The Koral Programming Language

Koral is an experimental compiled language that combines Go's aggressive escape analysis with Swift's Automatic Reference Counting (ARC). It targets C to deliver predictable, high-performance memory management without a garbage collector, while keeping the syntax clean and its core control flow expression-oriented.

This repository contains the compiler, standard library, formatter, language documentation, and sample projects.

Status: Koral is in an experimental stage and is not yet production-ready.

Reference note:

• README.md is a high-level overview, not the canonical grammar document.
• For syntax-sensitive details, use docs/grammar.bnf and current compiler behavior as the source of truth.
• If this README disagrees with the compiler, prefer compiler behavior and update the README.

The Core Idea: ARC + Escape Analysis

Most compiled languages make you choose: either you get high-level ergonomics with a tracing garbage collector, or you get manual control with verbose syntax. Koral offers a middle ground:

1. Escape Analysis First: Every allocation is analyzed at compile time. If the compiler can prove that an object does not escape its current scope, it is allocated on the stack. Stack allocation is practically free and completely bypasses ARC overhead.
2. ARC for the Rest: If an object does escape, it is allocated on the heap and managed via Automatic Reference Counting. This provides predictable, pause-free performance.

Because Koral compiles to C, stack allocations become standard C local variables. The backend compiler can heavily optimize them, often keeping them entirely in CPU registers and optimizing away reference counting operations for local data.

// The compiler sees this doesn't escape. 
// It's allocated on the stack. No ARC overhead.
let local_point = Point(1, 2)

// box(...) creates an owned mutable reference on the heap.
let heap_point = box(Point(3, 4))

// The 'ref' keyword borrows from an existing lvalue.
// Result mutability depends on the source: let mut → mut ref, let → ref.
let mut local_point2 = Point(3, 4)
let heap_point_ref = local_point2.ref  // mut ref (from let mut)

// Bumping the refcount, no deep copy
let shared_point = heap_point 

Language Highlights

• No GC, No Manual free: Automatic memory management based on reference counting and escape analysis.
• Expression-Oriented Control Flow: if, when, and blocks produce values; while and for keep the same surface style but remain statement-only.
• Zero-Cost Abstractions: Generics with trait constraints and monomorphization.
• Algebraic Data Types: Structs and enums with exhaustive pattern matching.
• C Interop: Foreign function interface (FFI) and a C backend for broad platform compatibility.

Syntax Quick Tour

Expression-oriented core control flow

let sign = if x > 0 then 1 else if x < 0 then -1 else 0

let label = when status in {
    .Active then "running",
    .Paused(reason) then "paused: " + reason,
    .Stopped then "done",
}

Blocks are also expressions, so branch bodies can stay local instead of forcing helper functions. When a branch body uses a block, that block still defaults to Void; use yield to produce the enclosing if or when expression's value from inside the block.

let label = if score >= 90 then {
    if score == 100 then yield "perfect"
    yield "A"
} else {
    yield "other"
}

while and for intentionally keep the same ... then ... surface shape, but they are statements rather than value-producing expressions.

Pattern matching built into if and while

Rules:

• is may destructure directly inside if and while conditions.
• Bound names from an earlier is clause remain visible to later and clauses.
• Condition chains evaluate left-to-right with normal short-circuit behavior.

if config.get("port") is .Some(v) then start_server(v)

while iter.next() is .Some(item) then process(item)

You can chain multiple condition clauses with and. Each clause runs only if previous clauses succeed, and bindings from earlier is clauses are visible to later clauses.

if load() is .Some(a) and parse(a) is .Ok(b) and b.is_valid() then use(b)

while source.next() is .Some(raw) and decode(raw) is .Ok(msg) then handle(msg)

Pattern combinators: or, and, not

when temperature in {
    > 0 and < 100 then "liquid",
    <= 0 then "solid",
    >= 100 then "gas",
}

or else / and then / or return — Error flow as keywords

let port = config.get("port") or else 8080

let name = user and then it.profile and then it.display_name or else "anonymous"

let read_config(path String) Result[Config] = {
    let text = read_text_file(path) or return
    let parsed = parse_json(text) or return
    return .Ok(parsed)
}

Generics

let nums = List[Int].new()
let scores = Dict[String, Int].new()
let max[T Ord](a T, b T) T = if a > b then a else b

Traits and given blocks

trait Greet {
    greet(self ref) String
}

type Bot(name String)

given Bot as Greet {
    greet(self ref) String = "beep boop, I'm " + self.name
}

let g ref Greet = box(Bot("K-9"))  // trait object

Algebraic data types with implicit member syntax

Rules:

