language basics

syntax

ghūl syntax is inspired by a number of non-brace languages, including ALGOL 68 and Pascal

identifiers and keywords

Identifiers in ghūl follow the convention of snake_case for variables, functions, methods, and properties, PascalCase for namespaces, traits, abstract classes, unions, and enums, and MACRO_CASE for concrete classes, structs, variants, and enum members. ghūl keywords are lowercase.

ghūl relies on keywords for block structure where other languages use braces or indentation. Keywords are context specific and generally come in pairs where the closing keyword is the reverse or mirror image of the opening keyword. In the examples below is introduces a method or class body and its block is closed by the reverse keyword si

ghul

let my_variable = 42;

print_something(thing: string) is

write_line("The thing is: {thing}");

print_something("a hello");

class PERSON is

name: string;

age: int;

The thing is: a hello

expressions and statements

Expressions in ghūl are constructs that return a value, while statements perform actions. All expressions can be used where statements are allowed, but only if statements can be used as expressions.

ghul

// variable declaration statement

let x = 10;

// expression used as part of a declaration statement

let y = x * 2;

// 'if' is a statement, 'x > 5' is an expression

if x > 5 then

write_line("x is greater than 5");

// 'if' can also be used as an expression

let z = if x > 5 then x else y fi;

x is greater than 5

function declarations

Functions in ghūl are declared with an optional return type, a name, a list of parameters in parentheses, and a body enclosed in is and si keywords

ghul

greet(name: string) -> void is

write_line("Hello, {name}!");

Functions can also have an expression body using => instead of is / si:

ghul

square(x: int) -> int => x * x;

control flow

ghūl supports various control flow constructs like if, else, while, for, and case expressions.

ghul

if x > 0 then

write_line("Positive");

elif x < 0 then

write_line("Negative");

else

write_line("Zero");

for item in my_list do

process(item);

Positive

types

ghūl is statically typed, with some support for type inference. Types can be explicitly specified using a colon : plus a type expression

ghul

let x: int = 42;

let y = "Hello";

User types are defined using class, struct, trait, enum, and union keywords.

built-in data types

ghūl's built-in data types are primitive types, arrays, tuples, and optionals.

primitive types

ghūl provides the following primitive data types:

integer types: byte, ubyte, short, ushort, int, uint, long, ulong, word, uword
floating-point types: single, double
fixed-point type: decimal
boolean type: bool
character type: char
void type: void

ghul

let my_int: int = 42;

let my_float: double = 3.14d;

let my_decimal: decimal = 19.99m;

let my_bool: bool = true;

let my_char: char = 'A';

These types are used to represent basic values in ghūl programs.

arrays

ghūl supports arrays, which are fixed-size, immutable collections of elements of the same type. Array types are denoted using square brackets [] after the element type.

ghul

let numbers: int[] = [1, 2, 3];

Arrays can be constructed with an array literal

ghul

let primes = [2, 3, 5, 7, 11];

Array elements can be read with indexer syntax

ghul

let p = primes[i];

tuples

Tuples in ghūl are lightweight, immutable data structures that can hold a fixed number of elements of different types. Tuple types use parentheses ( ), with elements separated by commas. Tuple literals are similarly constructed with ( ) and comma delimited elements. Tuples compare by structural equality: two tuples are equal when their corresponding elements are.

ghul

let point: (int, int) = (10, 20);

let person: (string, int) = ("Alice", 30);

Tuple elements can be accessed using the dot . notation followed by the element name:

ghul

let x = point.`0;

let y = point.`1;

let name = person.`0;

let age = person.`1;

Tuple elements can be given more descriptive names, either in the type or in the tuple literal:

ghul

let point: (x: int, y: int) = (10, 20);

let person = (name = "Alice", age = 30);

let x = point.x;

let y = point.y;

let name = person.name;

let age = person.age;

ghūl also supports tuple destructuring:

ghul

let (a, b) = point;

let (name, age) = person;

Destructuring also has a by-name form, (local = field, ...), that pulls each element from a named field rather than by position; the positional and by-name forms are covered with pattern matching.

optional types

A type followed by ? is an optional type: a value of T? can be present or absent. The same type written without the ? is non-optional, and a non-optional value is always there.

