i would expect to get back the Error from the *first* function in the sequence of functions in checkHeader (oggHeaderError from the oggHeader function). but instead i always see the Error from the *last* function in the sequence, OggPacketFlagError from the OggPacketFlag function. why is this? is there any way i can get the desired behavior...i.e. see the Error from the first function in the sequence that fails?
Hi You are essentially asking why this function: checkHeader handle = ((oggHeader handle) >> (oggStreamFlag handle) >> (oggHeaderFlag handle) >> (skipBytes handle 20) >> (oggPageSecCount handle) >> (oggPacketFlag handle)) returns the last error (OggPacketFlagError) instead of the first one. Some type annotations might help you see what is going on. So let's ask ghci the type of, e.g. oggHeaderFlag *File.Ogg> :t oggHeaderFlag oggHeaderFlag :: SIO.Handle -> IO (Either OggParseErrorType [Char]) oggHeaderFlag takes a handle, and computes either an error or a string. But since you are using >>, the computed value is not passed to the next function in the pipe! There is no way checkHeader can stop early simply because it is ignoring the intermediate results altogether. Since you are importing Control.Monad.Error, I believe you would probably want oggHeaderFlag et al to have type: SIO.Handle -> ErrorT OggParseErrorType IO [Char] This will propagate errors correctly. You can see a version of your code using ErrorT here: http://hpaste.org/12705#a1 Daniel