小编bic*_*885的帖子

在使用monad变换器构建monad堆栈来编写库时,我遇到了一个关于它的行为的问题.

以下代码不会传递类型检查器:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Foo (FooM, runFooM, foo) where

import Control.Applicative
import Control.Monad.Reader

newtype FooM m a = FooM { runFooM :: ReaderT Int m a }
  deriving (Functor, Applicative, Monad, MonadReader Int)

foo :: FooM m Int
foo = do
  x <- ask
  return x

错误是:

$ ghc foo.hs
[1 of 1] Compiling Foo              ( foo.hs, foo.o )

foo.hs:12:3:
    No instance for (Monad m) arising from a do statement
    Possible fix:
      add (Monad m) to the …