[Haskell-beginners] lazy mapM

[Kim-Ee Yeoh]

On Mon, Apr 1, 2013 at 6:26 AM, Ovidiu D <ovidiudeac at gmail.com> wrote:
> 1. Make f behave lazy
> Its input list is made of lines read from stdin and I want it to process
> lines one by one as they are entered by the user.

Eschewing laziness (which adds only complexity in this case), here's
something that'll work, if a little ugly:

import System.Exit

f :: String -> IO ()
f "exit" = exitSuccess
f a = putStrLn $ "you entered: " ++ a

main = do
   s <- getLine
   f s
   main

Going down this path would involve IORef's, among others from the "sin bin".

Something more pure and haskell-y would typically involve an analysis
of the DSL abstract syntax and state space and implementation using a
combination of State and Free monads.

-- Kim-Ee