Hi community, I am making my first steps in Haskell, reading Haskell wikibook and now stuck with one of the excercises, namely this one: Implement a function sequenceIO :: [IO a] -> IO [a]. Given a list of actions, this function runs each of the actions in order and returns all their results as a list. This is what I came with: ioOne :: Num a => IO a ioOne = do print guid return guid where guid = 2 ioTwo :: Num a => IO a ioTwo = do print guid return guid where guid = 3 sequenceIO :: Num a => [IO a] -> IO [a] sequenceIO [] = return [] sequenceIO (x : xs) = do result <- x return result : sequenceIO xs First two functions are there because of the invocation that I've planned: sequenceIO [getGuid, getNextGuid]. However, this could not be compiled (GHC): Couldn't match expected type `[m a]' against inferred type `IO [a]' In the second argument of `(:)', namely `sequenceIO xs' In the expression: return result : sequenceIO xs In the expression: do result <- x return result : sequenceIO xs Fine, I thought, something wrong with the type of the 'sequenceIO xs' (becasue I am sure the type of 'result' is fine). So I wrote another program to check what happens to the result of IO action evaluation (namely, which type is assigned): bar :: Num a => IO a bar = do print guid return guid where guid = 2 foo = do result <- bar result This could not be compiled either: No instance for (Num (IO b)) arising from a use of `bar' at auxil.hs:8:19-21 Possible fix: add an instance declaration for (Num (IO b)) In a stmt of a 'do' expression: result <- bar In the expression: do result <- bar result In the definition of `foo': foo = do result <- bar result I am a newbie, so I am interpreting this like "Haskell could not construct Num from the result of invocation of bar, which is of type IO a". But why do I need this at all? When doing console I/O with 'result <- getLine', I do not need to reconstruct String from the result. What am I doing wrong? Where is the failure in reasoning? Thanks, Sergey