I tried it, but of course you could be right.
It allocates a new list cell for every cell it finds in the input list. If the input list can be garbage collected then reverse takes constant space because each time it inspects a list cell from the input list that cell can be garbage collected. If the input list is being retained for some other reason then reverse will take linear space.
I don't think that's true. It must inspect the whole input list to give the first element of the output list, but it cannot garbage collect the input list because it needs to yield every element of it.
reverse creates the output list by pulling items from the head of the input list, and prefixing them to the output list. After each item has been pulled from the input, the list node can be GC'd from the input list. Also, if the output list is itself being consumed by a function which does not require the entire list (like length) then again the list node can be GC'd soon after consumption.
ghci> length $ reverse [1..10^7]
This is a compiler optimisation, I think, so will probably require ghc -O(2?). Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************