# A Generic State Transformer

Since state is a handy thing to have, the standard library includes a module `Control.Monad.State`

that defines a parameterized version of the state-transformer monad. This file is a simplified version of that library.

We will only allow clients to use the functions declared below.

`module State (State, get, put, runState, evalState, execState) where`

The type definition for a generic state transformer is very simple:

`data State s a = S { runState :: s -> (a, s) }`

It is a parameterized state-transformer monad where the state is denoted by type `s`

and the return value of the transformer is the type `a`

. We make the above a monad by declaring it to be an instance of the `monad`

typeclass

```
instance Monad (State s) where
return x = S (\s -> (x, s))
st >>= f = S (\s -> let (x, s') = runState st s
in runState (f x) s')
```

There are also two other was of evaluating the state monad. The first only returns the final result,

```
evalState :: State s a -> s -> a
evalState s = fst . runState s
```

and the second only returns the final state.

```
execState :: State s a -> s -> s
execState s = snd . runState s
```

## Accessing and Modifying State

Since our notion of state is generic, it is useful to write a `get`

and `put`

function with which one can *access* and *modify* the state. We can easily `get`

the *current* state via

```
get :: State s s
get = undefined
```

That is, `get`

denotes an action that leaves the state unchanged, but returns the state itself as a value. Note that although get *does not* have a function type (unless you peek under the covers of State), we consider it a monadic "action".

Dually, to *modify* the state to some new value `s'`

we can write the function

```
put :: s -> State s ()
put s' = undefined
```

which denotes an action that ignores (ie blows away the old state) and replaces it with `s'`

. Note that the `put s'`

is an action that itself yields nothing (that is, merely the unit value.)

## News :

Welcome to CIS 552!

See the home page for basic
information about the course, the schedule for the lecture notes
and assignments, the resources for links to the required software
and online references, and the syllabus for detailed information about
the course policies.