Pine Script v5中if块与else块为何需强制返回类型匹配？

我在研读TradingView Pine Script官方文档的数组章节示例代码时，注意到代码里有一行注释// Force the return type of this 'if' block to be the same as that of the next block.，想知道为什么必须这么做？

示例代码 #

//@version=5
indicator("Lows from new highs", "", true)
var lows = array.new<float>(0)
flushLows = false

// Remove last element from the stack when `_cond` is true.
array_pop(id, cond) => cond and array.size(id) > 0 ? array.pop(id) : float(na)

if ta.rising(high, 1)
    // Rising highs; push a new low on the stack.
    lows.push(low)
    // Force the return type of this `if` block to be the same as that of the next block.
    bool(na)
else if lows.size() >= 4 or low < array.min(lows)
    // We have at least 4 lows or price has breached the lowest low;
    // sort lows and set flag indicating we will plot and flush the levels.
    array.sort(lows, order.ascending)
    flushLows := true

// If needed, plot and flush lows.
lowLevel = array_pop(lows, flushLows)
plot(lowLevel, "Low 1", low > lowLevel ? color.silver : color.purple, 2, plot.style_linebr)
lowLevel := array_pop(lows, flushLows)
plot(lowLevel, "Low 2", low > lowLevel ? color.silver : color.purple, 3, plot.style_linebr)
lowLevel := array_pop(lows, flushLows)
plot(lowLevel, "Low 3", low > lowLevel ? color.silver : color.purple, 4, plot.style_linebr)
lowLevel := array_pop(lows, flushLows)
plot(lowLevel, "Low 4", low > lowLevel ? color.silver : color.purple, 5, plot.style_linebr)

if flushLows
    // Clear remaining levels after the last 4 have been plotted.
    lows.clear()

这是因为Pine Script对if-else分支的返回类型有严格要求：同一个if-else结构下的所有分支，必须返回相同类型的值，否则会触发编译错误。

拆解代码里的两个分支来看：

• 第一个if分支执行lows.push(low)，这是数组添加元素的操作，返回的是void（无返回值）；
• 第二个else if分支最后执行flushLows := true，这是赋值操作，返回的是bool类型的结果。

两者返回类型不匹配，编译器会直接报错。加上bool(na)后，第一个分支就会返回一个bool类型的空值（na是Pine Script中各类型通用的空值），让两个分支的返回类型统一，从而通过编译。

内容的提问来源于stack exchange，提问作者tino