With To Case Refactor

Overview

Elixir's with construct is designed for chaining multiple pattern-matching steps. When only one <- clause is present alongside an else branch, with adds no value over a plain case. Credo flags this as:

[R] → `with` contains only one <- clause and an `else` branch, consider using `case` instead

Always prefer case in this situation.

When to Use

• When a with expression has exactly one <- clause and one or more else arms.
• When refactoring code to address the Credo Credo.Check.Refactor.WithClauses warning.

Anti-Pattern (Avoid)

# ❌ BAD: single-clause with/else — should be a case
with {:ok, response} <- json_rpc(params, opts) do
  process(response)
else
  {:error, reason} ->
    Logger.error("RPC failed: #{inspect(reason)}")
    :error
end

# ❌ BAD: single-clause with/else wrapping a nested case
with {:ok, response} <- json_rpc(params, opts) do
  case parse(response) do
    {:ok, value} -> value
    _            -> :error
  end
else
  {:error, reason} ->
    Logger.error("RPC failed: #{inspect(reason)}")
    :error
end

Best Practice (Use Instead)

# ✅ GOOD: flat case replaces with/else
case json_rpc(params, opts) do
  {:ok, response} ->
    process(response)

  {:error, reason} ->
    Logger.error("RPC failed: #{inspect(reason)}")
    :error
end

# ✅ GOOD: nested case is fine when the outer with is replaced
case json_rpc(params, opts) do
  {:ok, response} ->
    case parse(response) do
      {:ok, value} -> value
      _            -> :error
    end

  {:error, reason} ->
    Logger.error("RPC failed: #{inspect(reason)}")
    :error
end

Transformation Rules

1. Move the expression on the right-hand side of <- to become the subject of case.
2. Turn the left-hand side of <- into the matching branch of case.
3. Move the body of the with block as the body of that case branch.
4. Move each arm of the else block as additional case branches.
5. Remove the with/else/end wrapper.

Real-World Example (from this codebase)

Before

with {:ok, response} <-
       params
       |> Map.merge(%{id: 0})
       |> Nonce.request()
       |> json_rpc(json_rpc_named_arguments) do
  case Nonce.from_response(%{id: 0, result: response}, id_to_params) do
    {:ok, %{nonce: 0}} -> handle_zero_nonce(...)
    {:ok, %{nonce: nonce}} when nonce > 0 -> handle_nonzero_nonce(...)
    _ -> retry(...)
  end
else
  {:error, reason} ->
    Logger.error("Error: #{inspect(reason)}")
    retry(...)
end

After

case params
     |> Map.merge(%{id: 0})
     |> Nonce.request()
     |> json_rpc(json_rpc_named_arguments) do
  {:ok, response} ->
    case Nonce.from_response(%{id: 0, result: response}, id_to_params) do
      {:ok, %{nonce: 0}} -> handle_zero_nonce(...)
      {:ok, %{nonce: nonce}} when nonce > 0 -> handle_nonzero_nonce(...)
      _ -> retry(...)
    end

  {:error, reason} ->
    Logger.error("Error: #{inspect(reason)}")
    retry(...)
end

Notes

• If the with has two or more <- clauses, keep it as with; this refactor only applies to the single-clause case.
• If there is no else branch at all, with is also acceptable for a single clause — but a case is still clearer and preferred.
• After refactoring, run mix format to ensure correct indentation.