条件判断

if..elif..else 条件分支

if condition {
    # ...
}

可以使用 back 关键字中断 if，返回上一作用域

if condition {
    back
    # here will not be executed
}

back 关键字可以携带返回值：

<variable-name> := if condition {
    back <backed-value>
    # here will not be executed
}
# now <variable-name> is defined with <backed-value>

if condition {
    # ...
} else {
    # ...
}

if condition {
    # ...
} elif condition {
    # ...
} else {
    # ...
}

match 条件分支

match <matched-value> {
    <expected-value1>: {
        # do something for <expected-value1>
        # no need to break
    }
    <expected-value2>: {
        # do something for <expected-value2>
    }
    _: {
        # default resolver
    }
}

match 表达式中也可以使用 back 关键字：

<variable-name> = match <matched-value> {
    <expected-value1>: {
        back <backed-value1>
        # here will not be executed
    }
    <expected-value2>: {
        back <backed-value2>
        # here will not be executed
    }
}

match 表达式中的 if..elif..else 语法糖：

match <matched-value> {
    <expected-value>: {
        if condition1 {
            # do something
        } elif condition2 {
            # do something
        } else {
            # do something
        }
    }
}

等效于：

match <matched-value> {
    <expected-value> if condition1 {
        # do something
    } elif condition2 {
        # do something
    } else {
        # do something
    }
}

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