In the last chapter, we discussed the P’’ computer architecture – a highly simplified computer consisting of two tapes. P’’ has a program tape, which contains the current program to run, and it also has a memory tape, where it can store Nats as a form of “scratch work.”

As a required step towards building a P’’ machine, we’ll obviously need to build this tape data structure. A data structure is a container for storing information, complete with an interface for interacting with it, and trade-offs for when one data structure should be used in place of another.


We’ve already seen a data structure, the List a. List a fits our description for what it means to be a data structure: it is a container of elements of type a, and its interface allows us to pull off the front of the list, getting back the element there, and the remainder of the list. We’ll look at trade-offs later when we discuss the Big-O notation, but that won’t be for some time yet.

Before we go ahead and build a tape, we should probably understand what a tape is, and how we’ll want to interact with it. Recall that a tape has a read head, which corresponds to the “active” element on the tape; we’re only ever allowed to look at the read head of the tape. In addition, we have two spools on either side of the read head, which act as a kind of “storage”. As we move the read head left and right, the value at the read head moves to one of the spools, and a new value comes under the read head from the other spool. We also said that tapes are infinitely long in both directions, a fact we’ll need to deal with at some point.

So, conceptually, we can represent a tape as some new type Tape a, consisting of two “spools containing as”, and a distinguished “read head”, itself of type a. But what should these “spools” be? Well, let’s go back to our use-cases and see what we need from the spools. The spools only ever absorb or provide a new element for the read head – meaning we need to either be able to put an element onto the spool, or take one off. Also, we’d like to get the elements out of the spool in the opposite order that we put them in on. Think about this like a VHS tape – the further back you rewind from a moment, the more you need to fast-forward to get back to where you were.

With all of this in our minds simultaneously, we see that the “spools” we wanted are nothing more than just List a. We can always pull the head off of a List a (provide an a to the read head), or “push” one in (absorbing the read head). Since we can only ever get the first thing in the list, if we add things to the front of the list, we’ll get the desired property that elements come out in the opposite order that they were added. This property is sometimes known as “last in, first out”.

We’re now ready to provide a type definition for tapes:

type Tape a = Tape (List a) a (List a)

A Tape a is nothing more than two List as representing the spools, and a value of type a sitting in the middle, representing our read head. We’ll provide a few helper functions to deal with a Tape a:

readHead : Tape a -> a
readHead (Tape _ a _) = a

readHead : Tape a -> a is nothing but a function that gets the current value “under focus” of the tape – which is to say, the read head. We also need some functions to move the read head left and right:

moveLeft : Tape a -> Tape a
moveLeft (Tape (Cons l ls) a rs) = Tape ls l (Cons a rs)
moveLeft (Tape Nil         a rs) = ???

moveLeft : Tape a -> Tape a takes the first thing off the left-most spool, and moves it to the current read head. It pushes the current read head into the right-most spool. But there’s a problem! What happens if our left-most spool is Nil (empty)? Conceptually, our spools are infinitely long, but we don’t have a notion of infinity to work with, unfortunately. The best we can do is to “magically” pull as out of nowhere, and pretend like they were in the spool to begin with. But which value of type a should we choose?

Because a is polymorphic, there’s no “obvious” answer for which a to use. It’s going to depend on the specific type that fills in the type variable a. For the program tape, we probably want to fill in the rest of the tape with Halt instructions. But for the memory tape, we want to fill it in with \(0\)s.

We can capture this “most interesting element” to fill the tapes with via a new class of patterns, just like we had the Kleisli class pattern!

class Point a where
  point : a

This should be read as “Point is a class of patterns, parameterized by some type a. This pattern is in effect whenever we have a value point : a.” You can think of a Point a as being some type a with a “point of interest”. With this point : a machinery in place, let’s revisit moveLeft:

moveLeft : Point a => Tape a -> Tape a
moveLeft (Tape (Cons l ls) a rs) = Tape ls  l     (Cons a rs)
moveLeft (Tape Nil         a rs) = Tape Nil point (Cons a rs)

Now, if the left-most spool runs out of elements, we can use our witness of a Point a pattern to “pull an a out of nowhere!” Recall that point : a is a value that exists whenever we have a witness of the Point a pattern.

The machine for moveRight is symmetric to the moveLeft example:

moveRight : Point a => Tape a -> Tape a
moveRight (Tape ls a (Cons r rs)) = Tape (Cons a ls) r     rs
moveRight (Tape ls a Nil)         = Tape (Cons a ls) point Nil

It’s worth thinking about this “pulling as out of nowhere” trick and why it works. Conceptually our spools have infinite length, but because the universe doesn’t have infinite size, we can’t directly make an equivalent analogue. However, what we can do is make a spool that is “as big as necessary”. Unfortunately, we don’t know how big “necessary” is in advance, so our next best bet is to make a spool that can become larger whenever more is requested. This solves the same problem, and so we’re good to go!

The P’’ Machine

Now, we know that a P’’ machine is defined as two tapes: one for instructions, and one for memory. We know that the memory tape is Tape Nat, but what type should we use for our instructions?

We made a list of seven possible instructions before. An particular instruction could be any one of Move Left, Move Right, Increment, Decrement, Enter Loop, Exit Loop, or Halt. The word “or” in this list should indicate that we want a sum type to describe these options:

type Instr = MoveLeft
           | MoveRight
           | Increment
           | Decrement
           | EnterLoop
           | ExitLoop
           | Halt

and we can define the “points of interest” for our Instructions and Nats:

instance Point Instr where
  point = Halt

instance Point Nat where
  point = 0

Given all of this, we can now define our P’’ machine as the product type of a Tape Instr and a Tape Nat.

type P'' = P'' (Tape Instr) (Tape Nat)

You should be convinced that this P'' type corresponds exactly to what we said we meant by a P’’ machine. We still need to write functions to “execute” the program described by our Tape Instr, but that shouldn’t be too hard.

In the next chapter, we’ll build this “executor” function, which looks like execute : P'' -> P'' – transforming a P’’ machine into another by evaluating the instruction at the read head of the program tape. When execute is run repetitively, until a Halt instruction is run, we’ll have our first working computer and everything will be groovy.


  1. Use equational reasoning to show that readHead (moveRight (moveLeft x)) = readHead x for every x : Point a => List a, and that readHead (moveLeft (moveRight x)) = readHead x.
  2. Why can’t we make the stronger claims of equational reasoning that moveRight (moveLeft x) = x and moveLeft (moveRight x) = x? Under which circumstances would this prove to not be true?
  3. Prove that Maybe a has an instance of the Point pattern, by writing the instance for it.