Hi, Just looking at the definitions for foldr and foldl I see that foldl is (apparently) tail recursive while foldr is not. Why? Is it because foldl defers calling itself until last whereas foldr evaluates itself as it runs? What, strictly speaking, is the definition of ”tail recursive” as opposed to just “recursive”? Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.27/2021 - Release Date: 24/03/2009 4:00 PM