Haskell/Understanding monads/State

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If you programmed in any language before, chances are you wrote some functions that "kept state". In case you did not encounter the concept before, a state is one or more variables that are required to perform some computation, but are not among the arguments of the relevant function. In fact, object-oriented languages like C++ make extensive usage of state variables in objects in the form of member variables. Procedural languages like C use variables declared outside the current scope to keep track of state. In Haskell, however, such techniques can not be applied in a straightforward way, as they require mutable variables and thus clash with the default expectation of functional purity. We can very often keep track of state by passing parameters from function to function or by pattern matching of various sorts, but in some cases it is appropriate to find a more general or convenient solution. We will consider the common example of generation of pseudo-random numbers with pure functions, and find out how the State monad can make such a task easier.

Pseudo-Random Numbers[edit]

Generating actual random numbers is a very complicated subject; we will consider pseudo-random numbers. They are called "pseudo" because they are not really random, they only look like it. Starting from an initial state (commonly called the seed), pseudo-random number generators produce a sequence of numbers that have the appearance of being random.

Every time a pseudo-random number is requested, a global state is updated: that's the part we have problems with in Haskell, since it is a side effect from the point of view of the function requesting the number. Sequences of pseudo-random numbers can be replicated exactly if the initial seed and the algorithm is known.

Implementation in Haskell[edit]

Producing a pseudo-random number in most programming languages is very simple: there is usually a function, such as C or C++'s rand(), that provides a pseudo-random value (or a random one, depending on the implementation). Haskell has a similar one in the System.Random module:

> :m System.Random
> :t randomIO
randomIO :: Random a => IO a
> randomIO

Obviously, save eerie coincidences, the value you will obtain will be different. A disadvantage of randomIO is that it requires us to utilise the IO monad, which breaks purity requirements. Usage of the IO monad is dictated by the process of updating the global generator state, so that the next time we call randomIO the value will be different.

Implementation with Functional Purity[edit]

In general, we do not want to use the IO monad if we can help it, because of the loss of guarantees on no side effects and functional purity. Indeed, we can build a local generator (as opposed to the global generator, invisible to us, that randomIO uses) using mkStdGen, and use it as seed for the random function, which in turn returns a tuple with the pseudo-random number that we want and the generator to use the next time:

> :m System.Random
> let generator = mkStdGen 0           -- "0" is our seed
> generator
1 1
> random generator :: (Int, StdGen)
(2092838931,1601120196 1655838864)

While we have now regained functional purity, there are new problems to bother us. First and foremost, if we want to use generator to get random numbers, the obvious definition...

> let randInt = fst . random $ generator :: Int
> randInt

...is useless, as it will always give back the same value, 2092838931, no matter how many times random is called, for the same generator is always used. Of course we can take the second member of the tuple (i.e. the new generator) and feed it to a new call to random:

> let (randInt, generator') = random generator :: (Int, StdGen)
> randInt                           -- Same value
> random generator' :: (Int, StdGen) -- Using new generator' returned from “random generator”
(-2143208520,439883729 1872071452)

That, however, keeps us from having a function which simply gives back a new value, without the fuss of having to pass the generator around. What we really need is a way to automate the extraction of the second member of the tuple (i.e. the new generator) and feed it to a new call to random; and that is where the State monad comes into the picture.

Definition of the State Monad[edit]


In this chapter we will use the state monad provided by the module Control.Monad.Trans.State of the transformers package. By reading Haskell code in the wild you will soon meet Control.Monad.State, a module of the closely related mtl package. The differences between these two modules need not concern us at the moment; everything we discuss here also applies to the mtl variant.

The Haskell type State is in essence a function that consumes state, and produces a result and the state after the result has been extracted. The function is wrapped by a data type definition, which comes along with a runState accessor so that pattern matching becomes unnecessary. For our current purposes, the definition is equivalent to:

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

Here, s is the type of the state, and a the type of the produced result. The name State for the type is arguably a bit of a misnomer, as the wrapped value is not the state itself but a state processor.

