可选

Marks a named declaration slot (parameter or 类 field) as voluntary — the caller or provider may omit the value.

Aliases: optional, voluntary

Syntax: <type> <name> 可选 [= default | 兜底 default]

Category#

declaration

Related#

• 兜底
• 空
• ∪
• 函数
• 类

Examples#

radix/corpus/sponte/sponte.fab (canonical · keyword)#

Marks a named declaration slot (parameter or 类 field) as voluntary — the caller or provider may omit the value.

# =============================================================================
# 可选 — Marks a named declaration slot (parameter or 类 field) as voluntary — the caller or provider may omit the value.
# =============================================================================
#
# What this teaches:
#   • Voluntary parameter syntax — using `可选` to mark function parameters or 类 fields as optional
#   • Default values — combining `可选` with `vel` to provide fallbacks when the value is omitted
#
# Common mistakes:
#   • confusing 可选 (optional parameter with a default) with ∪ 空类型 (nullable type) — 可选 marks omission, ∪ 空类型 marks absence
#
# See also: vel, 空类型, ∪, 函数, 类
# =============================================================================



# 可选 — optional parameter / field marker
#
# GRAMMAR:
#   optionalMarker :← '可选'
#
# EXPECTED OUTPUT:
#   No 入口 — declaration or test-runner surface only.
#
# BACKEND:
#   declaration-only optional slots (whitelist: 可选/sponte.fab). Cross-ref: optionalis/optionalis.fab.
#

fn greet(string 名称, string titulus optional) → string {
    return 名称
}

fn paginate(int pagina optional coalesce 1) → string {
    return "page"
}

class User {
    var string name
    var string email optional
}