嵌套在其他不执行的 monad 中的 IO 操作

Question

我有一个

foobar :: IO (ParseResult [(String,String)])

ParseResult 是这里定义的 monad：https://hackage.haskell.org/package/haskell-src-exts-1.13.5/docs/Language-Haskell-Exts-Parser.html#t:ParseResult

我想将这些字符串写入 https://hackage.haskell.org/package/HaTeX-3.17.1.0/docs/Text-LaTeX-Base-Writer.html

中定义的 LaTeXT m ()

运行此函数不会创建任何文件。

writeReport2 :: [Char] -> IO (ParseResult (IO ()))
writeReport2 name = do x <- foobar
                       return $ do y <- x
                                   return $ do z <- (execLaTeXT.docAndGraph) y
                                               renderFile fileName z
  where
    fileName = name ++ ".tex"

然而代码：

writeReport :: t -> LaTeXT IO a -> IO ()
writeReport name report = createLatex >>= renderFile fileName
  where
    createLatex = execLaTeXT report
    fileName = "AAAAA" ++ ".tex"


testFoo = [(" | HaskellExample Example File\n | Two examples are given below:\n\n >>> fib 10\n 55\n\n >>> putStrLn \"foo\nbar\"\n foo\n bar ","fib :: Int -> Int"),("\n | This is a thing: ","fib = undefined"),("\n | This is a thing:\n","fibar :: String -> Float")]

itWorks = writeReport "AAAA.txt" $ docAndGraph testFoo

将创建一个新文件。

两组代码类型检查。

Answer 1

我可以 writeReport2 无需修改即可工作。

我认为您的问题可能是 嵌套 IO 操作 在 writeResport2 的 return 值中！

为了展平嵌套的IO 操作，我不得不使用函数join :: Monad m => m (m a) -> m a from Control.Monad:

main :: IO ()
main = join $ fromParseResult <$> writeReport2 "test"

这是我的完整代码：

{-# LANGUAGE OverloadedStrings #-}

module Main where

import           Language.Haskell.Exts.Parser
import           Text.LaTeX.Base.Writer
import           Text.LaTeX
import           Data.String
import           Control.Monad

foobar :: IO (ParseResult [(String, String)])
foobar = return (ParseOk testFoo)

testFoo = [ ( " | HaskellExample Example File\n | Two examples are given below:\n\n >>> fib 10\n 55\n\n >>> putStrLn \"foo\nbar\"\n foo\n bar "
            , "fib :: Int -> Int"
            )
          , ("\n | This is a thing: ", "fib = undefined")
          , ("\n | This is a thing:\n", "fibar :: String -> Float")
          ]

docAndGraph :: Monad m => [(String, String)] -> LaTeXT m ()
docAndGraph x = do
    documentclass [] article
    document $
        raw (fromString (show x))

writeReport2 :: [Char] -> IO (ParseResult (IO ()))
writeReport2 name = do
    x <- foobar
    return $ do
        y <- x
        return $ do
            z <- (execLaTeXT . docAndGraph) y
            renderFile fileName z
  where
    fileName = name ++ ".tex"

main :: IO ()
main = join $ fromParseResult <$> writeReport2 "test"

正在加载到 GHCi：

$ stack ghci
io-action-nested-in-other-monads-not-executing-0.1.0.0: initial-build-steps (exe)
Configuring GHCi with the following packages: io-action-nested-in-other-monads-not-executing
Using main module: 1. Package `io-action-nested-in-other-monads-not-executing' component exe:io-action-nested-in-other-monads-not-executing with main-is file: /home/sven/dev/Whosebug-questions/io-action-nested-in-other-monads-not-executing/src/Main.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/sven/.ghc/ghci.conf
[1 of 1] Compiling Main             ( /home/sven/dev/Whosebug-questions/io-action-nested-in-other-monads-not-executing/src/Main.hs, interpreted )
Ok, modules loaded: Main.
Loaded GHCi configuration from /tmp/ghci22616/ghci-script

和运行它：

λ main

创建此文件：

$ cat test.tex 
\documentclass{article}\begin{document}[(" | HaskellExample Example File\n | Two examples are given below:\n\n >>> fib 10\n 55\n\n >>> putStrLn \"foo\nbar\"\n foo\n bar ","fib :: Int -> Int"),("\n | This is a thing: ","fib = undefined"),("\n | This is a thing:\n","fibar :: String -> Float")]\end{document}%

我知道这不是问题的范围，但如果你愿意，你可以绕过嵌套的 IO，通过 doinf this，例如：

writeReport3 :: [Char] -> IO ()
writeReport3 name = do
    let fileName = name ++ ".tex"
    x <- foobar
    case x of
      ParseOk y -> do
        z <- execLaTeXT (docAndGraph y)
        renderFile fileName z
      ParseFailed _ _ ->
        return ()

main :: IO ()
main = writeReport3 "test"

嵌套在其他不执行的 monad 中的 IO 操作

IO action nested in other monads not executing

haskell

io-monad