如何使MonadError与ghcjs/reflex一起工作

Question

我正在努力编译以下程序：

import           Data.Maybe
import qualified Data.Text       as T
import qualified Data.Text.IO    as T
import           Data.URLEncoded
import           Reflex.Dom
url :: T.Text
url = T.pack "parm1=one&parm2=two"
main = do
mainWidget body
body :: MonadWidget t m => m ()
body  = el (T.pack "div") $ do
-- let t = (T.pack "this program works if you replace the line below with this")
t <- fmap (T.pack . fromMaybe "" . Data.URLEncoded.lookup "parm2") (importString (T.unpack url))
text t

但是，这个类似的版本适用于香草GHC

import           Data.Maybe
import qualified Data.Text       as T
import qualified Data.Text.IO    as T
import           Data.URLEncoded
url :: T.Text
url = T.pack "parm1=one&parm2=two"
main = do
body
body  = do
t <- fmap (T.pack . fromMaybe "" . Data.URLEncoded.lookup "parm2") (importString (T.unpack url))
T.putStrLn t

编译器说有些东西模棱两可，我真的不确定如何实现这些工作。

The type variable ‘e0’ is ambiguous
Relevant bindings include body :: m () (bound at reflex.hs:14:1)
These potential instances exist:
instance [safe] Control.Monad.Error.Class.MonadError e (Either e)
-- Defined in ‘Control.Monad.Error.Class’
...plus 13 instances involving out-of-scope types
instance [safe] Control.Monad.Error.Class.MonadError
GHC.IO.Exception.IOException IO
-- Defined in ‘Control.Monad.Error.Class’
instance [safe] (Monad m, Control.Monad.Trans.Error.Error e) =>
Control.Monad.Error.Class.MonadError
e (Control.Monad.Trans.Error.ErrorT e m)
-- Defined in ‘Control.Monad.Error.Class’

仅供参考：我还没有完全掌握Monads，很容易被这些错误吓到。帮助！

Answer 1

在 ghc 版本中，importString在IOmonad do 语句的上下文中运行。importString能够在IOmonad 中返回一个值，因此编译器很高兴

在 ghcjs 版本中，importString在mmonad do 语句的上下文中运行(m在body的声明中指定(。importString无法在mmonad 中返回值，因此编译器会抱怨。

您可以通过使用liftIO将IOmonad 值更改为mmonad 值来解决此问题。 下面是包含此更改的代码以及我为帮助自己理解代码而进行的其他一些更改。

import           Data.Maybe
import qualified Data.Text       as T
import           Data.URLEncoded as DU
import           Reflex.Dom
import           Control.Monad.Trans as CMT 
url :: T.Text
url = T.pack "parm1=one&parm2=two"
main = do
mainWidget body
body :: MonadWidget t m => m ()
body  = el (T.pack "div") $ do
let istr = CMT.liftIO $ DU.importString (T.unpack url)
t <- fmap (T.pack . fromMaybe "" . DU.lookup "parm2") istr
text t