Converting a monadic function to an IO monadic function

parseSource :: String -> Either ParserError Mod.Module
parseSource src = do
    (imports, rest) <- parseImports (Lex.lexSource src)
    bindings <- mapM parseBinding rest
    buildModule imports bindings

I need to make the above return an IO (Either ParserError Mod.Module) as the buildModule statement at the end will need to perform some IO functions (reading files). The problem i have is that when i make i开发者_StackOverflow社区t an IO function, i can no longer do the bind(wrong term?) <- operations.

What is the simplest way to make this work?

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Take a look at defining your problem in terms of ErrorT ParseError IO.

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I couldn't find a combinator to lift a pure Either computation into the ErrorT monad, so I wrote one called liftError. I fleshed out your example with dummy types and implementations. The main runs the parser twice, once with input that throws a ParserError, and once which succeeds with an IO side-effect. In order for ErrorT ParserError IO to be a Monad, ParserError must be an instance of Error (so that it is possible to implement fail).

import Control.Monad.Error

type ParserMonad = ErrorT ParserError IO

data ParserError = ParserError1 | ParserError2 | ParserError3
                   deriving(Show)
data Module = Module
              deriving(Show)
data Import = Import
              deriving(Show)
data Binding = Binding
               deriving(Show)

instance Error ParserError where
    noMsg = undefined

-- lift a pure Either into the ErrorT monad
liftError :: Monad m => Either e a -> ErrorT e m a
liftError = ErrorT . return

parseSource :: String -> ParserMonad Module
parseSource src = do
    (imports, rest) <- liftError $ parseImports (lexSource src)
    bindings <- liftError $ mapM parseBinding rest
    buildModule imports bindings

lexSource :: String -> [String]
lexSource = return

parseImports :: [String] -> Either ParserError ([Import], [String])
parseImports toks = do{ when (null toks) $ throwError ParserError1
                      ; return ([Import], toks)
                      }

parseBinding :: String -> Either ParserError Binding
parseBinding b = do{ when (b == "hello") $ throwError ParserError2
                   ; return Binding
                   }

buildModule :: [Import] -> [Binding] -> ParserMonad Module
buildModule i b = do{ liftIO $ print "hello"
                    ; when (null b) $ throwError ParserError3
                    ; return Module
                    }

main = mapM (runErrorT . parseSource) ["hello", "world"]