Language Syntax

Language Syntax

CupidScript uses a C-like surface syntax.

Comments

Supported comment forms:

// line comment

/* block comment */

Comments are skipped by the lexer alongside whitespace.

Tokens

Identifiers

• Start: letter or _
• Continue: letters, digits, _

Integers

Decimal and hexadecimal integers with optional underscores:

0
12345
1_000_000
0xFF
0xFF_FF_FF

• Decimal: 0-9 digits
• Hexadecimal: 0x or 0X followed by hex digits (0-9, a-f, A-F)
• Underscores _ can appear anywhere in the number (ignored)

Floats

Floating-point literals with decimal point:

3.14
2.5
0.001
1.5e-3
2.0E+10

• Must contain a decimal point: .
• Optional exponent: e or E followed by optional +/- and digits
• Scientific notation: 1.5e-3 equals 0.0015

Strings

Double-quoted strings:

"hello"
"with \n escapes"
"quote: \""
"backslash: \\"

Raw (backtick) strings:

`C:\\temp\\file`
`line1
line2`

Notes:

• Backtick strings do not process escapes.
• Backtick strings can span multiple lines.
• String interpolation is not supported inside backtick strings.
• A backtick cannot appear inside a backtick string.

String interpolation is supported with ${...} inside a string:

let name = "Ada";
print("Hello ${name}");
print("sum=${1 + 2}");

Escape handling (in runtime unescape):

• \n, \t, \r, \", \\
• \e (ASCII ESC, 27)
• \xHH (single byte from two hex digits)
• Any unknown escape \x becomes literal x

List Literals

[]
[1, 2, 3]
["a", "b", "c"]

Trailing commas are allowed:

[1, 2, 3,]
[]

Map Literals

{}
{"name": "Frank", "age": 30}
{"key": value_expr}

Trailing commas are allowed:

{"a": 1, "b": 2,}
{}

Keys are expressions (any value is allowed). For convenience, an identifier before : is treated as a string key.

Functions

Named functions:

fn add(a, b = 1) {
  return a + b;
}

Function literals:

let inc = fn(x, step = 1) { return x + step; };

Defaults use = in the parameter list. Required parameters must come before default parameters. Missing arguments use their default values.

Arrow Functions

Arrow syntax supports concise single-expression functions:

let double = fn(x) => x * 2;
let add = fn(a, b) => a + b;

Block-bodied arrows require explicit return:

let f = fn(x) => { return x + 1; };

Spread & Rest

Spread (...) expands lists and maps:

let a = [1, 2, 3];
let b = [0, ...a, 4];

let defaults = {theme: "dark", size: 12};
let config = {...defaults, size: 14};

Rest parameters collect remaining arguments into a list:

fn log_all(prefix, ...items) {
  for item in items { print(prefix, item); }
}

Pipe Operator

The pipe operator passes the left value into the right call.

fn add(a, b) { return a + b; }

let r1 = 10 |> add(5);     // add(10, 5)
let r2 = 10 |> add(5, _);  // add(5, 10)

If _ is present, it marks where the left value should go. Otherwise, the left value becomes the first argument.

Classes

Class declarations define constructors and methods with single inheritance.

class File {
  fn new(path) { self.path = path; }
  fn is_hidden() { return starts_with(self.path, "."); }
}

class ImageFile : File {
  fn new(path) { super.new(path); self.is_img = ends_with(path, ".png"); }
  fn is_image() { return self.is_img; }
}

let f = File(".secret");
let img = ImageFile("cat.png");

• self refers to the instance inside methods.
• super refers to the parent class inside methods.
• Instances are created by calling the class as a function.

Structs

Structs define fixed-field data types with positional construction.

struct Point { x, y = 0 }
let p = Point(3);    // x=3, y=0
let q = Point(1, 2); // x=1, y=2

Enums

Enums define named integer constants:

enum Color { Red, Green = 5, Blue }
print(Color.Red);   // 0
print(Color.Green); // 5
print(Color.Blue);  // 6

Generators (yield)

Functions that use yield return a list of yielded values when called:

fn range(n) {
  let i = 0;
  while (i < n) {
    yield i;
    i += 1;
  }
}

let xs = range(4); // [0, 1, 2, 3]

Async / Await

Async functions and await are supported (currently synchronous execution):

async fn add(a, b) { return a + b; }
let v = await add(2, 3);

Operators & Punctuation

Punctuation

( ) [ ] { } , ; .

