Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC

Commits:

• c6cd2da1
  by Jappie Klooster at 2025-07-17T06:06:20-04:00

  Update interact docs to explain about buffering
  
  We need to tell the user to set to the
  appropriate buffer format.
  Otherwise, this function may get randomly stuck,
  or just behave confusingly.
  
  issue: https://gitlab.haskell.org/ghc/ghc/-/issues/26131
  
  NB, I'm running this with cabal *NOT* ghci. ghci messes with buffering anyway.
  
  ```haskell
  interaction :: String -> String
  interaction "jappie" = "hi"
  interaction "jakob" = "hello"
  interaction x = "unkown input: " <> x
  
  main :: IO ()
  main = interact  interaction
  ```
  so in my input (prefixed by `>`) I get:
  
  ```
  > jappie
  unkown input: jappie
  ```
  
  we confirmed later this was due to lack of \n matching.
  Anyway movnig on to more unexpected stuff:
  
  ```haskell
  main :: IO ()
  main = do
    interact  (concatMap interaction . lines)
  ```
  
  get's stuck forever.
  actually `^D` (ctrl+d) unstucks it and runs all input as expected.
  for example you can get:
  ```
  > sdfkds
  > fakdsf
  unkown input: sdfkdsunkown input: fakdsf
  ```
  
  This program works!
  ```haskell
  interaction :: String -> String
  interaction "jappie" = "hi \n"
  interaction "jakob" = "hello \n"
  interaction x = "unkown input: " <> x <> "\n"
  
  main :: IO ()
  main = do
    interact  (concatMap interaction . lines)
  ```
  the reason is that linebuffering is set for both in and output by default.
  so lines eats the input lines, and all the \n postfixes make sure the buffer
  is put out.
  

1 changed file:

• libraries/ghc-internal/src/GHC/Internal/System/IO.hs

Changes:

 --

 -- This operation may fail with the same errors as 'getContents' and 'putStr'.

 --

+-- If it doesn't produce output the buffering settings may not be

+-- correct, use ^D (ctrl+D) to close stdin which forces

+-- the buffer to be consumed.

+--

+-- You may wish to set the buffering style appropriate to your program's

+-- needs before using this function, for example:

+--

+-- @

+-- main :: IO ()

+-- main = do

+--   hSetBuffering stdin LineBuffering

+--   hSetBuffering stdout NoBuffering

+--   interact (concatMap (\str -> str ++ str) . L.lines)

+-- @

+--

 -- ==== __Examples__

 --

 -- >>> interact (\str -> str ++ str)