Before You Ask...[edit]

What in the world did we mean by "for our current purposes" two paragraphs above? The subtlety is that the transformers package implements the State type in a somewhat different way. The differences do not affect how we use or understand State; as a consequence of them, however, Control.Monad.Trans.State does not export a State constructor. Rather, there is a state function,

state :: (s -> (a, s)) -> State s a

which does the same job. As for why the implementation is not the obvious one we presented above, we will get back to that a few chapters down the road.


You will also have noticed that we defined the data type with the newtype keyword, rather than the usual data. newtype can be used only for types with just one constructor and just one field; it ensures the trivial wrapping and unwrapping of the single field is eliminated by the compiler. For that reason, simple wrapper types such as State are usually defined with newtype. One might ask whether defining a synonym with type would be enough in such cases. type, however, does not allow us to define instances for the new data type, which is what we are about to do...

Instantiating the Monad[edit]

Note also that, in contrast to the monads we have met thus far, State has two type parameters. This means that, when we instantiate the monad, we are actually leaving the parameter for the state type:

instance Monad (State s) where

This means that the "real" monad could be State String, State Int, or State SomeLargeDataStructure, but not State on its own.

The return function is implemented as:

return :: a -> State s a
return x = state ( \st -> (x, st) )

In words, giving a value to return produces a function, wrapped in the State constructor: this function takes a state value, and returns it unchanged as the second member of a tuple, together with the specified result value.

Binding is a bit intricate:

(>>=) :: State s a -> (a -> State s b) -> State s b
processor >>= processorGenerator = state $ \st -> 
                                   let (x, st') = runState processor st
                                   in runState (processorGenerator x) st'

The idea is that, given a state processor and a function that can generate another processor given the result of the first one, these two processors are combined to obtain a function that takes the initial state, and returns the second result and state (i.e. after the second function has processed them).

Loose schematic representation of how bind creates a new state processor (pAB) from the given state processor (pA) and the given generator (f). s1, s2 and s3 are actual states. v2 and v3 are values. pA, pB and pAB are state processors. The diagram ignores wrapping and unwrapping of the functions in the State wrapper.

The diagram shows this schematically, for a slightly different, but equivalent form of the ">>=" (bind) function, given below (where wpA and wpAB are wrapped versions of pA and pAB).

-- pAB = s1 --> pA --> (v2,s2) --> pB --> (v3,s3)        
wpA >>= f = wpAB          
    where wpAB = state $ \s1 -> let pA = runState wpA
                                    (v2, s2) = pA s1
                                    pB = runState $ f v2
                                    (v3,s3) = pB s2
                                in  (v3,s3)

Setting and Accessing the State[edit]

The monad instantiation allows us to manipulate various state processors, but you may at this point wonder where exactly the original state comes from in the first place. State s is also an instance of the MonadState class, which provides two additional functions:

put newState = state $ \_ -> ((), newState)

This function will generate a state processor given a state. The processor's input will be disregarded, and the output will be a tuple carrying the state we provided. Since we do not care about the result (we are discarding the input, after all), the first element of the tuple will be, so to say, "null".[1]

The specular operation is to read the state. This is accomplished by get:

get = state $ \st -> (st, st)

The resulting state processor is going to produce the input st in both positions of the output tuple - that is, both as a result and as a state, so that it may be bound to other processors.

Getting Values and State[edit]

From the definition of State, we know that runState is an accessor to apply to a State a b value to get the state-processing function; this function, given an initial state, will return the extracted value and the new state. Other similar, useful functions are evalState and execState, which work in a very similar fashion.

