I think I first “got” monad transformers after reading the wonderful article Grok Haskell Monad Transformers by Dan Piponi. But it wasn’t without some frustration, and I think that was due to the use of arbitrary functions (test1, test2, go1, go2…). I found it hard to wrap my mind around the bigger issue of the transformer mechanics when I didn’t have a firm grasp on what the purpose of these individual functions were.
So, I wrote the ubiquitous “Guess a number” program, that many probably first wrote in BASIC, using Haskell and a State Transformer instead.
module Main where import System.Random import Control.Monad.State main = do answer <- getStdRandom (randomR (1,100)) -- think of a number putStrLn "I'm thinking of a number between 1 and 100, can you guess it?" guesses <- execStateT (guessingSession answer) 0 putStrLn $ "Success in " ++ (show guesses) ++ " tries." guessSession :: Int -> StateT Int IO () guessSession answer = do gs <- lift getLine -- get guess from user let g = read gs -- convert to number modify (+1) -- increment number of guesses case compare g answer of LT -> do lift $ putStrLn "Too low" guessSession answer GT -> do lift $ putStrLn "Too high" guessSession answer EQ -> lift $ putStrLn "Got it!"
It illustrates a few things rather nicely I think, especially given it is only 20 lines long.
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