(This is the same issue as http://www.haskell.org/pipermail/haskell/ 2004-March/013739.html but there was no follow-up at that time.) The intersperse library function is not as lazy as it could be. The current definition of intersperse is intersperse :: a -> [a] -> [a] intersperse _ [] = [] intersperse _ [x] = [x] intersperse sep (x:xs) = x : sep : intersperse sep xs For any list (x:xs) not containing _|_, intersperse sep (x:xs) is a list of the form (x:...); yet intersperse sep (x:_|_) = _|_ because the pattern match on the second equation diverges. A better definition would be intersperse _ [] = [] intersperse sep (x:xs) = x : intersperseWithPrefix xs where intersperseWithPrefix [] = [] intersperseWithPrefix (x:xs) = sep : x : intersperseWithPrefix xs (slightly modified from http://www.haskell.org/pipermail/haskell/2004- March/013741.html) An application: There was a question on #haskell about how to compute the "ruler" sequence [1,2,1,3,1,2,1,4,1,2,1,3,1,2,1,5,...]. The definition ruler = fix ((1:) . intersperse 1 . map (1+)) works with the properly lazy intersperse, but not with the intersperse in Data.List. Comments on this new definition? Can it get added to Data.List? Regards, Reid Barton