Cassette is a small functional programming language. It looks like this:

import IO, Net, String (crlf), List

; keeps reading from a connection while there's any data
def read_resp(conn) do
  def loop(received)
    let
      chunk = IO.read_chunk(conn, 1024)   ; get the next chunk
    in
      if #chunk == 0, received    ; no more data
      else loop(received <> chunk)

  loop("")
end

let
  conn = Net.connect("cassette-lang.com", "80")   ; open a network connection
  req = List.join([                               ; form an HTTP request
    "GET / HTTP/1.0",
    "Host: cassette-lang.com",
    "",
    ""
  ], crlf)
in do
  IO.write(conn, req)         ; send the request
  IO.print(read_resp(conn))   ; read the response and print it
end

iThis is version 2 of Cassette. Version 1 is described here.

I made Cassette as a simple language for personal programming. It's designed for solo programmers working on non-enterprise projects. It's DIY, roll your own, batteries-not-included. It's for fun.

Here are some features of Cassette:

• Functional
• Immutable values
• Garbage collected
• Efficient tail-call recursion
• Module based

Getting Started

This project requires a C toolchain and SDL2.

1. Get the project's dependencies
   • On macOS with Homebrew, run brew install llvm git sdl2
   • On Debian, run apt install build-essential clang git libsdl2-dev
2. Build Cassette
   • Clone Cassette with git clone https://github.com/protestContest/Cassette (and then cd Cassette)
   • Run make to build the project. This creates the executable bin/cassette.
3. Try the example with ./bin/cassette test/test.ct.

Syntax

Values

Cassette has only four value types: integers, pairs, tuples, and binaries.

Integers are signed, 30-bit numbers (-536,870,912 to 536,870,911). Integers can be written in decimal, hexadecimal, or a literal byte value. The keyword true is shorthand for 1 and the keyword false is shorthand for 0.

1                 ; decimal integer
0x1F              ; hex integer
$a                ; => 0x61
true              ; => 1
false             ; => 0

Symbols are arbitrary values. (Some languages call them atoms.) At runtime, these become the integer hash value of the symbol name.

:hello
:ok
:not_found_error

Pairs are Lisp-style cons cells, which are used to create linked lists. The keyword nil is shorthand for a special pair, which is the empty list. The pair operator, :, is right-associative.

100 : 200         ; pair
nil               ; empty list
[1, 2, 3]         ; list, same as 1 : 2 : 3 : nil

Tuples are fixed-size arrays. They're less flexible than lists, but they use less memory and are more efficient to access. The maximum size of a tuple is the maximum integer size.

{1, 2, 3}

Binaries are byte vectors. Strings are represented as UTF-8 encoded binaries. The maximum size of a binary is the maximum integer size.

"Hello!"

Operators

Cassette supports several built-in operators on values. Most operators only work on certain types.

Basic arithmetic and comparison operators work on integers.

-24               ; unary negation
73 + 4            ; addition
87 - 41           ; subtraction
43 * 12           ; multiplication
17 / 4            ; division (truncating)
400 % 12          ; modulus
256 >> 3          ; bit shift
1 << 27
0xAA | 1          ; bitwise or
1036 & 0xFF       ; bitwise and
~7                ; bitwise not
12 < 3            ; comparison
12 <= 3
12 > 3
12 >= 3

Some operators only work with pairs, tuples, or binaries.

; get the head of a pair
@(1 : 2)          ; => 1
@[1, 2, 3]        ; => 1

; get the tail of a pair
^(1 : 2)          ; => 2
^[1, 2, 3]        ; => [2, 3]

; join two tuples or binaries
{1, 2} <> {3, 4}  ; => {1, 2, 3, 4}
"ab" <> "cd"      ; => "abcd"

; get the length of a tuple or binary
#{:foo, :bar}     ; => 2
#"hello"          ; => 5

; get an element of a tuple or binary
{1, 2, 3}[0]      ; => 1
"test"[2]         ; => $s (an integer)

; slice a tuple or binary
{1, 2, 3, 4}[1,3] ; => {2, 3}
"hello"[1,4]      ; => "ell"

Logic and equality operators work on any type. Only the values 0 (a.k.a. false) and nil evaluate as false. Logic operators short-circuit and evaluate to one of their operands. Equality is compared structurally, and returns true or false.

false or :ok      ; => :ok
true and nil      ; => nil
not nil           ; => true
not {0, 0}        ; => false
3 == 3            ; => true
[1, 2] == [1, 2]  ; => true

Conditionals

An if expression is a list of predicate/consequent pairs. It tests each predicate until one is true, then evaluates that predicate's consequent. If none are true, the else expression is evaluated.

if true, :ok else :error

if
  x >= 10,  :ten_plus
  x >= 1,   :one_plus
else
  :less_than_one

Variables

A let expression is a list of assignments and a result expression. Each assigned variable is in scope in the subsequent assignments (but not in its own) and in the result expression.

let
  x = 3
  y = x * 2 + 1
in
  x - y

let
  result = try_this()
  result = try_that(result)   ; the argument refers to the previous assignment
in
  done(result)

An assignment in a let expression can have an except clause. If the except test evaluates true, the clause's alternative expression becomes the result of the let expression, and the rest of the let expression is ignored.

let
  conn = Net.connect(host, "80") except error?(conn), conn
  result = IO.write(conn, req) except error?(result), result
in
  IO.read(conn)

Functions

Functions can be defined as lambdas. Function calls look similar to other languages.

let foo = \a, b -> a + b
in foo(1, 2)

Blocks

A do block is a list of expressions. The result of the last expression is the result of the block.

do
  some_work()   ; executed, but ignored
  other_work()  ; block result
end

A do block also allows functions to be defined with def. def assigns a function to a variable, which is in scope for the whole block. Functions defined with def can refer to themselves recursively.

do
  inf_loop()    ; this is fine since the function is defined for the whole block

  def bar(x) {:bar, x}

  def inf_loop()
    inf_loop()
end

Modules

Cassette programs are composed of modules. The body of the module is a do block, and can define functions with def. A module can export some of its defined functions and import other modules. A module can reference imported functions by qualifying them with the module name or alias. Module declaration, import, and export statements must appear first in a module (its "header").

module Foo
import Bar (bar_fn), LongModuleName as LMN
export foo, foo2

def foo(x)
  let y = Bar.parse(x)
  in LMN.run(y)

def foo2(x) :unimplemented

bar_fn(x)    ; no qualifier needed

Primitives

Built-in functions are executed via the trap pseudo-function. These trap calls are usually wrapped in a module function at runtime for convenience. A reference of currently-implemented traps is available here.

trap(:print, "Test")
trap(:sdl_line, 10, 20, 100, 200, window)

Standard Library

Cassette provides some standard modules for common tasks. These modules are documented here.