I'm working on exercise 6.16 in Thompson's "Haskell; the Craft of Functional Programming" My goal for the moment is to call printPicture to see my results: type Picture = [[Char]] printPicture :: Picture -> IO () printPicture = putStr . concat . map (++"\n") And everything is encouraging. The return type of my function syncs up: *Main> :t superimposeImage imgA imgB superimposeImage imgA imgB :: [[Char]] Now, the only issue is that I have put a number of trace statements in my program whose output is *interspersed* with the return data. The return data is simply lists of characters, where each character is "." or "#" *Main> printPicture $ superimposeImage imgA imgB siPointLogic i1 i2 0 15 .siPointLogic i1 i2 1 15 .siPointLogic i1 i2 2 15 .siPointLogic i1 i2 3 15 .siPointLogic i1 i2 4 15 .siPointLogic i1 i2 5 15 .siPointLogic i1 i2 6 15 .siPointLogic i1 i2 7 15 .siPointLogic i1 i2 8 15 .siPointLogic i1 i2 9 15 .siPointLogic i1 i2 10 15 .siPointLogic i1 i2 11 15 When I remove my various trace statements, I get what I want: *Main> printPicture $ superimposeImage imgA imgB ................ ................ ................ ................ ...........##... So the question is, how can I leave the trace statements in the code and yet get the printPicture output after all the debugging output has output? It only makes sense for it to work that way. The trace output occurs during the program and in particular, during the program *prior* to the call to printPicture. Therefore all of the trace output should have finished *prior* to printPicture outputting it's rows. -- Carter's Compass: I know I'm on the right track when, by deleting something, I'm adding functionality.