How to reliably determine if a Handle is a terminal in Haskell? - haskell

Question

I am using System.IO.hIsTerminalDevice to determine if a Handle is a terminal and apply colorization if this is the case. I noticed that when forking a process using CreatePipe as stream for new process stdin and stdout, this function returns True which seems to be the wrong answer: A pipe should not be considered as a terminal. I have tried to track down the issue looking at System.IO and System.POSIX.IO source code but it ends up in C pipe function and lead me nowhere.
Is there a better way to tell if a handle is a terminal? Or am I doing something wrong?
Update
Here are 2 programs that are supposed to expose the behaviour I observed:
import Control.Monad
import System.IO
import System.Process
main = do
(in_, Just out, Just err, h) <- createProcess $ (proc "./test2" [])
{ std_in = CreatePipe
, std_err = CreatePipe
, std_out = CreatePipe }
dump out
where
dump h = forever $ do
ln <- hGetLine h
putStrLn ln
Then `test2 :
import System.IO
main = do
print =<< hIsTerminalDevice stderr
print =<< hIsTerminalDevice stdout
print =<< hIsTerminalDevice stdin
Then running the first program:
$ ./test
False
False
False
I know what's happening: What I am forking is not the program itself but a docker container! And I explicitly add -t parameter which allocates a tty for the container...

Answer 1

Do you have a minimal example to illustrate the problem? The following program prints "False" twice, suggesting that handles created with CreatePipe are not misidentified as terminal devices:
import System.Process
import System.IO
main = do (Just in_, Just out, err, h) <-
createProcess $ (shell "cat") { std_in = CreatePipe,
std_out = CreatePipe }
print =<< hIsTerminalDevice in_
print =<< hIsTerminalDevice out