haskell - 冲突教程示例中 'pure' 关键字的用途是什么？

Question

在Clash官方网站上，有以下示例:

>>> sampleN @System 4 (register 0 (pure (8 :: Signed 8)))

我知道什么是纯函数，但为什么这里有 this 关键字？如果我删除它，我会收到错误:

Clash.Prelude> sampleN @System 4 (register 0 (8 :: Signed 8))

<interactive>:2:32: error:
    * Couldn't match expected type `Signal System a'
                  with actual type `Signed 8'
    * In the second argument of `register', namely `(8 :: Signed 8)'
      In the third argument of `sampleN', namely
        `(register 0 (8 :: Signed 8))'
      In the expression: sampleN @System 4 (register 0 (8 :: Signed 8))
    * Relevant bindings include it :: [a] (bound at <interactive>:2:1)

有什么线索吗？

Answer 1

Signal 有一个 pure 所属的 Applicative 实例。 pure::a -> Signal dom a 将 Signed 8 类型的值提升为 Signal dom (Signed 8)。它与该上下文中的Signal System a 相结合，形成Signal System (Signed 8)。

使用的引用:http://hackage.haskell.org/package/clash-prelude-1.0.0/docs/Clash-Signal.html