The following prints 1, as you'd expect: ----------------- import GHC.Stack foo :: HasCallStack => IO () foo = do print (length (getCallStack callStack)) main = foo ------------------- But when I make it this: ... l <- getCallStack callStack ; print (length l) I get all this: ------------------------------------------ ... • Couldn't match type ‘[]’ with ‘IO’ Expected type: IO ([Char], SrcLoc) Actual type: [([Char], SrcLoc)] • In a stmt of a 'do' block: l <- getCallStack callStack In the expression: do l <- getCallStack callStack print (length l) In an equation for ‘foo’: foo = do l <- getCallStack callStack print (length l) | 5 | l <- getCallStack callStack ; | ^^^^^^^^^^^^^^^^^^^^^^ -------------------------------------------- What am I not seeing? Regards, Michael Turner Executive Director Project Persephone 1-25-33 Takadanobaba Shinjuku-ku Tokyo 169-0075 Mobile: +81 (90) 5203-8682 turner@projectpersephone.org Understand - http://www.projectpersephone.org/ Join - http://www.facebook.com/groups/ProjectPersephone/ Donate - http://www.patreon.com/ProjectPersephone Volunteer - https://github.com/ProjectPersephone "Love does not consist in gazing at each other, but in looking outward together in the same direction." -- Antoine de Saint-Exupéry