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I'm trying to write a framework for real-time interactive graphics in Haskell. I've been trying to get a handle on things by using Netwire 5, but I don't seem to have a good handle on how things "depend" on one another. For example, the following code should produce val for two seconds before switching to (val + 1), and then continuing on indefinitely.

someWire :: Num a => a -> Wire s e m a a
someWire x = (-->) ((pure x) &&& (periodic 2) >>> until) (someWire (x + 1))

However, this creates some sort of memory leak where my program stalls and just keeps allocating memory until something on my system crashes. Alternatively, this definition

someWire :: Num a => a -> Wire s e m a a
someWire x = (-->) ((pure x) &&& (at 2) >>> until) (someWire (x + 1))

behaves the way that I expect it to: count from val onwards having the value change every two seconds. Can someone please explain this behavior?

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2 回答 2

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关键洞察力是立即periodic产生事件。

因此，当我们从这条线产生一个值时，我们必须将其评估为以下内容：

someWire x
(-->) ((pure x) &&& (periodic 2) >>> until) (someWire (x + 1))
(-->) (pure (x, Event _) >>> until) (someWire (x + 1))
(-->) *inhibition* (someWire (x + 1))
someWire (x + 1)

由于这不是尾递归，因此不允许垃圾收集器清理为线路分配的先前实例，并且我们会耗尽内存（而不是获得无限循环）。

于 2014-05-17T20:15:09.653 回答
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让我们看一下工作版本：

import Control.Wire hiding (until)
import qualified Control.Wire (until) as W

someWire :: Num a => a -> Wire s e m a a
someWire x = (-->) (pure x &&& at 2 >>> W.until) (someWire (x + 1))

-- To test in GHCi: testWire clockSession_ $ someWire 3

Wires 是Arrows，因此如果我们能够遵循箭头组合器的操作，将更容易理解发生了什么。

(&&&) :: Arrow a => a b c -> a b c' -> a b (c, c')

(&&&)构建一个箭头，将其输入（在我们的例子中，可以被认为是驱动事件的心跳）成对，将一个箭头应用于每个组件。在这里，我们用一个总是产生x的箭头和一个返回将在两秒内发生的事件的箭头进行分叉。

(>>>) :: Category cat => cat a b -> cat b c -> cat a c

(>>>)只是箭头的组合（Arrow是 的子类Category），以先到后继的顺序编写。所以我们正在发送pure x &&& at 2to的输出W.until。

W.until :: Monoid e => Wire s e m (a, Event b) a

组合使用W.until将产生与事件配对的值的箭头（例如pure x &&& at 2）转换为在事件发生之前产生值的箭头。一旦发生这种情况，我们使用 . 切换到另一根线(-->)。

现在应该更容易理解为什么不能使用periodic而不是at. at只发生在一个瞬间，而periodic不断发生，因此W.until永远不会终止。（如果您深入研究源代码，您会发现它的W.until作用类似于折叠事件发生，这与直观的解释相符。）

于 2014-05-17T19:04:20.407 回答