将函数定义更改为无点样式

Question

我在编写教授给我的这段代码时遇到了问题：

Write a function called digit7 which takes an Int and returns a Bool saying whether or not 7 is one of the digits. (Hint: use show to turn the number into a list of characters.) Use digit7 to create a function of no parameters called square7 which returns the smallest number whose square contains a 7 as a digit.

我的代码是：

digit7 l = elem '7' (show l)

这可行，但是，我需要以无点样式编写的代码。我也无法弄清楚 square7 函数。

Answer 1

对于digit7函数，您可以使用函数组合将您的定义转换为无点样式：

digit7 = (elem '7') . (show)

这是因为：

   digit7 l
-> ((elem '7') . (show)) l     By substitution
-> (elem '7') ((show) l)       By definition of (.)
-> elem '7' (show l)           By operator precedence

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至于square7功能，我推荐使用dropWhile和head。