Printf not working in debug=true

Challenges
1

I am using Haskell for the redis server
I am trying to debug, but it doesn’t work

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2

@dawkrish Could you upload your code to GitHub and share the link? It will be much easier to debug if I can run it directly.

I’ll try to run your code and get back to you tomorrow.

3

Okay sure, just wait a minute

4

I dont know how I can
because it already has a remote set

5

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6
1 Like
7

@dawkrish Printing to stderr should work:

import System.IO (stderr, hPutStrLn)

main :: IO ()
main = do
    let port = "6379"
    hPutStrLn stderr "Logs from your program will appear here"
    hPutStrLn stderr $ "Redis server listening on port " ++ port
    TCP.serve TCP.HostAny port $ \(socket, address) -> do
        hPutStrLn stderr $ "Client connected: " ++ show address
        loop socket
        hPutStrLn stderr $ "Client disconnected: " ++ show address

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8

Okay thanks ! But why is not printing to stdout working? Is it due to Haskell or is it same for every language ??!

9

TBH, I’m not quite sure why stdout is not working for Haskell, because it works for other languages.

I need to investigate a bit and get back to you later.

11

Thanks a lot ! I will use stderr till then. You have saved me big time

1 Like
12

@dawkrish Turns out the issue is due to buffering. You can manually flush stdout like this:

import System.IO (hFlush, stdout)

main :: IO ()
main = do
    let port = "6379"
    putStrLn $ "Redis server listening on port " ++ port
    hFlush stdout
13

I don’t understand this but I guess you flushing means clearing the previous stdout ?

14

Here’s an explanation from ChatGPT:

In many programming languages, stdout output is buffered. This buffering can delay the appearance of printed content until:

• The buffer is full
• The program ends
• A newline character (\n) is encountered (in some cases)

When you flush stdout, you ensure that everything written to the output buffer is immediately sent to the terminal.

15

Okie ! Thanks .
I hope the Haskell issue fixes

16

@andy1li can you raise an issue to get this fix because this way of doing is a bit irritating

17

This is my code right now

{-# OPTIONS_GHC -Wno-unused-top-binds #-}
{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE OverloadedStrings #-}

module Main (main) where

import qualified Network.Simple.TCP as TCP
import qualified Data.ByteString as BS
import System.IO (stderr, hPutStrLn, hPrint)
import qualified RESP

main :: IO ()
main = do
    let port = "6379"
    putStrLn $ "Redis server listening on port " ++ port
    TCP.serve TCP.HostAny port $ \(socket, address) -> do
        printToStdOut $ "Client connected: " ++ show address
        loop socket
        printToStdOut $ "Client disconnected: " ++ show address

loop :: TCP.Socket -> IO()
loop sock = do
    msg <- TCP.recv sock 1024
    case msg of
        Nothing -> printToStdOut "Client has exited!"
        (Just bs) -> do
            handleCommand sock bs
            loop sock
            
handleCommand :: TCP.Socket -> BS.ByteString -> IO()
handleCommand sock cmd 
    | cmd == "PING\n" = TCP.send sock "+PONG\r\n"
    | otherwise = do
        printToStdOut cmd
        TCP.send sock cmd


printToStdOut :: Show a => a -> IO()
printToStdOut = hPrint stderr
1 Like
18

can you raise an issue to get this fix because this way of doing is a bit irritating

@dawkrish Could you clarify a bit more on what you’d like to get fixed?

19

using “putStrLn” (that goes to stdout) to use in debug mode

1 Like
22

@dawkrish You can use hSetBuffering to disable buffering like this:

+ import System.IO (stderr, stdout, hSetBuffering, BufferMode(NoBuffering))

main :: IO ()
main = do
+   hSetBuffering stdout NoBuffering
+   hSetBuffering stderr NoBuffering
    let port = "6379"
    putStrLn $ "Redis server listening on port " ++ port
    TCP.serve TCP.HostAny port $ \(socket, address) -> do
        printToStdOut $ "Client connected: " ++ show address
        loop socket
        printToStdOut $ "Client disconnected: " ++ show address
24

Hm, for now I am just using my custom function which is there in my above code snippet “printToStdout” . I don’t understand buffering so I am avoiding that solution

By the way for other languages like python, the simple “print”, for C++, the “cout” are working fine ?