sqlite - Haskell 变量 "not in scope: type variable ' m'"

Question

我不清楚为什么类型声明无法编译并且我无法解释编译器错误，我错过了什么？下面是带有类型定义和编译器错误的代码。

{-# LANGUAGE RankNTypes, FlexibleContexts #-}
module API.Models.RunDB where

import Control.Monad.IO.Class (MonadIO, liftIO)
import Database.Persist.Sql (ConnectionPool, runSqlPool, SqlPersistT)
import Web.Scotty (ActionM)

type RunDB = MonadIO m => (forall a. SqlPersistT IO a -> m a)

runDB' :: ConnectionPool -> RunDB
runDB' pool q = liftIO $ runSqlPool q pool

RunDB.hs:8:22: error:
    Not in scope: type variable ‘m’
  |
8 | type RunDB = MonadIO m => (forall a. SqlPersistT IO a -> m a)
  |                      ^

Answer 1

在类型同义词的右侧，您不能像在大多数其他地方那样突然组成类型变量名。您需要通过指定 forall m 来显式量化它，如下所示:

type RunDB = forall m. MonadIO m => (forall a. SqlPersistT IO a -> m a)

还有两种替代解决方案。一种方法是将 m 作为参数传递给类型同义词，然后更改 runDB' 的类型以返回 RunDB m:

type RunDB m = MonadIO m => (forall a. SqlPersistT IO a -> m a)
runDB' :: ConnectionPool -> RunDB m

另一种方法是去掉类型同义词，然后这样写:

runDB' :: MonadIO m => ConnectionPool -> SqlPersistT IO a -> m a