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SCL - Simple Callback Language

This language was my attempt to solve some of the pains I experienced writing internet facing JavaScript and Python software. The acronym and file extension is SCL so the language can also be pronounced like 'Sickle'.


The only system requirements should be a modern C++ compiler as I've only used things in the standard library.

$ mkdir build
$ cd build
$ cmake ..
$ make -j `nproc`

This will give you a binary which you can do ./scl help for documentation on how to use. The commands are as follows:

  • build: generate a bytecode file for given source code
  • exec: run a bytecode file
  • eval: compile and run given source code
  • minify: Minify input source code
  • debug: generate bytecode text for given source code Notice that all of these commands take one argument as their input file.

Language Structure

  • Statements end with semicolons (;)
    • Automatic Semicolon Insertion: semicolons are optional, but can help to add meaning
  • Not whitespace dependent
  • Comments: // and /* ... */


Builtin Global Variables

All of these values are reassignable and can be referenced and called within other scopes.

  • i: command line arguments (Note: only at global scope, see closures section)
  • o: leave current scope with return value provided as argument (this is known as the return operator in most other languages)
  • print: write values to terminal (ie: print("Hello, world!"))
  • input: read values from terminal as a string (ie: age = Num(input()))
  • if: performs functionality of ternary and branching
  • Str: converts given value to a string representation
  • Num: Parses a number (output is either Int or Float)
  • vars: debugging tool
  • async: run closure in async context (see section)
  • import: Load a native function or module
  • size: Gives size of given value, equivalent to len in Python
  • copy: Deep-copies given value


Variables are declared with the let operator which has similar syntax to JavaScript. References also behave as you would expect from JavaScript.

let name = "John Smith";
let age = 30, vehicle;
vehicle = "Hot rod"; 

WiP: metaprogramming

You can define macros that expand to larger expressions


Supports any valid JSON data

Type Literal Use
Str "Hello, world!" Holds character sequences
Int 10 Holds whole numbers (64 bits)
Float 1.2 64bit floating point numbers
a -> b (: i + 2 ) first-class functions, alternatives to blocks
List [1, 2.5, 'cat'] Hold series of values
Obj { temp: 98.6 } Dictionary with strings as keys


Closures are first class functions but more important here as they're used to replace code blocks.

Defining a Closure

  • Closure literals are enclosed in (: )
  • Variables can reference macros just like any other data, however code cannot modify their internals
let say_hello = (:
    print("hello, " + i);

print("what's your name?");
let name = input();
say_hello(name); // greets user

Input and Output

Input is accessible via the local variable i. Use the local variable o to return a value. Although you can use i and o themselves, declaring variables (let) or aliases (using) for them can improve clarity and is required when they get shadowed by a previous scope.

let greeting = (:
    let name = i;
    let return = o;
    return("Hello, " + name);

Everything is a Closure

Because of the increased tools for control flow, in this langauge, everything is a closure. Where other langauges use operators like if, return, await, etc, this language can just use closures (often even with user-level implementations). Further, this langauge doesn't have blocks (usually in curly braces), because closures can serve the same purpose as them.

What does this mean?

To emphasize this point further, it can be thought that program files are wrapped in (: ) by the compiler. Command line arguments are passed as the closure's input (i) and it's output (o) is eqivalent to sys.exit. So a simple echo program can be written as such

// Echo command line arguments

// Exit success

This simplicity also applies to modules. There's no reason to have special operator for exports.


Control Flow

These are currently defined as builtins/standard library functions, but in the future they might be converted to operators.


For now if is just a function.

  • Note: comma separated arguments implicitly converted to list
let gpa = Num(input()); // 3.86

if (gpa > 4 || gpa < 0, (:
    print("seems rigged");
), gpa >= 2, (:
), (:

You could also implement a less useful if like so

let tern =  (: i[1 + Int(i[0] != 0)] );
let if = (: tern(i)() );


While Loops

Pretty standard apart from it not being an operator.

let n = 0;
while ((: n < 5 ), (: 
    n += 1;

You can implement while on your own like shown below. This will be required until conditional jumps get added to the VM bytecode.

let while = (:
    let args = i, break = o
    if (args[0](break), (:
  • Calling o() from the body will skip to next cycle
  • Calling i() from body or condition will break out of the loop
  • Calling i(true) will make while(...) return true when you break out

Range Based For

The following definitions (among others) will eventually be included in the standard library. In the near future I'll add range with similar functionality to python's version but without iterators.

let foreach = (:
	let list = i[0], action = i[1]
	let index = 0, end = size(list)
	while((: index < end), (:
		action(list[index], index, i)
		index = index + 1	
let map = (:
	let list = i[0], fn = i[1]
	list = copy(i[0])
	let ret = foreach(i[0], (: list[i[1]] = fn(i) ))
	if(ret == empty, list, ret)


This was the main reason I made this language. JavaScript, Python, C· and most other languages featuring the most popular async/await syntax require you to change your program depending on sync/async context and in doing so adds excess and/or confusing features. In this language I only needed to add a single built-in global async in order to provide equivalent functionality.

Running code in a new thread

Lets walk through an exmaple that gets main points across. Imagine we have a function request that takes a url and fetches it's content over the internet.

let request = import('')

We can call request like any normal function and as we're awaiting the results, the VM can work on other tasks

let text = request('')
print(text); // x

However we can also perform the function call in a separate thread! We first make an async wrapper for the request function and then call it, receiving an eventual. Which we can call later to get the results.

// Alternatively we can make the request in a new thread
let eventual = async(request)('')

// So that we can do other things while we wait on the download

// And then we can simply invoke the eventual to get the same behavior as before
print(eventual()) // x


See async demo to see how easy it is to convert between functions that return promises and functions with callbacks


By default functions will implicitly return when they reach the end, however this behavior can be overridden by changing the value of o.

  • This is dangerous because the thread won't return unless you already passed o to something that can explicitly call it.
  • Feature may be removed in future implementation and is usually wrong to use
// Function that freezes thread for given duration
let delay = import(delay)

// This is equivalent to JavaScript's window.setTimeout
let set_timeout = (:
	let duration = i[0], action = i[1], arg = i[2]

// This function does same thing as `delay`
let delay2 = (:
	// After i ms, set_timeout will call o
	set_timeout(i, o)
	// Prevent implicit return
	o = 0

// Note that in this example, it's reccomended to simply do
let delay3 = (: set_timeout(i, o)() )

Core values

  • Unclear operators should be avoided, use functions instead
    • ie - return, break, continue, export, etc. don't exist here
  • No redundant language features
  • Avoid strong opinions

More coming soon

Most of these features are at least working. There are some things that are implemented haven't made their way into this guide and even more that I haven't implemented but have planned. If there's anything you want to see added, lmk.


A programming language with a comfy syntax which answers the question "what if everything was call/cc?"








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