Can Haskell's monads be thought of as using and returning a hidden state parameter?

Question

I don't understand the exact algebra and theory behind Haskell's monads. However, when I think about functional programming in general I get the impression that state would be modelled by taking an initial state and generating a copy of it to represent the next state. This is like when one list is appended to another; neither list gets modified, but a third list is created and returned.

Is it therefore valid to think of monadic operations as implicitly taking an initial state object as a parameter and implicitly returning a final state object? These state objects would be hidden so that the programmer doesn't have to worry about them and to control how they gets accessed. So, the programmer would not try to copy the object representing the IO stream as it was ten minutes ago.

In other words, if we have this code:

main = do
    putStrLn "Enter your name:"
    name <- getLine
    putStrLn ( "Hello " ++ name )

...is it OK to think of the IO monad and the "do" syntax as representing this style of code?

putStrLn :: IOState -> String -> IOState
getLine :: IOState -> (IOState, String)
main :: IOState -> IOState

-- main returns an IOState we can call "state3"
main state0 = putStrLn state2 ("Hello " ++ name)
    where (state2, name) = getLine state1
        state1 = putStrLn state0 "Enter your name:"

Answer 1

Not monads in general, but for the IO monad, yes — in fact, the type IO a is often defined as the function type RealWorld -> (RealWorld, a). So in this view, the desugared type of putStrLn is String -> RealWorld -> (RealWorld, ()) and getChar is RealWorld -> (RealWorld, Char) — and we only partially apply it, with the monadic bind taking care of fully evaluating it and passing the RealWorld around. (GHC's ST library actually includes a very real RealWorld type, though it's described as "deeply magical" and not for actual use.)

There are many other monads that don't have this property, though. There's no RealWorld being passed around with, for example, the monads [1,2,3,4] or Just "Hello".

Answer 2

Is it therefore valid to think of monadic operations as implicitly taking an initial state object as a parameter and implicitly returning a final state object?

This seems to be a common sticking point for learning monads, i.e., trying to figure out how a single magical monad primordial soup is simultaneously useful for representing stateful computations, computations that can fail, nondeterministic computations, exceptions, continuations, sequencing side effects, and so on.

Threading state through a sequence of stateful computations is one single example of an operation that satisfies the monad laws.

You're correct in observing that the State and IO monads are close cousins, but your analogy will fall apart if you try inserting, say, the list monad.

Answer 3

No, that's not what monads in general do. However, your analogy is in fact exactly correct with regards to the data type State s a, which happens to be a monad. State is defined like this:

newtype State s a = State { runState :: s -> (a, s) }

...where the type variable s is the state value and a is the "regular" value that you use. So a value in "the State monad" is just a function from an initial state, to a return value and final state. The monadic style, as applied to State, does nothing more than automatically thread a state value through a sequence of functions.

The ST monad is superficially similar, but uses magic to allow computations with real side-effects, but only such that the side effects can't be observed from outside particular uses of ST.

The IO monad is essentially an ST monad set to "more magic", with side effects that touch the outside world and only a single point where IO computations are run, namely the entry point for the entire program. Still, on some conceptual level, you can still think of it as threading a "state" value through functions the way regular State does.

However, other monads don't necessarily have anything whatsoever to do with threading state, or sequencing functions, or whatnot. The operations needed for something to be a monad are incredibly general and abstract. For instance, using Maybe or Either as monads lets you use functions that might return errors, with the monadic style handling escaping from the computation when an error occurs the same way that State threads a state value. Using lists as a monad gives you nondeterminism, letting you simultaneously apply functions to multiple inputs and see all possible outputs, with the monadic style automatically applying the function to each argument and collecting all the outputs.

Answer 4

I prefer to think about monads as objects that represents delayed actions (runXXX, main) with result which can be combined according to that result.

return "somthing"
actionA >>= \x -> makeActionB x

And those action not necessary to be state-full. I.e. you can think about monad of function construction like this:

instance Monad ((->) a) where
    m >>= fm = \x -> fm (m x) x
    return = const

sqr = \x -> x*x
cube = \x -> x*x*x

weird = do
    a <- sqr
    b <- cube
    return (a+b)

Answer 5

Absolutely not. This is not what monads are in general. You can use monads to implicitly pass around data, but that is just one application. If you use this model of monads then you are going to miss out on a lot of the really cool things monads can do.

Instead, think about monads as being pieces of data that represent a computation. For example, there is a sense in which implicitly passing around data isn't pure because pure languages insist that you be explicit about all of your arguments and return types. So if you want to pass around data implicitly you can do this: define a new data type that is a representation of doing something impure, and then write a piece of code to work with that.

An extreme example (just a theoretical example, you're unlikely to want to do this) would be this: C allows impure computations, so you could define a type that represents a piece of C code. You can then write an interpreter that takes one of these C structures and interprets it. All monads are like this, though usually much simpler than a C interpreter. But this view is more powerful because it also explains the monads that aren't about passing around hidden state.

You should probably also try to resist the temptation to see IO as passing around a hidden world state. The internal implementation of IO has nothing to do with how you should think about IO. The IO monad is about building representations of the I/O you want to perform. You return one of these representations to the Haskell run-time system and it's up to that system to implement your representation however it wants.

Any time you want to do something, and you can't see how to directly implement it in a pure way, but you can see how to build a pure data structure to describe want you want, you may have an application for a monad, especially if your structure is naturally a tree of some sort.