ghul

let ►name: string? = "Alice"; // present

let nickname: string? = null; // absent

The postfix ? operator tests whether an optional has a value. A plain if x? narrows x to its non-optional form inside the branch, so the value reads directly:

ghul

if ►name? then

write_line("name is {name}"); // name is non-optional here

name is Alice

A non-optional type never holds the absent case, so a T? is not assignable to a T. The compiler rejects it rather than warning:

ghul

// rejected: a string? is not assignable to a string

let title: string = maybe;

string? is not assignable to string

[non-optional] string expected but maybe may not hold a value

To pass a T? where a T is wanted, make the value present first: narrow it with if x? or if let (see control flow), assert it with x! (which throws when absent), or supply a fallback with x ?? other. Optional types work for reference and value types alike - and beyond those two, for generic code that doesn't know which one it has, and for user-defined types that never mention T? at all. The optional types page covers all of that, along with the ?? and ?. operators and the warnings that keep optional handling honest.

type conversions

ghūl does not perform implicit type conversion (coercion) between scalar types; all scalar type conversions must be explicitly cast. However, ghūl supports polymorphic behavior by allowing upcasting, where instances of derived classes or interfaces can be automatically coerced to compatible ancestor types in the class/interface hierarchy.

ghul

// OK: both addends are type double

let d = 1.0d + 1.0d;

// compile time error: addends are mixed types

// (double vs int)

let e = 1.0d + 1;

// OK with explicit cast

let e = 1.0d + cast double(1);

// OK: "hello" is a string, and string derives

// from object

let o: object = "hello";

variables

ghūl has three kinds of variables: locals declared within the body of a function or method, function or method arguments and variables captured by a function literal.

locals

Local variables are declared with let followed by the variable name, an optional explicit type, and an initializer:

ghul

let i = 1234;

let j: int = 0;

let k: int = 5678;

arguments

Arguments will be covered in detail with functions and methods, but the basic form is the argument name followed by its type.

ghul

some_function(argument: type)

captured variables

Variables captured by a function literal will be covered with function literals. They are not explicitly declared but inferred from each function literal's body.

scope

The scope of all variable definitions is from the point of declaration to the end of the innermost block that contains the declaration. Blocks will be covered later, but generally a block is a control flow statement or a function or method body.

type inference and explicit types

ghūl infers the type of a local variable from its initializer. An explicit type can be given alongside; it's a compile time error if the initializer is not assignment compatible with it.

literals

Literal expressions represent fixed values of a specific type.

ghul

let integer_literal = 42;

let floating_point_literal = 3.14;

let string_literal = "Hello, world!";

let boolean_literal_true = true;

let boolean_literal_false = false;

operators and expressions

arithmetic operators

ghul

let add = 1 + 2;

let sub = 3 - 1;

let mul = 3 * 3;

let div = 12 / 3;

let mod = 13 % 4;

comparison and logical operators

ghul

let gt = 3 > 1; // true

let gte = 4 >= 4; // true

let lt = 3 < 1; // false

let lte = 4 <= 4; // true

let eq = 1 == 2; // false

let neq = 1 != 2; // true

ghul

let list = [1, 2, 3];

let index = 4;

let search_value = 3;

// false

let and_then =

index < list.count /\ list[index] == search_value;

// true

let or_else =

index >= list.count \/ list[index] != search_value;

assignment

variables and properties can be updated via assignment statements

ghul

let i mut = 0;

let j = 10;

let s mut = "Hello";

i = i + j;

s = "{s} World!";

thing.property = i + j;

write_line("i = {i}, s = {s}, thing.property = {thing.property}");

i = 10, s = Hello World!, thing.property = 20

language basics ​

syntax ​

identifiers and keywords ​

expressions and statements ​

function declarations ​

control flow ​

types ​

built-in data types ​

primitive types ​

arrays ​

tuples ​

optional types ​

type conversions ​

variables ​

locals ​

arguments ​

captured variables ​

scope ​

type inference and explicit types ​

literals ​

operators and expressions ​

arithmetic operators ​

comparison and logical operators ​

assignment ​

language basics

syntax

identifiers and keywords

expressions and statements

function declarations

control flow

types

built-in data types

primitive types

arrays

tuples

optional types

type conversions

variables

locals

arguments

captured variables

scope

type inference and explicit types

literals

operators and expressions

arithmetic operators

comparison and logical operators

assignment