Elixir错误处理管道选项

Question

The following code is an invalid changeset which errors, however it took me a long time to find the cause because the error message was not originally being matched and logged.

I added a case statement to the end of the pipe, is this the best way to pickup errors in pipes?

    User.changeset(%User{}, %{username: "username_is_test", password: "test", password_confirmation: "test", email: "test@test.com"})
    |> Repo.insert
    |> case  do
        {:ok, result} -> IO.puts("result")
        {:error, error} -> IO.inspect error
    end

Answer 1

Pipelines and error tuples don't work very well together. You can handle an error at the end of a pipeline with a case as you have, but it only works at the last stage.

For operations returning error tuples, I prefer to use with / else syntax:

with changeset <- User.changeset(%User{}, %{username: "username_is_test", password: "test", password_confirmation: "test", email: "test@test.com"})
     {:ok, result} <- Repo.insert(changeset) do
  IO.puts("result")
else
  {:error, error} -> IO.inspect error
end

You can add as many failable operations as required in the with block, and handle all the error cases with pattern matching in the else block.

https://hexdocs.pm/elixir/Kernel.SpecialForms.html#with/1

Answer 2

You have a couple options. If you don't want to explicitly handle the error conditions, you should use Repo.insert! instead. At least this will raise an exception if the changeset is not valid.

Otherwise, you should be using a case handle handling the {:error, changeset} by checking the changeset.action in your template.

In more complicated pipelines that can error part way through, I've started using the with special form.

with result when not is_nil(result) <- fun1, 
     {:ok, result} <- fun2(result), 
     {:ok, result} <- fun3(result) do
  success_handling(result)
else
  nil -> # handle first failure
  {:error, error} -> # handle other errors
  _ -> # catch all failure
end