提升 Monad Reader 本地

Question

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

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.

Answer 1

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

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