elixir - 异常名称的模式匹配

Question

我正在探索 Elixir 并且有疑问。假设我有这样的代码:

defmodule Drop2 do
    def fall_velocity(planemo, distance) do
        gravity = case planemo do
            :earth -> 9.8
            :moon -> 1.6
            :mars -> 3.71
        end
        :math.sqrt(2 * gravity * distance)
    end
end

我将一个负数传递为 distance使功能失败:

iex(8)> Drop2.fall_velocity(:earth, -20)
** (ArithmeticError) bad argument in arithmetic expression
    (stdlib) :math.sqrt(-392.0)
             drop2.ex:9: Drop2.fall_velocity/2

这可以通过添加异常处理来改善:

defmodule Drop2 do
    def fall_velocity(planemo, distance) do
        try do
            gravity = case planemo do
                :earth -> 9.8
                :moon -> 1.6
                :mars -> 3.71
            end
            :math.sqrt(2 * gravity * distance)
        rescue
            ArithmeticError -> {:error, "Distance must be non-negative number"}
            CaseClauseError -> {:error, "Unknown planemo: #{planemo}"}
        end
    end
end

现在我们有:

iex(9)> Drop2.fall_velocity(:earth, -20)
{:error, "Distance must be non-negative number"}

很好，但我看不到模式 ArithmeticError火柴。在前面的示例中，生成的异常主要是文本， (ArithmeticError)括在括号中。这不是 Elixir 中常见的模式匹配。这是怎么回事？

Answer 1

In the previous example, the exception generated was mostly text, with (ArithmeticError) wrapped in parenthesis.

异常(exception)不是文本，只是 iex正在打印异常的文本表示，而不是确切的值 ( source)。这是确切的异常(exception):

iex(1)> try do
...(1)>   :math.sqrt(-1)
...(1)> rescue
...(1)>   e -> e
...(1)> end
%ArithmeticError{message: "bad argument in arithmetic expression"}

这是一个结构 ArithmeticError与 message字段，如 Kernel.SpecialForms.try/1 的文档中所述:

Rescue clauses

Besides relying on pattern matching, rescue clauses provides some conveniences around exceptions that allows one to rescue an exception by its name.