• .Member(...) requires an expected type from context.
• It may construct enum cases or call static methods.
• If the expected type is not known, the expression is rejected.

type Result[T Any] {
    Ok(value T),
    Error(error ref Error),
}

let parse_int(s String) Result[Int] =
    if s == "42" then .Ok(42) else .Error(box("bad input"))

Lazy streams

let result = list.iterator()
    .filter((x) -> x > 0)
    .map((x) -> x * 2)
    .take(10)
    .fold(0, (acc, x) -> acc + x)

Language Capabilities

Type System

• Primitive types: Bool, Int, UInt, Int8–Int64, UInt8–UInt64, Float32, Float64, Never
• Structs (product types): type Point(x Int, y Int)
• Enums (sum types / tagged enums): type Shape { Circle(r Float64), Rectangle(w Float64, h Float64) }
• Type aliases: type Name = TargetType
• Generic types and functions: Type[T], func[T Constraint](...)
• Function types: Func[Int, Int, Int] — (Int, Int) -> Int
• Reference types: ref (read-only), mut ref (mutable), ptr (read-only), mut ptr (mutable), weakref (read-only weak), mut weakref (mutable weak)

Control Flow

• if / then / else expressions (with pattern matching via is)
• while statements (with pattern matching via is)
• for statements over any Iterable
• when expressions for exhaustive pattern matching
• finally for deterministic cleanup
• break, continue, return
• yield inside if / when branch bodies for branch values and early branch exit

Pattern Matching

• Wildcard (_), literal, variable binding, comparison (> n, <= n)
• Struct/Pair/Enum destructuring (including nested)
• Logical patterns: or, and, not

Traits and Generics

• Trait definitions with inheritance: trait Ord Eq { ... }
• Generic trait declarations use postfix type parameters: trait Iterator[T Any] { ... }
• Implementations via given blocks
• Trait objects for runtime polymorphism: ref Greet, mut ref Greet
• Operator overloading through algebraic traits (Add, Sub, Mul, Div, Index, etc.)

Functions and Lambdas

• Top-level and generic functions
• Named parameters: let connect(host: String, port: Int) = ... called as connect(host: "localhost", port: 8080)
• Lambda expressions: (x Int) Int -> x * 2
• Closures with captured variables
• Literals: strings use "..."; rune literals use '...' (default Rune, can infer to UInt8 in explicit byte context)
• Duration suffix literals: 10s, 250ms, 30min, 2h, 150us, 42ns
• Pair literal: (a, b) (equivalent to Pair(a, b))
• Pair destructuring: let (a, b) = pair (binds Pair fields to separate variables)
• Collection literals:
  • List: [1, 2, 3] (defaults to List[T] when no explicit type context exists)
  • Set: let s Set[Int] = [1, 2, 3]
  • Dict: ["k": 1, "v": 2]
  • Empty literal [] requires explicit type context (e.g. let xs List[Int] = [])
• String interpolation: "value = \(x)"
• Multiline string literals: """...""" with Swift-style indentation stripping

Memory Management

• Automatic reference counting with copy-on-write semantics
• Escape analysis for stack vs. heap allocation decisions
• Weak references (weakref / mut weakref) for breaking reference cycles
• finally for deterministic resource cleanup

Reference creation rules:

• .ref result type depends on the source's mutability: let mut binding → mut ref T, let binding → ref T, mutable path → mut ref T.
• mut ref T implicitly converts to ref T (widening). The reverse is not allowed.
• .ref on rvalues is rejected by the compiler.
• Method/subscript receiver adjustment is a special case: a call whose receiver is declared as self ref may use an rvalue receiver. The compiler materializes a stable temporary for the duration of that call.
• This receiver-only rule does not make expr.ref on rvalues legal.
• self mut ref still requires a writable lvalue receiver; rvalues are rejected.
• Calling a self ref method on an rvalue can introduce hidden retain/allocation cost due to temporary materialization.
• Trait objects follow the same mutability split as ordinary refs: ref Trait can call only self ref requirements, while mut ref Trait can call both self mut ref and self ref requirements.
• ref T is read-only: .val read only. mut ref T supports .val read and .val = expr assignment.
• ptr T is read-only: .val read only. mut ptr T supports .val read, .val = expr, and p[i] = expr.
• Use box(expr) for owned heap references from literals/temporaries — returns mut ref T.

Weak reference rules:

• .weakref on a ref T produces weakref T; on a mut ref T produces mut weakref T. It is only valid on ref types.
• .to_ref() on a weakref T returns Option[ref T]; on a mut weakref T returns Option[mut ref T].
• mut weakref T implicitly converts to weakref T (widening).

let strong mut ref Int = box(42)
let weak mut weakref Int = strong.weakref   // mut ref → mut weakref

when weak.to_ref() in {
    .Some(r) then println(r.val),
    .None    then println("expired"),
}

Module System

Module rules summary:

• using "path" merges another file into the current module scope.

