# MonadCont under the hood

This tutorial is a response to the following Stack Overflow question. There's a short but useful description of `Cont` and `MonadCont` operations in the Control.Monad.Cont documentation, but it doesn't really describe how the continuation monad does its thing. This is an attempt at much more detailed explanation of what Cont and MonadCont are doing under the hood.

This tutorial assumes a working knowledge of Haskell, though of course it doesn't assume that you understood the implementation of `Control.Monad.Cont`

the first time you read it!

## Contents

# Introducing Continuations and the Cont type

Continuations are functions that represent "the remaining computation to do." Their representation here is `a -> r`

, which is simply a function that takes some value produced by the current computation, of some type `a`

, and returns the final result of type `r`

from it.

The type `Cont r a`

(instances of which I will, in this tutorial, refer to as `Cont`* objects*) represents a *continuation-passing-style function that takes a single continuation as its only input*. In other words, its guts are a function that:

- takes a continuation as an argument
- does whatever it needs to do
- produces a value of type
`r`

at the end, presumably by invoking the continuation.

Note that whatever it needs to do, i.e. whatever values it needs to be able to use to do its thing, must already be bound up into the `Cont`

object. So, generally, we won't be dealing with `Cont`

objects directly, but with functions that can ultimately produce one.

# Sequencing Continuation-Style Computations

## Basic Sequencing

`Cont`

objects can be chained together, so that the continuation you pass in threads through the guts of all the `Cont`

objects in the chain before it's finally invoked. The way they chain is the way `Cont`

works: each object in the chain invokes a continuation that *has the next object's computation prepended to the final continuation*. Let's say we have a chain of `Cont`

objects `f1 -> f2 -> f3`

, and let's say you had a continuation `c3`

that you want to pass to the chain. Then:

`f3`

needs to invoke`c3`

when it's done.`f2`

needs to invoke a continuation`c2`

that will invoke`f3`

, which will invoke`c3`

.`f1`

needs to invoke a continuation`c1`

,

which will invoke`f2`

,

which will invoke`c2`

,

which will invoke`f3`

,

which will finally invoke`c3`

.

To chain the `Cont`

objects together, then, we need to create the appropriate continuation functions `c1`

and `c2`

and make sure they get passed as the continuation argument to `f1`

and `f2`

respectively.

## Extending to Monad

Extending this idea to the `Monad`

class in general, there's an extra wrinkle: we allow for the value of one computation to affect *which* `Cont`

object gets invoked next. In this world:

`return`

takes a value and produces a`Cont`

object that just passes that value to its continuation.- The bind operator
`(>>=)`

takes a`Cont`

object, and a*function that produces another*, and chains them together into one`Cont`

object given a value from the first`Cont`

object. That object, when invoked, is going to:- take a single continuation object
`C`

, - produce an intermediate value,
- use that intermediate value to select/create the next
`Cont`

object to invoke, - invoke that
`Cont`

object with`C`

- take a single continuation object

# Understanding the Monad

## Return

The code:

```
return a = Cont ($ a)
```

is equivalent to the following code:

```
return a = Cont $ \c -> c a
```

Why? The code `($ a)`

is a *slice* of the operator `$`

, which represents application. In other words, `($ a)`

can be equivalently written `\f -> f a`

, or "take a function as input and apply it to a."

Thus, `return`

in the `Cont`

monad passes the result of its computation directly to the continuation it's given.

## Bind

The code:

```
m >>= k = Cont $ \c -> runCont m $ \a -> runCont (k a) c
```

is a terse way of saying the following:

```
m >>= k = let s c = runCont m c
t c = \a -> runCont (k a) c
in Cont $ \c -> s (t c)
```

Do you see what's happening? `(k a)`

has become part of the continuation that `m`

is given, and `m`

passes its value to `k`

by simply passing its value to its continuation. The `Cont`

objects are being created "just in time" to be used, based on the computation so far.

# Exploring the Monad

Here's a simple example that should help illustrate the monad in action:

```
f :: Int -> Cont r Int
f x = Cont $ \c -> c (x * 3)
g :: Int -> Cont r Int
g x = Cont $ \c -> c (x - 2)
```

These are simple functions that produce `Cont`

objects, given an intermediate value `x`

. You can see that the value being passed to the continuation is an Int, though we don't put any restrictions on what that continuation can ultimately produce.