Function evalState, given a State a b and an initial state, will return the extracted value only, whereas execState will return only the new state; it is possibly easiest to remember them as defined as:

evalState :: State s a -> s -> a
evalState processor st = fst ( runState processor st )
execState :: State s a -> s -> s
execState processor st = snd ( runState processor st )

Example: Rolling Dice[edit]

randomRIO (1,6)

Suppose we are coding a game in which at some point we need an element of chance. In real-life games that is often obtained by means of dice, which we will now try to simulate with Haskell code. For starters, we will consider the result of throwing two dice: to do that, we resort to the function randomR, which allows to specify an interval from which the pseudo-random values will be taken; in the case of a die, it is randomR (1,6).

In case we are willing to use the IO monad, the implementation is quite simple, using the IO version of randomR:

import Control.Monad
import System.Random
rollDiceIO :: IO (Int, Int)
rollDiceIO = liftM2 (,) (randomRIO (1,6)) (randomRIO (1,6))

Here, liftM2 is used to make the non-monadic two-argument function (,) work within a monad, so that the two numbers will be returned (in IO) as a tuple.

  1. Implement a function rollNDiceIO :: Int -> IO [Int] that, given an integer, returns a list with that number of pseudo-random integers between 1 and 6.

Getting Rid of the IO Monad[edit]

Now, suppose that for any reason we do not want to use the IO monad: we might want the function to stay pure, or need a sequence of numbers that is the same in every run, for repeatability.

To do that, we can produce a generator using the mkStdGen function in the System.Random library:

> mkStdGen 0
1 1

The argument to mkStdGen is an Int that functions as a seed. With that, we can generate a pseudo-random integer number in the interval between 1 and 6 with:

> randomR (1,6) (mkStdGen 0)
(6,40014 40692)

We obtained a tuple with the result of the dice throw (6) and the new generator (40014 40692). A simple implementation that produces a tuple of two pseudo-random integers is then:

clumsyRollDice :: (Int, Int)
clumsyRollDice = (n, m)
        (n, g) = randomR (1,6) (mkStdGen 0)
        (m, _) = randomR (1,6) g

When we run the function, we get:

> clumsyRollDice
(6, 6)

The implementation of clumsyRollDice works, but we have to manually write the passing of generator g from one where clause to the other. This is pretty easy now, but will become increasingly cumbersome if we want to produce large sets of pseudo-random numbers. It is also error-prone: what if we pass one of the middle generators to the wrong line in the where clause?

  1. Implement a function rollDice :: StdGen -> ((Int, Int), StdGen) that, given a generator, return a tuple with our random numbers as first element and the last generator as the second.

Introducing State[edit]

We will now try to solve the clumsiness of the previous approach introducing the State StdGen monad. For convenience, we give it a name with a type synonym:

import Control.Monad.Trans.State
import System.Random
type GeneratorState = State StdGen

Remember, however, that a GeneratorState Int is in essence a StdGen -> (Int, StdGen) function, so it is not really the generator state, but a processor of the generator state. The generator state itself is produced by the mkStdGen function. Note that GeneratorState does not specify what type of values we are going to extract, only the type of the state.

We can now produce a function that, given a StdGen generator, outputs a number between 1 and 6:

rollDie :: GeneratorState Int
rollDie = do generator <- get
             let (value, newGenerator) = randomR (1,6) generator
             put newGenerator
             return value

The do notation is in this case much more readable; let's go through each of the steps:

  1. First, we take out the pseudo-random generator with <- in conjunction with get. get overwrites the monadic value (The 'a' in 'm a') with the state, and thus generator is bound to the state. (If in doubt, recall the definition of get and >>= above).
  2. Then, we use the randomR function to produce an integer between 1 and 6 using the generator we took; we also store the new generator graciously returned by randomR.
  3. We then set the state to be the newGenerator using the put function, so that the next call will use a different pseudo-random generator;
  4. Finally, we inject the result into the GeneratorState monad using return.

