如何通过（Int-> Int）使用Reader Monad？

Question

I would like to learn, how to use the Reader Monad. Unfortunately only a small amount of example code is available

I would like to create a Reader, where the environment and the retrived values are Int . I defined the type like this:

type IntRead = Reader Int Int

The code I tried is this:

addStuff :: Reader Int Int
addStuff = do  
    a <- (*2)  
    b <- (+10)
    return (a+b)  

I get an error, because ReaderT is expected. How to create a function like addStuff using the Reader Monad? Where should I provide the environment to this function?

Answer 1

You can convert functions to readers and back with these two isomorphisms:

reader :: (r -> a) -> Reader r a
runReader :: Reader r a -> r -> a

Eg

addStuff :: Reader Int Int
addStuff = do  
    a <- reader (*2)  
    b <- reader (+10)
    return (a+b)

and then you can test your code with runReader addStuff 5 .

This is OK for learning purposes. For more serious code, you shouldn't use the isomorphisms that much, but instead rely on ask or asks . Eg

addStuff :: Reader Int Int
addStuff = do  
    x <- ask   -- fetch the implicit Int value
    let a = (*2) x
        b = (+10) x
    return (a+b)

or, better

addStuff :: Reader Int Int
addStuff = do  
    a <- asks (*2) -- fetch the implicit Int value, and apply the function
    b <- asks (+10)
    return (a+b)

or, even better, using applicative style:

addStuff :: Reader Int Int
addStuff = (+) <$> asks (*2) <*> asks (+10)

The whole point of the reader abstraction is not to think about the underlying function. You can just pretend to have access to a read-only variable, which is accessible through the ask primitive.

Usually, only at the very last step you use runReader to actually use your monadic reader action.