Reference

This section documents Yulang's syntax and semantics in detail. The guide is where you go to learn the language; this is where you go to look something up.

Program structure

A Yulang program is a sequence of top-level statements. Statements are declarations (my, our, pub, struct, enum, act, role, impl, error, cast, type, use, mod) and bare expressions whose values are shown by the CLI and playground.

Visibility

Keyword	Meaning
my	Private binding, local or top-level
our	Public — appears in the enclosing module's companion
pub	Exported binding; also shown in the playground's type pane

Comments

// Line comment.

/* Block comment. */

-- Single-line doc comment (not a line comment).

---
Multi-line doc block.
May contain markdown and ```yulang fences.
---

// and /* ... */ are ordinary comments. -- and --- blocks are documentation comments — they appear in syntax trees and tooling, so they are not interchangeable with //.

By topic

Surface syntax

• Syntax Style — when and how to drop parentheses
• Application & Operators — how f x, f(x), f: x, and f.method differ
• Operator Declarations — infix, prefix, suffix, precedence

Values and types

• Values & Types — the type universe
• Strings — string layout, escapes, interpolation
• Patterns — every pattern form
• Structs & Roles — struct, with:, role, impl
• Casts — cast(x: A): B, where the compiler inserts them

Computation

• Functions — declarations, currying, named arguments
• Control Flow — for, sub:, case, references
• Effects — act, catch, handler shapes
• Errors — error, fail, from, up, wrap

Style

• Idioms — what idiomatic Yulang looks like

Theory

• Type Inference Theory — what the inferer is doing under the hood

Standard library

• Core, list, str
• opt, result
• fs, undet