with_clauses.ex

defmodule Credo.Check.Refactor.WithClauses do
  use Credo.Check,
    base_priority: :high,
    explanations: [
      check: ~S"""
      `with` statements are useful when you need to chain a sequence
      of pattern matches, stopping at the first one that fails.

      However, there are a few cases where a `with` can be used incorrectly.

      ## Starting or ending with non-pattern-matching clauses

      If the `with` starts or ends with clauses that are not `<-` clauses,
      then those clauses should be moved either outside of the `with` (if
      they're the first ones) or inside the body of the `with` (if they're the
      last ones). For example look at this code:

          with ref = make_ref(),
               {:ok, user} <- User.create(ref),
               Logger.debug("Created user: #{inspect(user)}") do
            user
          end

      This `with` should be refactored like this:

          ref = make_ref()

          with {:ok, user} <- User.create(ref) do
            Logger.debug("Created user: #{inspect(user)}")
            user
          end

      # Using only one pattern matching clause with `else`

      If the `with` has a single pattern matching clause and no `else`
      branch, it means that if the clause doesn't match than the whole
      `with` will return the value of that clause. However, if that
      `with` has also an `else` clause, then you're using `with` exactly
      like a `case` and a `case` should be used instead. Take this code:

          with {:ok, user} <- User.create(make_ref()) do
            user
          else
            {:error, :db_down} ->
              raise "DB is down!"

            {:error, reason} ->
              raise "error: #{inspect(reason)}"
          end

      It can be rewritten with a clearer use of `case`:

          case User.create(make_ref()) do
            {:ok, user} ->
              user

            {:error, :db_down} ->
              raise "DB is down!"

            {:error, reason} ->
              raise "error: #{inspect(reason)}"
          end

      Like all `Readability` issues, this one is not a technical concern.
      But you can improve the odds of others reading and liking your code by making
      it easier to follow.
      """,
      params: []
    ]

  alias Credo.Code

  @message_only_one_pattern_clause "`with` contains only one <- clause and an `else` " <>
                                     "branch, use a `case` instead"
  @message_first_clause_not_pattern "`with` doesn't start with a <- clause, " <>
                                      "move the non-pattern <- clauses outside of the `with`"
  @message_last_clause_not_pattern "`with` doesn't end with a <- clause, move " <>
                                     "the non-pattern <- clauses inside the body of the `with`"

  @doc false
  @impl true
  def run(%SourceFile{} = source_file, params) do
    issue_meta = IssueMeta.for(source_file, params)
    Code.prewalk(source_file, &traverse(&1, &2, issue_meta))
  end

  # TODO: consider for experimental check front-loader (ast)
  defp traverse({:with, meta, [_, _ | _] = clauses_and_body} = ast, issues, issue_meta)
       when is_list(clauses_and_body) do
    # If clauses_and_body is a list with at least two elements in it, we think
    # this might be a call to the special form "with". To be sure of that,
    # we get the last element of clauses_and_body and check that it's a keyword
    # list with a :do key in it (the body).

    # We can hard-match on [maybe_body] here since we know that clauses_and_body
    # has at least two elements.
    {maybe_clauses, [maybe_body]} = Enum.split(clauses_and_body, -1)

    if Keyword.keyword?(maybe_body) and Keyword.has_key?(maybe_body, :do) do
      {ast, issues_for_with(maybe_clauses, maybe_body, meta[:line], issue_meta) ++ issues}
    else
      {ast, issues}
    end
  end

  defp traverse(ast, issues, _issue_meta) do
    {ast, issues}
  end

  defp issues_for_with(clauses, body, line, issue_meta) do
    issue_if_one_pattern_clause_with_else(clauses, body, line, issue_meta) ++
      issue_if_not_starting_with_pattern_clause(clauses, line, issue_meta) ++
      issue_if_not_ending_with_pattern_clause(clauses, line, issue_meta)
  end

  defp issue_if_one_pattern_clause_with_else(clauses, body, line, issue_meta) do
    pattern_clauses_count = Enum.count(clauses, &match?({:<-, _, _}, &1))

    if pattern_clauses_count <= 1 and Keyword.has_key?(body, :else) do
      [format_issue(issue_meta, message: @message_only_one_pattern_clause, line_no: line)]
    else
      []
    end
  end

  defp issue_if_not_starting_with_pattern_clause(
         [{:<-, _meta, _args} | _rest],
         _line,
         _issue_meta
       ) do
    []
  end

  defp issue_if_not_starting_with_pattern_clause(_clauses, line, issue_meta) do
    [format_issue(issue_meta, message: @message_first_clause_not_pattern, line_no: line)]
  end

  defp issue_if_not_ending_with_pattern_clause(clauses, line, issue_meta) do
    if length(clauses) > 1 and not match?({:<-, _, _}, Enum.at(clauses, -1)) do
      [format_issue(issue_meta, message: @message_last_clause_not_pattern, line_no: line)]
    else
      []
    end
  end
end