2015 Nov 09

I must confess: up until this morning, I had no idea what any tutorial on the State monad was talking about. I could only split them into two categories:

1. The posts which solve a (possibly artificial) problem using State/StateT
2. The (intended to be) introductory material which actually explained nothing

and without an actual explanation, you still have to do all the work yourself.

This post belongs in category #1. Consider yourself warned. If you want to know how `State` works, I really can’t explain it (yet), go use it to solve a problem, and \$GOD help you. Here, I’m only telling the excruciating story of how I heard of FizzBuzz, then years later heard of `State`, and today I decided to combine them.

You might be aware of the “FizzBuzz test”. It’s a cutesy little puzzle designed to teach toddlers division. At some point, it’s been used as an interview question, to help companies stop employing toddlers - because this is illegal in many jurisdictions.

Like many others, I started trying to grok `StateT` from Justin Le’s blog, in which he uses `StateT` to implement a constraint solver. `State` I found out from the “Learn You A Haskell For Great Good!” book. The book says:

A State s a is a stateful computation that manipulates a state of type s and has a result of type a.

On its own, this sentence says nothing. Previously, however, the chapter explains what “stateful computations” are: functions `s -> (a, s)`, which means functions which take a state `s`, perform work and return a result of type `a`, and a possibly changed state `s`. In Justin’s post, this computation is dependent selection from a sample. Now it should make more sense. If not, READ THE BOOK, there’s no “tl;dr”.

Alright, so let’s draw a parallel: if our state is an integer, and our computation is a function `fizzbuzz` which implements the FizzBuzz test then returns a “new” state, yada yada, seems like FizzBuzz can be modeled in terms of a `State` instance, yeah? Let’s test that claim.

I will first implement the “computation”:

``````fizzbuzz :: Int -> (String, Int)
fizzbuzz n | n `mod` 5 == 0 && n `mod` 3 == 0 = ("FizzBuzz", n + 1)
| n `mod` 3 == 0 = ("Fizz", n + 1)
| n `mod` 5 == 0 = ("Buzz", n + 1)
| otherwise = (show n, n + 1)
``````

In all cases, the “new state” is the next integer. A contrived example for our contrived need.

Now, how do we “wrap” this computation in a `State`? I sort of understood how to wrap one in a `StateT` from Justin’s article, and I sort of grokked Brandon Simmons’ version, which led me down the path of compiler errors. So I bit the unavoidable bullet: dive into the transformers package.

You’ll see `State` is defined in terms of `StateT`. Rather than going to `StateT` first, instead I’m thinking: if this level of abstraction makes any sense, you could read a little bit of the source code and understand even a little bit of it. So let’s read the “constructor”, `state`:

``````-- | Construct a state monad computation from a function.
-- (The inverse of 'runState'.)
=> (s -> (a, s))  -- ^pure state transformer
-> StateT s m a   -- ^equivalent state-passing computation
state f = StateT (return . f)
``````

Alright, already this makes a lot of sense! `state` takes a “stateful computation”, and wraps it in a `StateT s m a`. Only a few lines above, `StateT s m a` is aliased to ```State s```. So `state` must be the way to wrap functions in a `State`. I think. Let’s test our newfound confidence:

``````state_ints_fizzbuzz :: State Int String
state_ints_fizzbuzz = state fizzbuzz
``````

This compiles, so at least the type gods are satisfied. Sweet.

Now, in various tutorials, you’d find something like: `stateIntsFizz = State fizzbuzz`. Unfortunately, those tutorials are outdated. Since `State` now is a type alias:

``````type State s = StateT s Identity
``````

(taken directly from hackage, see above), you must use the `state` function. If this is not the case at the time you read this, I’m really sorry. And kind of angry at the same time. Let’s at least find some solace in our mutual feeling of frustration.

Now, how do we use this monad to solve FizzBuzz? Well, presumably we need to run the computation wrapped inside it 100 times, starting from 1. So 1 is our initial state. Further along the source code, we read:

``````-- | Evaluate a state computation with the given initial state
-- and return the final value, discarding the final state.
--
-- * @'evalState' m s = 'fst' ('runState' m s)@
``````

This seems to be exactly what we want. The values (of type `a`), are the desired output according to our implementation of `fizzbuzz`. We evaluate the state 100 times, and gather all the values, print them, done! Sounds good in theory. Let’s test:

``````evalState (replicateM 100 state_ints_fizzbuzz) 1
``````

`replicateM` performs an action `n` times, and gathers the results. The crucial question is “why replicateM and not replicate?”, and the answer is because the context is a monad, the one we so painstakingly constructed.

Now to put all this in the context of, say, `main :: IO ()`, I will simply print all the results we’ve gathered, like so:

``````main :: IO ()
main = mapM_ putStrLn \$ evalState (replicateM 100 state_ints_fizzbuzz) 1
``````

Compiles, and works as expected!

Up next, using `State`/`StateT` to solve a not-so-trivial problem. Maybe there’ll be more revelations there.

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