无法将预期类型 'Control.Monad.Trans.Reader.ReaderT MongoContext IO a0' 与实际类型 'IO ()' 匹配

Question

我想打印取自 mongoDB 的点图，然后转换成图像。

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

您需要 liftIO $ 在 Prelude.putStrLn 的左侧，但下次粘贴带有行号等的完整错误。您的 do 块位于 ReaderT MongoContext IO monad 中，其中包含 IO 因此您可以在其中执行 IO 操作，但您必须先解除它们。

无法将预期类型 'Control.Monad.Trans.Reader.ReaderT MongoContext IO a0' 与实际类型 'IO ()' 匹配

Couldn't match expected type 'Control.Monad.Trans.Reader.ReaderT MongoContext IO a0' with actual type 'IO ()'

haskell

dot

graphviz

mongodb