BTW, they can be equivalently written as:

```
f x = return (x * 3)
g x = return (x - 2)
```

where they look very similar to normal functions. I'm writing them longhand to show you explicitly what the functions are doing.

```
h :: Int -> Cont r Int
h x | x == 5 = f x
| otherwise = g x
```

This is a more complicated function that chooses between two other `Cont`

objects, based on the input it's given.

Now let's create a top-level `Cont`

object that does some chaining:

```
doC :: Cont r Int
doC = return 5 >>= h
```

And we'll invoke it like this:

```
finalC :: Show a => a -> String
finalC x = "Done: " ++ show(x)
runCont doC finalC
```

Note that `runCont doC`

produces a function of type `(Int -> a) -> a`

, which is invoked on a continuation of type `Show a => a -> String`

, which reduces in this context to `Int -> String`

. The final value produced will be a `String`

. Can you guess what it will say? What if you changed `return 5`

to `return 4`

?

Let's see if you're right:

`return 5`

produces a `Cont`

object that basically looks like this: `Cont $ \c -> c 5`

. So that part is easy.

`h`

is a function that takes a value and produces a `Cont`

object depending on the value it's given.

*Lemma:* The sequence of terms `runCont Cont $`

effectively cancel out, i.e. `runCont (Cont $ \c -> ...)`

is simply the function `\c -> ...`

. This is because `runCont`

is a field selector of `Cont`

objects, and `Cont`

objects only have that one field.

Therefore, `(return 5) >>= h`

expands and simplifies to:

```
doC = let s c = c 5
t c = \a -> runCont (h a) c
in Cont $ \c -> s (t c)
```

And finally, `runCont doC finalC`

evaluates to:

runCont doC finalC => runCont (Cont $ \c -> s (t c)) finalC -- unfold doC => s (t finalC) -- simplify with lemma and apply to finalC => (t finalC) 5 -- unfold s => (\a -> runCont (h a) finalC) 5 -- unfold t => runCont (h 5) finalC -- apply \a... to 5 => runCont (f 5) finalC -- unfold h => runCont (Cont $ \c -> c (5*3)) finalC -- unfold f => (\c -> c (5*3)) finalC -- simplify with lemma => finalC (5*3) -- apply \c... to finalC => "Done: 15" -- apply *; apply finalC to final value!

If you changed doC to `return 4 >>= h`

, the derivation would be almost identical to the above, except that 4 would pass through to h, which would unfold to g instead. "Done: 2" should be the result.

# MonadCont and callCC

One final extension to this monad, which can be extremely useful in practice, is the `MonadCont`

class, which provides a `callCC`

operation. `callCC`

creates a `Cont`

object that invokes a function to construct a `Cont`

object, and then runs it with the continuation it's given. However, it provides that function an *alternate* continuation that can be invoked to "break out" of the computation and simply pass a value to the continuation that was active when `callCC`

was invoked. This function's operation is definitely easier to understand by seeing it in action. Evaluate the following code, replacing the corresponding functions above:

```
h :: Int -> (Int -> Cont r Int) -> Cont r Int
h x abort | x == 5 = f x
| otherwise = abort (-1)
doC n = return n >>= \x ->
callCC (\abort -> h x abort) >>= \y ->
g y
```

Run `runCont (doC 5) finalC`

. h should invoke f, and g will be invoked afterward, so you should get 13 as the final answer.

Now change `(doC 5)`

to `(doC 4)`

. In this case, h will call abort, which passes -1 to g. -3 should be the final answer.

Now change `doC`

to move g inside the callCC abort context:

```
doC n = return n >>= \x ->
callCC (\abort -> h x abort >>= \y ->
g y)
```

and run with `(doC 4)`

. In this case, h invokes abort and g is never invoked! -1 is the final answer.

Once you've converted all your operations to continuation-passing style by putting them in the `Cont`

monad, and have a handle on how `>>=`

works in that monad, understanding how `callCC`

works is surprisingly simple:

```
callCC f = Cont $ \c -> runCont (f (\a -> Cont $ \_ -> c a )) c
```

can be written as

```
callCC f = let backtrack a = Cont $ \_ -> c a
in Cont $ \c -> runCont (f backtrack) c
```

The key is `backtrack`

, which takes whatever "inner" continuation is active when backtrack is invoked, completely ignores it, and simply passes its value to the "outer" continuation `c`

. (Compare this to the definition of `return`

, which always uses the continuation it's given.) `f`

is the function passed to `callCC`

, whose extent provides the context under which `backtrack`

can be used.