Question

<Haskell>

Using getCh, define an action readLine :: IO String that behaves in the same way as getLine, except that it also permits the delete key to be used to remove characters.

Hint: the delete character is ’\DEL’, and the control character for moving the cursor back one space is ’\b’.

Answer 1

Solution

> getLine' :: IO String
> getLine' = do x <- getChar
> if x == '\n' then
> return []
> else
> do xs <- getLine
> return (x:xs)

Note: `getLine` above allows for the use of the backspace key.
It works as you'd expect. Further, GHCI won't allow me to enter '\DEL'
as a character.

As a result, on windows, this solution doesn't do anything that
`getLine` doesn't despite the added complexity.

On a linux system, the behaviour is thus:

> getLine'
this^?is
"this\DELis"
> readLine
this^?is
"thiis"

> readLine :: IO String
> readLine = get ""

> get :: String -> IO String
> get xs = do x <- getChar
> case x of
> '\n' -> return xs
> '\DEL' -> if null xs then
> get xs
> else
> do putStr "\ESC[1D \ESC[ID"
> get $ init xs
> _ -> get $ xs ++ [x]
> getLine' :: IO String
> getLine' = do x <- getChar
> if x == '\n' then
> return []
> else
> do xs <- getLine
> return (x:xs)

Note: `getLine` above allows for the use of the backspace key.
It works as you'd expect. Further, GHCI won't allow me to enter '\DEL'
as a character.

As a result, on windows, this solution doesn't do anything that
`getLine` doesn't despite the added complexity.

On a linux system, the behaviour is thus:

> getLine'
this^?is
"this\DELis"
> readLine
this^?is
"thiis"

> readLine :: IO String
> readLine = get ""

> get :: String -> IO String
> get xs = do x <- getChar
> case x of
> '\n' -> return xs
> '\DEL' -> if null xs then
> get xs
> else
> do putStr "\ESC[1D \ESC[ID"
> get $ init xs
> _ -> get $ xs ++ [x]

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all the best