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https://github.com/zesterer/atto

An insanely simple self-hosted functional programming language
https://github.com/zesterer/atto

functional interpreter language recursion rust self-hosted

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An insanely simple self-hosted functional programming language

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README 

        
# Atto



Atto is an insanely simple functional programming language.



It features a syntax driven entirely by polish notation and no delimiters to speak of (it ignores all non-separating whitespace).

What do you get for this simplicity? Well... an insanely simple language with a ~200 line self-hosted interpreter.



Despite these design limitations, it's actually possible to write quite pleasing code in Atto.

That, combined with the extraordinarily extendable syntax (you can define your own operators, or overload those defined in the `core` library) make it

ideal for solving a whole class of programming problems that are normally awkward to solve in more imperative languages.



## Design



Atto's design is stupidly simple. There are two kinds of structure:



Functions: `fn  [args] is `



Expressions: ` [expr]`



That's it. Expressions, function calls, literals and operations are all considered to be the same thing.



I leave you with a quick factorial calculation example demonstrating the compact expressiveness of Atto at work.



```

fn f n is

  if = n 0

    1

  * n f - n 1

```



Yes, that's it.



## Atto Interpreter Written In Atto



In `examples/self-hosted.at`, I've written a fully-functioning REPL-based interpreter for Atto.

It supports function declaration, function calling, and all of the evaluation operators that Atto does, including I/O.

It has a minor issues, such as behaving unpredictably with invalid input. However, it should be able to successfully run any valid Atto program (provided your stack is big enough).



Which reminds me: I need to use a non-recursive interpretation algorithm in the Rust interpreter. Also, tail-call optimisation would be nice.



## Core Library



Atto comes with a `core` library. It provides a series of non-intrinsic functions and utilities that are themselves written in Atto.

In addition, it provides all of the operators common to Atto usage.

The Atto interpreter implicitly inserts the `core` library above whatever you run, similar in nature to C's `#include`.



- `# x y`: Ignore the first value, evaluate to only the second (useful for comments)

- `@ x y`: Ignore the second value, evaluate to only the first

- `! x`: Negate a boolean

- `wrap x`: Wrap a value in a list

- `empty`: Produces the empty list

- `debug_enabled`: Can be overriden to enable debugging utilities

- `debug i x`: Display the value of `x` with the information tag `x`

- `assert i x`: Assert that `x` is true

- `assert_eq x y`: Assert that `x` and `y` are equivalent

- `is_atom x`: Determine whether a value is atomic (i.e: null, bool or a number)

- `is_str x`: Determine whether a value is a string

- `is_list x`: Determine whether `x` is a list

- `is_bool x`: Determine whether `x` is a bool

- `is_num x`: Determine whether `x` is a number

- `is_null x`: Determine whether `x` is null

- `len l`: Determine the length of a list

- `skip n l`: Skip the first `n` values in a list

- `nth n l`: Get the `n`th item in a list

- `in x l`: Determine whether `x` is in a list

- `split i l`: Split a list into two separate lists at the `i`th index



You can check `src/atto/core.at` for full documentation about what `core` provides.



## Tutorial



### Basic numeric operators:



```

fn main is

	+ 5 7

```



Yields: `12`



```

fn main is

	- * 3 3 5

```



Yields: `4`



### Printing values to the console:



```

fn main is

	print "Hello, world!"

```



```

fn main is

	print str 1337

```



### Receiving inputs from the user and converting them to a value:



```

fn main is

    print + "Product = " str

    	* litr input "second: "

          litr input "first: "

```



### Pairing values together into a two-component list:



```

fn main is

	pair 3 17

```



Yields `[3, 17]`



### Fusing lists together:



```

fn main is

	fuse pair 3 17 pair 5 8

```



Yields: `[3, 17, 5, 8]`



### Conditional expressions:



```

fn main is

	if true

		10

		5

```



Yields: `10`



```

fn main is

	if false

		10

		5

```



Yields: `5`



### Selecting the first value in a list:



```

fn main is

	head pair 3 17

```



Yields: `3`



### Selecting values trailing after the head of a list:



```

fn main is

	tail fuse 3 fuse 17 9

```



Yields: `[17, 9]`



### Converting a string into a value:



```

fn main is

	- 7 litr "3"

```



Yields: `4`



```

fn main is

	+ 7 litr "8.5"

```



Yields: `15.5`



```

fn main is

	= null litr "null"

```



Yields: `true`



### Defining a function with parameters:



```

fn add x y is

	+ x y



fn main is

	add 5 3

```



Yields: `8`



### Recursion to find the size of a list:



```

fn size l is

	if = null head

		0

		+ 1 size tail l

fn main is

	size fuse 1 fuse 2 3

```



Yields: `3`



## Optimisation



Currently, Atto's Rust interpreter performs virtually no optimisations. Despite that, I'll attempt to talk below about some ideas I've had that seem promising.



Atto's design does not permit the realiasing of values within a function, nor does it permit mutation. This, and the fact that the syntax is incredibly

quick to parse, makes it an extremely good potential target for a lot of optimisations. Inlining, constant propagation, CSE detection and tail call

optimisations are naturally easy to implement on top of Atto's design.



The lack of realiasing also means that Atto has an affine type system *by design*, without ever requiring compiler

support for move semantic analysis or anything like that.



The only real obstacles to some really impressive optimisation is its dynamic type system. However,

in a significant number of cases it's likely that types can be inferred at compile-time with specialized machine code emitted for each function depending on

the types passed to it.



I'm also working on ideas for a statically-typed version of Atto. However, I've yet to settle on a design that is sufficiently simple as to compliment the

current design.