Editable default string in Haskell's terminal input

user_1349009 0
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I want to be able to prompt the user for input (let's say a FilePath), but also to offer a mutable/interactive string as a default, so instead of having the user type the full path, I can prompt with:

C:\Users\John\project\test

and have them be able to backspace 4 times and enter final to yield C:\Users\John\project\final, rather than type the entire path.

However printing a default string with putStr or System.IO.hPutStr stdout does print this default to the terminal, but does not allow me to alter any of it. E.g.

import System.IO

main = do
  hSetBuffering stdout NoBuffering
  putStr "C:\\Users\\John\\project\\test"
  l <- getLine
  doSomethingWith l

I suspect Data.Text.IO's interact may be able to do what I want but I could not get it to work.

Any suggestions would be greatly appreciated.

haskell
1 Answers

getLine doesn’t offer any facility for line editing. For this you can use a library like haskeline instead, for example:

import System.Console.Haskeline

main :: IO ()
main = do
  runInputT defaultSettings $ do
    mInput <- getInputLineWithInitial "Enter path: "
      ("C:\\Users\\John\\project\\test", "")
    case mInput of
      Nothing -> do
        outputStrLn "No entry."
      Just input -> do
        outputStrLn $ "Entry: " ++ show input

An alternative is to invoke the program with a wrapper that provides line editing, such as rlwrap. For building a more complex fullscreen text UI, there is also brick, which provides a simple text editing component in Brick.Widgets.Edit.

user_2468860
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