NEPLg2 Tutorial - 05 - Option（値が有る/無い）
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Option（値あたいが有ある/無ない）

Option<T> は「値が有る (Some) / 値が無い (None)」を表す型です。

NEPL では core/option に Option と基本操作が入っています。

Some / None と match

TEST
#entry main
#indent 4
#target std

#import "core/option" as *
#import "core/result" as *
#import "std/test" as *

fn main <()*>i32> ():
    let a <Option<i32>> some<i32> 10
    let b <Option<i32>> none<i32>
    let mut checks <Vec<Result<(),str>>> checks_new

    match a:
        Option::Some v:
            set checks checks_push checks check_eq_i32 10 v
        Option::None:
            set checks checks_push checks Result<(),str>::Err "a was None"

    match b:
        Option::Some v:
            set checks checks_push checks Result<(),str>::Err "b was Some"
        Option::None:
            set checks checks_push checks Result<(),str>::Ok ()
    let shown <Vec<Result<(),str>>> checks_print_report checks
    checks_exit_code shown
ret0

option_unwrap_or で既定値を使う

unwrap は None で失敗するため、入門では option_unwrap_or を推奨します。

TEST
#entry main
#indent 4
#target std

#import "core/option" as *
#import "core/result" as *
#import "std/test" as *

fn main <()*>i32> ():
    let some_v <Option<i32>> some<i32> 77
    let none_v <Option<i32>> none<i32>
    let checks <Vec<Result<(),str>>>:
        checks_new
        |> checks_push check_eq_i32 77 option_unwrap_or<i32> some_v 0
        |> checks_push check_eq_i32 123 option_unwrap_or<i32> none_v 123
    let shown <Vec<Result<(),str>>> checks_print_report checks
    checks_exit_code shown
ret0