The example given in the article can be addressed by using foldl' instead of foldr. The issue is discussed in http://www.haskell.org/haskellwiki/Foldr_Foldl_Fo

The example given in the article can be addressed by using foldl' instead of foldr. The issue is discussed in http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl A Haskell programmer needs to understand when non-strict evaluation can/should be avoided. foldr is usually more elegant but sometimes less efficient.