在clash tutorial示例中，'pure‘关键字的用途是什么？

Question

在Clash官网中，有以下示例：

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

我知道什么是纯函数，但为什么要在这里使用这个关键字？如果我删除它，我会得到一个错误：

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，从而产生了Signal System (Signed 8)。

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