Hello! Sorry that I bother you again with my problem.
It may be you'd profit from reading some of the Monad tutorials out there.
I followed your advice and read the "All about monads" tutorial. After I've done that I could fix one problem. But now there is another one. In the main program, there is following piece of code <code-snippet> let classifiedImagesWithData = ((return trainingSet) >>= readClassifiedImages) let allImages = (getImages classifiedImagesWithData) </code-snippet> Signatures of readClassifiedImages and getImages are readClassifiedImages :: [ClassifiedImage] -> [IO (ClassifiedImage, Image)] getImages :: [(ClassifiedImage, Image)] -> [Image] readClassifiedImages does some file I/O, so it is OK when its result is in the IO monad. getImages does not do any IO related stuff, so I declared it without the IO monad. The line let allImages = (getImages classifiedImagesWithData) of the above code snippet leads to following error: <error> ExperimentalYaleFaceDb.hs:43: Couldn't match `(ClassifiedImage, Image)' against `IO (ClassifiedImage, Image)' Expected type: [(ClassifiedImage, Image)] Inferred type: [IO (ClassifiedImage, Image)] In the first argument of `getImages', namely `classifiedImagesWithData' In the definition of `allImages': allImages = (getImages classifiedImagesWithData) Failed, modules loaded: TestLik, Lik, HUnit, HUnitText, HUnitBase, HUnitLang. </error> Is there a way to fix this problem apart from changing signature of getImage? Thanks in advance Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com