Question

I like to parse strings ad hoc in Python by just pasting into the interpreter.

>>> s = """Adams, John
... Washington,George
... Lincoln,Abraham
... Jefferson, Thomas
... """
>>> print "\n".join(x.split(",")[1].replace(" ", "")
                    for x in s.strip().split("\n"))
John
George
Abraham
Thomas

This works great using the Python interpreter, but I’d like to do this with Haskell/GHCi. Problem is, I can’t paste multi-line strings. I can use getContents with an EOF character, but I can only do it once since the EOF character closes stdin.

Prelude> s <- getContents
Prelude> s
"Adams, John
Adams, John\nWashington,George
Washington,George\nLincoln,Abraham
Lincoln,Abraham\nJefferson, Thomas
Jefferson, Thomas\n^Z
"
Prelude> :{
Prelude| putStr $ unlines $ map ((filter (`notElem` ", "))
Prelude|                         . snd . (break (==','))) $ lines s
Prelude| :}
John
George
Abraham
Thomas
Prelude> x <- getContents
*** Exception: <stdin>: hGetContents: illegal operation (handle is closed)

Is there a better way to go about doing this with GHCi? Note – my understanding of getContents (and Haskell IO in general) is probably severely broken.

UPDATED

I will be playing with the answers I have received. Here are some helper functions I made (plagiarized) that simulate Python’s """ quoting (by ending with """, not starting) from ephemient’s answer.

getLinesWhile :: (String -> Bool) -> IO String
getLinesWhile p = liftM unlines $ takeWhileM p (repeat getLine)

getLines :: IO String
getLines = getLinesWhile (/="\"\"\"")

To use AndrewC’s answer in GHCi –

C:\...\code\haskell> ghci HereDoc.hs -XQuasiQuotes
ghci> :{
*HereDoc| let s = [heredoc|
*HereDoc| Adams, John
*HereDoc| Washington,George
*HereDoc| Lincoln,Abraham
*HereDoc| Jefferson, Thomas
*HereDoc| |]
*HereDoc| :}
ghci> putStrLn s
Adams, John
Washington,George
Lincoln,Abraham
Jefferson, Thomas
ghci> :{
*HereDoc| putStr $ unlines $ map ((filter (`notElem` ", "))
*HereDoc|                         . snd . (break (==','))) $ lines s
*HereDoc| :}
John
George
Abraham
Thomas

Answer 1

getContents == hGetContents stdin. Unfortunately, hGetContents marks its handle as (semi-)closed, which means anything attempting to read from stdin ever again will fail.

Does it suffice to simply read up to an empty line or some other marker, never closing stdin?

takeWhileM :: Monad m => (a -> Bool) -> [m a] -> m [a]
takeWhileM p (ma : mas) = do
    a <- ma
    if p a
      then liftM (a :) $ takeWhileM p mas
      else return []
takeWhileM _ _ = return []

ghci> liftM unlines $ takeWhileM (not . null) (repeat getLine)
Adams, John
Washington, George
Lincoln, Abraham
Jefferson, Thomas

"Adams, John\nWashington, George\nLincoln, Abraham\nJefferson, Thomas\n"
ghci>