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.
Atto’s design is stupidly simple. There are two kinds of structure:
fn <name> [args] is <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
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.
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
# 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
xwith the information tag
assert i x: Assert that
assert_eq x y: Assert that
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
xis a list
is_bool x: Determine whether
xis a bool
is_num x: Determine whether
xis a number
is_null x: Determine whether
len l: Determine the length of a list
skip n l: Skip the first
nvalues in a list
nth n l: Get the
nth item in a list
in x l: Determine whether
xis in a list
split i l: Split a list into two separate lists at the
You can check
src/atto/core.at for full documentation about what
Basic numeric operators:
fn main is + 5 7
fn main is - * 3 3 5
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
Fusing lists together:
fn main is fuse pair 3 17 pair 5 8
[3, 17, 5, 8]
fn main is if true 10 5
fn main is if false 10 5
Selecting the first value in a list:
fn main is head pair 3 17
Selecting values trailing after the head of a list:
fn main is tail fuse 3 fuse 17 9
Converting a string into a value:
fn main is - 7 litr "3"
fn main is + 7 litr "8.5"
fn main is = null litr "null"
Defining a function with parameters:
fn add x y is + x y fn main is add 5 3
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
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.