hint runStmt no output when redirected

Question

Following this link and trying to stack run this module:

module Main where

import Language.Haskell.Interpreter

main :: IO ()
main = do
  _ <- runInterpreter
     $ setImports ["Prelude"]
    >> runStmt "x <- pure 42"
    >> runStmt "print x"
  return ()

there is no output when redirected from the command line:

user@localhost ~/hs-project $ stack run > 42.txt

I can only guess that - unlike in an answer from the mentioned post - the output of hint goes through pipes, because "print" runs inside the interpreter, doesn't come from the module.

This is the result of ls -l /proc/<pid>/fd.

lrwx------ 1 user user 64 31 mei 20:50 0 -> /dev/pts/4
lrwx------ 1 user user 64 31 mei 20:50 1 -> /dev/pts/4
lrwx------ 1 user user 64 31 mei 20:50 2 -> /dev/pts/4
lrwx------ 1 user user 64 31 mei 20:50 3 -> 'anon_inode:[timerfd]'
lr-x------ 1 user user 64 31 mei 20:50 4 -> 'pipe:[2705380]'
lr-x------ 1 user user 64 31 mei 20:50 6 -> 'pipe:[2705381]'

@DanielWagner Thanks. Is it possible to redirect?

sjb
– sjb

2024-05-31 17:48:20 +00:00
Commented May 31, 2024 at 17:48 — sjb
– sjb, Commented May 31, 2024 at 17:48

K. A. Buhr · Accepted Answer · 2024-05-31 17:46:05Z

2

Try having the interpreter flush its output before exiting:

_ <- runInterpreter
   $ setImports ["Prelude", "System.IO"]
   >> runStmt "x <- pure 42"
   >> runStmt "print x"
   >> runStmt "hFlush stdout"

or turn off buffering before generating output:

_ <- runInterpreter
   $ setImports ["Prelude", "System.IO"]
   >> runStmt "hSetBuffering stdout NoBuffering"
   >> runStmt "x <- pure 42"
   >> runStmt "print x"

Both approaches seemed to work for me.

answered May 31, 2024 at 17:46

K. A. Buhr

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