Team: zaph0d

- farrellm23: language design and implementation
- ZebraFish: moral support and brownies

Source: github

### Disclosures:

- I wrote most of the type checker for Zaphod back in May, then dropped the project until taking it up again for the Jam at the beginning of August (see commit history).
- While I did most of my coding in emacs, I did play with the repl.it environment enough to get Haskell compiling and running.

# Zaphod

A two headed language

How is Zaphod a two headed language? In several ways:

- Zaphod is like Scheme with Haskell's type system[1] bolted on
- Zaphod's type system includes an explicit
`Top`

type, so "untyped"

functions and macros are possible - [todo] Zaphod supports an alternative syntax that looks much more

like Python

## Why does Zaphod exist?

### Dual foundation

Haskell's type system is great, but metaprogramming with Template

Haskell is clunky. Scheme has a great metaprogramming via macros, but

is untyped. Zaphod combines the two, with a powerful type system for

normal programming and macros for metaprogramming.

### Dual syntax [todo]

Lisps are great for manipulating ASTs, but many programmers do not

enjoy the parenthesis heavy syntax. Python has a nice, clean syntax,

but is clunky for AST manipulations. Zaphod uses the same AST for

both Scheme and Python syntaxes, so code can be written with Python

syntax while AST manipulations (macros) can be written with Scheme

syntax.

## What does Zaphod look like?

### Basics

```
;; line comments
(* block
comment *)
;; the unit value
()
;; a (quoted) symbol
'zaphod
;; a tuple of three symbols - the following 4 produce the same value
['a 'b 'c]
(tuple 'a 'b 'c)
(cons 'a (cons 'b (cons c ())))
'(a b c)
;; a lambda expression
(lambda (x) [x x])
```

### Types

```
() ;; the unit type
Symbol ;; the type for symbols
Top ;; universal return type
(x . y) ;; pair type
;; tuple types, the following two are equivalent
[x y z]
(x . (y . (z . ())))
;; function types, eg, a function from one symbol to another symbol
(-> [Symbol] Symbol)
;; universal quantification, for parametric polymorphism
(forall a (-> [a] a)) ;; the type of the identity function
;; hard to demonstrate, but symbols can also be types, eg, the type 'Bool
```

### Defining a value

```
;; Explicit type
(def name
Symbol
'Zaphod)
;; Inferred type
(def unit ())
```

### Defining a function

```
(defn (id x)
(forall a (-> [a] a))
x)
```

### Macros

```
(def defn
(-> [(Symbol . Top) Type Top] Top)
(macro (p t e)
['def (car p) t ['lambda (cdr p) e]]))
```

### ADTs

```
(data Bool
True
False)
(data (Maybe a)
Nothing
(Just a))
(defn (not p)
(-> [Bool] Bool)
(if p
False
True))
```

## Why is Zaphod interesting?

Zaphod takes the minimalism of Scheme and applies it to Haskell's type

system. Take the `data`

"special form" is not a special form at all,

but a macro! `Bool`

is a symbol masquerading as a type, and `Maybe`

is a function from a type to tuple type. `True`

, `False`

and `Nil`

are all symbols that have had their types overwritten, and `(Just x)`

is, similarly a coerced tuple.

Consider Haskell's

maybe

function. Normally, we would implement this with pattern matching,

but Zaphod doesn't have pattern matching yet, so we can abuse the

underlying representation.

```
(defn (maybe d f m)
(forall a (forall b (-> [b
(-> [a] b)
(Maybe a)]
b)))
(if (is-symbol m)
d
(f (unsafe-coerce (cadr m)))))
```

## Future work

- Records (via a macro)
- Type classes (via macros, after adding implicit arguments)
- Numeric, string, character types

## References

<a id="1">[1]</a>

Dunfield, Joshua, and Neelakantan R. Krishnaswami. “Complete and Easy Bidirectional Typechecking for Higher-Rank Polymorphism.” Proceedings of the 18th ACM SIGPLAN international conference on Functional programming - ICFP ’13 (2013): n. pag. Crossref. Web.

If you're interested in Haskell-meets-Lisp, Kadena's Pact might be of interest. I haven't played around with it (it's for blockchain applications), but it's an example of combining type systems with macros.

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