We can finally use our monadic die:

> evalState rollDie (mkStdGen 0)

At this point, a legitimate question is why we have involved monads and built such an intricate framework only to do exactly what fst $ randomR (1,6) does. The answer is illustrated by the following function:

rollDice :: GeneratorState (Int, Int)
rollDice = liftM2 (,) rollDie rollDie

We obtain a function producing two pseudo-random numbers in a tuple. Note that these are in general different:

> evalState rollDice (mkStdGen 666)

That is because, under the hood, the monads are passing state to each other. This used to be very clunky using randomR (1,6), because we had to pass state manually; now, the monad is taking care of that for us. Assuming we know how to use the lifting functions, constructing intricate combinations of pseudo-random numbers (tuples, lists, whatever) has suddenly become much easier.

  1. Similarly to what was done for rollNDiceIO, implement a function rollNDice :: Int -> GeneratorState [Int] that, given an integer, returns a list with that number of pseudo-random integers between 1 and 6.

Pseudo-random values of different types[edit]

Until now, absorbed in the die example, we considered only Int as the type of the produced pseudo-random number. However, already when we defined the GeneratorState monad, we noticed that it did not specify anything about the type of the returned value. In fact, there is one implicit assumption about it, and that is that we can produce values of such a type with a call to random.

Values that can be produced by random and similar function are of types that are instances of the Random class (capitalised). There are default implementations for Int, Char, Integer, Bool, Double and Float, so you can immediately generate any of those.

Since we noticed already that the GeneratorState is "agnostic" in regard to the type of the pseudo-random value it produces, we can write down a similarly "agnostic" function, analogous to rollDie, that provides a pseudo-random value of unspecified type (as long as it is an instance of Random):

getRandom :: Random a => GeneratorState a
getRandom = do generator <- get
               let (value, newGenerator) = random generator
               put newGenerator
               return value

Compared to rollDie, this function does not specify the Int type in its signature and uses random instead of randomR; otherwise, it is just the same. What is notable is that getRandom can be used for any instance of Random:

> evalState getRandom (mkStdGen 0) :: Bool
> evalState getRandom (mkStdGen 0) :: Char
> evalState getRandom (mkStdGen 0) :: Double
> evalState getRandom (mkStdGen 0) :: Integer

Indeed, it becomes quite easy to conjure all these at once:

allTypes :: GeneratorState (Int, Float, Char, Integer, Double, Bool, Int)
allTypes = liftM (,,,,,,) getRandom
                     `ap` getRandom
                     `ap` getRandom
                     `ap` getRandom
                     `ap` getRandom
                     `ap` getRandom
                     `ap` getRandom

Here we are forced to used the ap function, defined in Control.Monad, since there exists no liftM7. As you can see, its effect is to fit multiple computations into an application of the (lifted) 7-element-tuple constructor, (,,,,,,). To understand what ap does, look at its signature:

>:type ap
ap :: (Monad m) => m (a -> b) -> m a -> m b

remember then that type a in Haskell can be a function as well as a value, and compare to:

>:type liftM (,,,,,,) getRandom
liftM (,,,,,) getRandom :: (Random a1) =>
                          State StdGen (b -> c -> d -> e -> f -> (a1, b, c, d, e, f))

The monad m is obviously State StdGen (which we "nicknamed" GeneratorState), while ap's first argument is function b -> c -> d -> e -> f -> (a1, b, c, d, e, f). Applying ap over and over (in this case 6 times), we finally get to the point where b is an actual value (in our case, a 7-element tuple), not another function. To sum it up, ap applies a function-in-a-monad to a monadic value (compare with liftM, which applies a function not in a monad to a monadic value).

So much for understanding the implementation. Function allTypes provides pseudo-random values for all default instances of Random; an additional Int is inserted at the end to prove that the generator is not the same, as the two Ints will be different.

> evalState allTypes (mkStdGen 0)
  1. If you are not convinced that State is worth using, try to implement a function equivalent to evalState allTypes without making use of monads, i.e. with an approach similar to clumsyRollDice above.


  1. The technical term for the type of () is unit.