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 puting the content in a list of tuplessomething like [([Char],[Char])], but I do not know how to implement the Reader Monad for making the environment available to all the components of my program without using imperative style, In particular I do not know how to set and use the environment, as far as I understood I should give it as parameter to all the functions that need the environment with runReader function env, but I am very confused, can somebody give me some indications or a good tutorial? thanks in advance

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.