On 3 February 2016 at 11:52, derek riemer
Doesn't foldr have to "reach" to the far right of the list of infinite primes to get the last number since it consumes from the right?
foldr does not consume from the right. It is right-associative. See these simulations: http://foldr.com http://foldl.com Notice that the essential difference between foldr and foldl is the placement of the parentheses in the result, which the simulations show very well. Whether that expression "short-circuits" or not depends on the operator you used with your fold. Operators that are lazy in their second argument (at least some of the time), like "||" and ":", can short-circuit when used with foldr. A common mistake is to think the parentheses force an evaluation order, like "inside-out" for example. This is not the case for lazy languages like Haskell. This unfortunate confusion probably arises because parentheses often do prescribe evaluation order in basic arithmetic and strict languages like Java. Hope this helps, Thomas Koster