putStrLn不打印到控制台 [英] putStrLn doesn't print to console

问题描述

我正在试验wxHaskell。我无法在ghci下运行应用程序，所以我必须使用应用程序来测试它。我想用println调试来测试程序。但是，似乎putStrLn在GUI中不起作用：

  { - ＃LANGUAGE Haskell2010＃ - } 
 
模块Main其中
 
 import Graphics.UI.WX 
 
 drawUI dc view = do 
 circle dc（点10 10）5 [penKind：= PenSolid ，color：= red] 
 putStrLn画
 
 helloGui :: IO（）
 helloGui = do 
f<  -  frame [
 text： =Example，
 resizeable：= False，
 bgcolor：= white，
 layout：= space 400 300，
 on paint：= drawUI] 
 return（ ）
 
 main :: IO（）
 main = do 
 putStrLnStarted
 start helloGui 
  

如果我注释掉开始helloGui，一切都打印好。但是，如果我将其返回，则不会打印任何内容，但会显示窗口。这里有什么不对？

解决方案

这可能是输出缓冲;

显式清除：

  putStrLnStarted
 hFlush stdout 
  

或者打开行缓冲：

  hSetBuffering stdout LineBuffering  - 甚至NoBuffering 
 putStrLnStarted
  

I am experimenting with wxHaskell. I wasn't able to run the app under ghci, so I have to use application to test it. I wanted to test the program with println debugging. However, it seems that putStrLn doesn't work in GUI:

{-# LANGUAGE Haskell2010 #-}

module Main where

import Graphics.UI.WX

drawUI dc view = do 
  circle dc (point 10 10) 5 [penKind := PenSolid, color := red]
  putStrLn "painted"  

helloGui :: IO ()
helloGui = do
  f <- frame [
    text := "Example", 
    resizeable := False, 
    bgcolor := white,
    layout := space 400 300,
    on paint := drawUI]
    return ()

main :: IO ()
main = do
  putStrLn "Started"
  start helloGui

If I comment out start helloGui, everything is printed well. However, if I return it, nothing is printed but the window is displayed. What's wrong here?

解决方案

This is probably output buffering; the output is not written until the program exits.

Either flush explicitly:

  putStrLn "Started"
  hFlush stdout

Or turn on line buffering:

  hSetBuffering stdout LineBuffering -- or even NoBuffering
  putStrLn "Started"

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