• using module::path { Symbol, Other as Alias } imports explicit public symbols from another module.

• using module::path { .. } imports all public symbols from that module, and .. must be the only item.

• Entry file basenames must match [a-z][a-z0-9_]*.

• File merge (using "file_name" / using "./helpers" / using "../shared/format") is resolved relative to the current file directory

• Modules are declared in koral.json; std modules are declared in std/koral.json

• Imports are file-local bindings and never re-export automatically

• Removed forms: using "file" as Name, using Super..., bare-path imports such as using Std.Io, alias imports such as using Std.Io as Io, batch imports such as using Std.Io.*, and public/protected/private using ...

• Access control: public, protected (default), private

• Direct Type(...) construction requires constructor field visibility at call site; non-public fields should be initialized via public factory methods

• Module entry file basename must match [a-z][a-z0-9_]*

• String in using "file" is the literal file name (no case conversion); file is resolved relative to the current file's directory

• Type aliases must start with an uppercase letter (type Name = ...)

FFI

• foreign let for binding C functions
• foreign type for opaque or layout-compatible C types
• Native library linking is configured in koral.json / std/koral.json via links, not via source syntax

Standard Library Overview

The standard library (std/) ships with the compiler and is loaded automatically unless --no-std is specified.

Commonly used pieces:

• Core types: Int, Float64, String, Rune, Bool
• Collections: List[T], Dict[K, V], Set[T]
• Error flow: Option[T], Result[T], or else, and then, or return
• Runtime and system modules: Io, Os, Proc, Time, Async, Sync, Net
• Utility modules: Math, Rand, Text, Container

Minimal examples:

let nums List[Int] = [1, 2, 3]
let scores Dict[String, Int] = ["alice": 10, "bob": 8]

let port = Option[Int].Some(8080) or else 80
let doubled = Option[Int].Some(21) and then it * 2

let parse_port(text String) Result[Int] = {
    let port = parse_int(text) or return
    return .Ok(port)
}

let ok = Result[Int].Ok(42)
let err = Result[Int].Error(box("failed"))

For full module-by-module API documentation, see docs/std/.

Repository Layout

• compiler/ — Swift compiler project (koralc, KoralCompiler)
• bootstrap/ — self-hosting compiler implementation and bootstrap tests
• std/ — standard library modules and runtime C files
• docs/ — language and developer documentation
• toolchain/fmt/ — formatter implementation and tests
• samples/ — example projects
• test/ — ad-hoc language playground and cases

Prerequisites

• Swift toolchain (for building koralc)
• A C compiler in PATH (clang recommended)

On Windows, ensure clang.exe is available from terminal:

clang --version

Build from Source

cd compiler
swift build -c debug

Run the Compiler

# Build a manifest-declared target module
swift run koralc build --package-config path/to/koral.json --target-module app::main

# Build and run
swift run koralc run --package-config path/to/koral.json --target-module app::main

# Emit C only
swift run koralc emit-c --package-config path/to/koral.json --target-module app::main -o out

Common Options

• -o, --output <dir>: output directory
• --package-config <path>: build from a package manifest
• --target-module <name>: choose the manifest target module, for example app::main
• --deps-root <path>: dependency root directory for manifest-driven builds
• --std-config <path>: explicit std manifest path
• --no-std: compile without loading the std manifest
• -m / -m=<N>: print escape analysis diagnostics (Go-style; -m -m or higher level currently same output)

Test

The only supported test entry lives under tests/.

cd compiler
swift build -c debug
cd ..
compiler/.build/debug/koralc build --package-config tests/compiler-runner/koral.json --target-module compiler_runner -o bin/compiler-test-runner
./bin/compiler-test-runner/compiler_runner.exe --compiler swift --swift-koralc compiler/.build/debug/koralc.exe -j=8

See tests/README.md for bootstrap mode, custom compiler mode, and other runner flags.

Documentation

• Language Guide (English)
• 语言文档（中文）
• Grammar (BNF)
• Compiler Developer Guide

Standard Library Resolution (KORAL_HOME)

If koralc cannot find std/std.koral / std/koral.json due to your working directory, set KORAL_HOME to the repository root.

# macOS / Linux
export KORAL_HOME=/path/to/koral

# Windows PowerShell
$env:KORAL_HOME = "C:\path\to\koral"

Contributing

Issues and pull requests are welcome. If you change parser/type-checker/codegen behavior, please add or update integration test cases under tests/compiler-cases/.