With block in Plug server 抛出 MatchError 而不是使用 else 块

Question

以前，我会在 Elixir Plug 服务器中使用 with 块来解析来自请求的参数，如果失败，我会 return 一个理智的响应。但是，这似乎不再有效（Elixir 1.11）。谁能指出发生了什么事？

这是一个极简主义的 Plug 服务器，它会显示问题

defmodule MatchTest.Router do
  use Plug.Router

  plug(:match)
  plug(:dispatch)

  get "/" do
    other_with =
      with {:ok, _} <- Map.fetch(%{}, "test") do
        :ok
      else
        :error -> :error
      end

    with conn <- Plug.Conn.fetch_query_params(conn),
         {:ok, a} = Map.fetch(conn.query_params, "a") do
      Plug.Conn.send_resp(conn, 200, "a = #{a}; other_with = #{other_with}")
    else
      :error -> Plug.Conn.send_resp(conn, 400, "Incorrect Parameters")
    end
  end

  match _ do
    Plug.Conn.send_resp(conn, 404, "Not Found")
  end
end

defmodule MatchTest do
  use Application

  def start(_type, _args) do
    Supervisor.start_link(
      [{Plug.Cowboy, scheme: :http, plug: MatchTest.Router, options: [port: 4000]}],
      strategy: :one_for_one,
      name: RateLimitedServer.Supervisor
    )
  end
end

正如预期的那样，当我在 GET 请求中包含 a 参数时一切正常：

ddrexler@Drexbook-Pro:temp|$ http -v get localhost:4000/ a=="test"
GET /?a=test HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:4000
User-Agent: HTTPie/2.3.0



HTTP/1.1 200 OK
cache-control: max-age=0, private, must-revalidate
content-length: 28
date: Mon, 15 Feb 2021 22:40:14 GMT
server: Cowboy

a = test; other_with = error

特别是，第一个 with 子句按预期工作，当我们尝试从空映射 Map.fetch() 时，它跳转到 else 子句（您可以看到这是因为字符串“other_with = error”).

但是，当我尝试排除参数时，我得到了 500:

ddrexler@Drexbook-Pro:~|$ http -v get localhost:4000/
GET / HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:4000
User-Agent: HTTPie/2.3.0



HTTP/1.1 500 Internal Server Error
content-length: 0

并且服务器收到未捕获的 MatchError：

ddrexler@Drexbook-Pro:match_test|$ mix run --no-halt
Compiling 1 file (.ex)
warning: "else" clauses will never match because all patterns in "with" will always match
  lib/match_test.ex:15


14:40:16.341 [error] #PID<0.350.0> running MatchTest.Router (connection #PID<0.349.0>, stream id 1) terminated
Server: localhost:4000 (http)
Request: GET /
** (exit) an exception was raised:
    ** (MatchError) no match of right hand side value: :error
        (match_test 0.1.0) lib/match_test.ex:16: anonymous fn/2 in MatchTest.Router.do_match/4
        (match_test 0.1.0) lib/plug/router.ex:284: MatchTest.Router.dispatch/2
        (match_test 0.1.0) lib/match_test.ex:1: MatchTest.Router.plug_builder_call/2
        (plug_cowboy 2.4.1) lib/plug/cowboy/handler.ex:12: Plug.Cowboy.Handler.init/2
        (cowboy 2.8.0) /Users/ddrexler/src/elixir/match_test/deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
        (cowboy 2.8.0) /Users/ddrexler/src/elixir/match_test/deps/cowboy/src/cowboy_stream_h.erl:300: :cowboy_stream_h.execute/3
        (cowboy 2.8.0) /Users/ddrexler/src/elixir/match_test/deps/cowboy/src/cowboy_stream_h.erl:291: :cowboy_stream_h.request_process/3
        (stdlib 3.13.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

另请注意警告 "else" clauses will never match because all patterns in "with" will always match - 显然不正确！有一种情况是它们不匹配，因为我得到了 MatchError。 else 块包含一个 :error 选项，应该捕获此结果！

Answer 1

你可能已经从@sbacarob 的评论中算出来了。

您的错误即将到来，因为当没有 a 参数时：{:ok, a} = Map.fetch(conn.query_params, "a") 被评估为 {:ok, a} = :error 并引发 MatchError.

要避免 MatchError，您需要使用特殊的 with-特定运算符 <-。您可以将其视为“软匹配”运算符（其中 = 是“硬匹配”）。软匹配让匹配失败（在 with 中）并下降到 else/end，而硬匹配将引发 MatchError.

With block in Plug server 抛出 MatchError 而不是使用 else 块

With block in Plug server throws MatchError instead of using else block

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