无法匹配期望类型'Control.Monad.Trans.Reader.ReaderT MongoContext IO a0'与实际类型'IO（）'

Question

I want to print my dot graph taken from mongoDB and then convert into an image.

run = do

    docs <- timeFilter -- function to fetch [Document] from mongoDB
    let dot = onlyDot docs -- exclude extra field from the documents
    let dotObject = getObjId dot -- convert into an object
    -- converting dot graph to string and then string to text to pass it on to parseDotGraph function
    let xDotGraph = parseDotGraph (B.pack (show dotObject)) :: G.DotGraph String
    Prelude.putStrLn $ B.unpack $ renderDot $ toDot xDotGraph -- this is not working, want to print
    -- addExtension (runGraphviz xDotGraph) Png "graph" -- this is not working, want to draw as an image
    printDocs dot

Answer 1

You need liftIO $ to the left of Prelude.putStrLn , but next time paste the complete error with line numbers and such. Your do block is in the ReaderT MongoContext IO monad, which contains IO so you can do IO actions in it but you have to lift them first.