本地提升Monad Reader [英] Lift Monad Reader local

问题描述

import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.Error

data Context
data Memory 
data Functions

data InterpreterM a = ExeInterpreter a | PropInterpreter a

newtype InterpreterMT m a = InterpreterMT { runInterpreterMT :: m (InterpreterM a) }
type Interpreter = InterpreterMT (StateT (Memory, Functions) (ReaderT (Context, Context) (ErrorT String IO)))

data Stmt
data Stmts = EmptyStmts | Statements Stmt Stmts

interpretStmt :: Stmt -> Interpreter Context

interpreter :: Stmts -> Interpreter ()
interpreter EmptyStmts = return () 
interpreter (Statements s stmts) = do
    currEnv <- interpretStmt s
    local (\(prev, _) -> (prev, currEnv)) $ interpreter stmts

问题出在最后一行-没有提起-我知道.但是我不知道该怎么做，因为我的实验也给我带来了错误. 我正在寻求帮助.

The problem is in last line- there is no lifting- I know it. But I don't know how to put lift here because my experiments also gives me errors. I am asking for help.

推荐答案

如果要将诸如local之类的功能提升为内部monad，则需要map...T类的功能.

If you want a function such as local to be lifted to an inner monad you need the map...T class of functions.

例如，如果您有WriterT [String] (ReaderT Int (Either String)) a，并且想在其他阅读器环境中运行something:

For example, if you have a WriterT [String] (ReaderT Int (Either String)) a and you want to do run something with a different reader environment:

mapWriterT (local (+1)) something

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