# Lambda calculus challenges for ASU Undergrads

## TL;DR

This was a challenge written for a programming languages and compilers course at ASU. This is meant to test your skill in lambda calculus.## Rules

This challenge is broken into three parts, each with growing difficulty. To
complete these challenges you *ARE NOT* expected to re-write common lambda
calculus functions such as:

```
pred
succ
pair
fst
snd
tru
fls
church numerals
...
```

You get the idea. All common lambda functions are game and do not need to be expanded to lambda header form. For a more expansive list of common Lambda functions check out the CSE340 lectures. TODO: Add an external link here, open to suggestions.

## Challenge Description

Consider a stack implementation in \(\lambda\)-calculus using pairs:

```
(a, (b, (c, (d, fls) ) ) )
```

We define a stack to recursively have the form:

```
stack = (element, stack_body)
```

To put this in a legitimate syntax, we can say:

```
stack = pair element stack_body
```

A stack body is empty if it is `fls`

. Otherwise, the `stack_body`

will follow
the same form.

### Part 1

Any good stack should have a basic function, poping! Write a `pop`

function that
gets the element on the top of the stack.

For instance:

```
pop (a, (b, (c, (d, fls) ) ) ) -> a
```

If a stack is empty, return `fls`

.

### Part 2

How can we pop things if we can’t push things! Write a `push`

function that,
when given a stack and element, pushes the element on the stack.

For instance:

```
push a (b, (c, (d, fls) ) ) ) -> (a, (b, (c, (d, fls) ) ) )
```

If a stack is empty, return `(pair element fls)`

.

### Part 3

Time to turn up the heat! Now that we have a working stack, let’s get a little
creative with it. Write a `reverse`

function that reverses the entire stack.

For instance:

```
reverse (a, (b, (c, (d, fls) ) ) ) -> (d, (c, (b, (a, fls) ) ) )
```

If a stack is empty, return `fls`

.

### Part 4

This will be the holy grail of the challenges. Write a `sort`

function that
fully sorts a stack, assuming the stack is full of church numerals.

For instance:

```
sort (7, (3, (1, (3, fls) ) ) ) -> (1, (3, (3, (7, fls) ) ) )
```

If a stack is empty, return `fls`

.

## Solutions

The solutions will be provided on request, or posted here if it becomes too pain staking to give everyone the solution. Good luck!