Operators

Assignment:

• =
• +=, -=, *=, /= (compound assignment, works on variables, fields, and index expressions)

Arithmetic:

• + - * / %

Unary:

• ! -

Comparison:

• == != < <= > >=

Boolean:

• && ||

Nullish coalescing:

• ?? (returns left operand if not nil, otherwise right)

Range:

• .. (exclusive range, e.g., 0..5 → [0,1,2,3,4])
• ..= (inclusive range, e.g., 0..=5 → [0,1,2,3,4,5])

Precedence (high to low)

1. Primary: literals, identifiers, parenthesized (expr), list literals [...], map literals {...}
2. Postfix: calls f(...), indexing a[b], field access a.b (field access works after any expression, e.g. (expr).field)
3. Unary: !expr, -expr
4. Multiplicative: * / %
5. Additive: + -
6. Range: start..end, start..=end
7. Comparison: < <= > >=
8. Equality: == !=
9. AND: &&
10. OR: ||
11. Nullish: a ?? b
12. Ternary: cond ? then : else

Optional chaining

Optional chaining accesses a field only if the target is not nil:

user?.name

If the target is nil, the result is nil (no error). Otherwise, it behaves like normal field access.

switch Statement

CupidScript supports switch as a statement:

switch (x) {
  case 1 { print("one"); }
  case 2 { print("two"); }
  default { print("other"); }
}

Notes:

• Cases are compared using the same equality rules as == for basic types.
• Fallthrough: once a case matches, execution continues into subsequent cases until a break is hit or the switch ends.
• default can appear anywhere; if no case matches, execution starts at default and falls through to later cases.

Pattern cases are supported (no guards):

switch (value) {
  case [a, b, ...rest] { print(a, b, rest); }
  case {x, y: yy} { print(x + yy); }
  case int(x) { print("int", x); }
  default { print("other"); }
}

defer Statement

Schedule cleanup work to run when the current block exits:

defer close(file);
defer { print("cleanup"); }

Notes:

• Defers run in LIFO order when the block exits.
• Defers run even if the block exits via return, break, continue, or throw.

match Expression

match is like switch, but returns a value and is used inside expressions.

let label = match (x) {
  case 1: "one";
  case 2: "two";
  default: "other";
};

Patterns and guards:

let label = match (value) {
  case [a, b]: a + b;
  case {x, y: z}: x + z;
  case n if n > 10: "big";
  case _: "small";
};

Notes:

• Patterns can bind identifiers; bindings are visible in the guard and case result.
• Guards (if expr) are optional and run only when the pattern matches.
• The first matching case expression is evaluated and returned.
• default is optional; if omitted and no case matches, the result is nil.

try / catch / finally Statements

Syntax:

try {
  // body
} catch (e) {
  // handler
} finally {
  // cleanup
}

// finally can also be a single expression:
try { work(); } catch (e) { print(e); } finally cleanup();

Notes:

• catch is required; finally is optional.
• finally executes after the try/catch regardless of return or throw.
• finally can be a block or a single expression statement.

export Statements

Syntax:

export name = expr;
export {name, local as exported};

Defines a value in the module's implicit exports map. require() returns this map.

import Statements

Syntax:

import "./path/to/module.cs";           // side effects only
import mod from "./path/to/module.cs";  // bind exports map
import {foo, bar as baz} from "./path/to/module.cs";
import mod, {foo} from "./path/to/module.cs";

Notes:

• import is syntax sugar over require().
• Named imports read fields from the module's exports map.
• If an imported name is missing, the binding is set to nil.

const Bindings

Use const to create an immutable binding:

const x = 10;

Notes:

• Reassignment to a const binding raises a runtime error.
• const requires an initializer.
• Destructuring also works with const:

const [a, b] = [1, 2];
const {name, age} = {"name": "Ada", "age": 36};

Destructuring let

List and map destructuring are supported in let declarations.

List destructuring

let [a, b, c] = [1, 2];
// a=1, b=2, c=nil

Map destructuring

let {x, y} = {"x": 10, "y": 20};
// x=10, y=20

let {key: alias} = {"key": "value"};
// alias="value"

Notes:

• Missing list elements or map keys produce nil.
• _ can be used to ignore a binding.
• Destructuring requires an initializer.
• List destructuring expects a list; map destructuring expects a map (runtime error otherwise).