help with reader monad

Question

I am new at haskell, I have to write a program context-aware,so I thought I can use the Reader Monad for keeping the context read from a file, I know how to read the file p

Answer 1

The basic scheme for using any "normal" monad^[0] is pretty much the same across the board. Essentially:

• Write functions that return a value of monadic type, using do notation if you like, just like you'd write an IO function like main.
• Make use of any specific functions for the monad you're working with.
• Call these functions from each other, using the standard rule:
  • Bind a value from the same monad using a <- to get at the value "inside", causing the other value to be "run".
  • Bind any other value using let, leaving it independent of the monadic structure.
• Use a particular monad's specialized "run" function to evaluate the monadic computation and get the final result.

Do that, and all the messy details of the extra functionality described by the monad (in this case, passing an extra environment parameter around) are handled automatically.

Now, the usual Reader operations are ask and local:

• ask is a monadic value holding the environment; in a do block you use it the same way you'd use something like getLine in the IO monad.
• local takes a function that provides a new environment and a computation in the Reader monad, runs the latter in an environment modified by the former, then takes the result and puts it into the current function. In other words, it runs a sub-computation with a locally modified environemnt.

The "run" function is the creatively-named runReader, which simply takes a computation in the Reader monad and an environment value, runs the former using the latter, and returns the final result outside of the monad.

As an example, here's some functions doing some meaningless calculation in a Reader monad, where the environment is a "maximum value" that says when to stop:

import Control.Monad.Reader

computeUpToMax :: (Int -> Int) -> Int -> Reader Int [Maybe Int]
computeUpToMax f x = do 
    maxVal <- ask
    let y = f x
    if y > maxVal
        then return []
        else do zs <- local (subtract y) (computeUpToMax f y)
                z <- frob y
                return (z:zs)

frob :: Int -> Reader Int (Maybe Int)
frob y = do
    maxVal <- ask
    let z = maxVal - y
    if z == y 
        then return Nothing
        else return $ Just z

To run it, you'd use something like this:

> runReader (computeUpToMax (+ 1) 0) 9
[Just 8, Just 6, Nothing]

...where 9 is the initial environment.

Almost exactly the same structure can be used with other monads, such as State, Maybe, or [], though in the latter two cases you'd typically just use the final monadic result value instead of using a "run" function.

[0]: Where normal means not involving compiler magic, the most obvious "abnormal" monad of course being IO.

Answer 2

I think it's easiest if you look at how you would solve this problem without using Reader, then compare the translated version. Here's a trimmed-down example from a program I'm working on where the environment is a set of callback functions to update the display. It's slightly more complicated because it uses ReaderT instead of Reader, but everything works in basically the same way.

runProcess :: Env -> State -> Action -> IO State
runProcess env state action = do
  newstate <- processAction state action
  let ufunc = mainUFunc env              -- get the callback to update the display
  ufunc newstate                         -- update the display
  return newstate

Now I'll change it to use the Reader monad to pass along the environment. Since the code was already in IO, it's necessary to use the monad transformer version, ReaderT.

runProcessR :: State -> Action -> ReaderT Env IO State
runProcessR state action = do
  newstate <- lift $ processAction state action
  env <- ask                              -- get the environment from the reader
  liftIO $ (mainUFunc env) newstate       -- updating is in IO; it needs to be lifted
  return newstate

At this point, the program's main loop will essentially be:

loop :: State -> ReaderT Env IO ()
loop = do
  action <- liftIO getAction
  if action == EndLoop
    then return ()
    else do
      st' <- processActionR st action
      loop st'

mainLoop :: IO ()
mainLoop = do
  env <- setUpCallbacks
  let st = initState
  runReaderT $ loop st

So that's how you can use Reader. Every function that used to take an environment parameter no longer needs to. Functions that don't take the environment can be used directly or lifted if they're monadic.

Answer 3

This is IMHO best monad resource - All About Monads, and here's part for Reader monad.