Elixir Expert

Expert guidance for writing idiomatic, type-safe Elixir code.

Topics

This skill covers multiple areas of Elixir expertise. Reference the appropriate guide based on the task:

Best Practices

See below for:

•Data access patterns (Access behaviour vs Map.get)
•Error handling and control flow (with, case, error tuples)
•Higher-order functions and composition
•Guards and function expectations
•Testing patterns

Use this when writing or reviewing general Elixir code, handling errors, or working with collections.

Typespecs

See typespecs.md for:

•Writing effective type specifications
•Being specific with types (avoiding any())
•User-defined types and parameterized types
•Behaviours and callbacks
•Dialyzer integration
•Common typespec patterns and pitfalls

Use this when adding type annotations, defining behaviours, preparing code for Dialyzer analysis, or documenting function contracts.

Absinthe Dataloader

See dataloader.md for:

•Solving N+1 query problems in GraphQL
•Setting up Dataloader with Absinthe
•Basic and advanced usage patterns
•Custom batch functions and query customization
•Authorization and context patterns
•KV source for non-Ecto data
•Common gotchas and testing strategies

Use this when building GraphQL APIs with Absinthe, optimizing resolver performance, implementing authorization rules, or working with nested GraphQL queries.

Quick Reference

Common patterns to apply immediately:

Best Practices

Data Access:

elixir

# Use bracket syntax for flexibility
opts[:key]  # instead of Map.get(opts, :key)

Error Handling:

elixir

# Use with for sequential operations
with {:ok, data} <- fetch(),
     {:ok, result} <- process(data) do
  result
end

# Use case for critical error handling
case call_service(id) do
  {:ok, result} -> result
  {:error, error} -> handle_error(error)
end

Type Specifications:

elixir

# Be specific, avoid any()
@spec process(String.t(), keyword()) :: {:ok, result()} | {:error, atom()}

# Document custom types
@typedoc "User configuration options"
@type options :: [timeout: pos_integer(), retry: boolean()]

Elixir Best Practices

Core patterns for idiomatic Elixir code.

Data Access

Use bracket access syntax instead of Map.get/2 or Keyword.get/2:

elixir

# Avoid - locks you into specific data structure
opts = %{foo: :bar}
Map.get(opts, :foo)

# Prefer - works with maps, keywords, and Access behaviour
opts[:foo]

Error Handling and Control Flow

Never pipe side-effecting function results

Side-effecting functions return {:ok, term()} | {:error, term()}. Use with or case to handle results explicitly:

elixir

# Avoid - spreads error handling across functions
def main do
  data
  |> call_service()
  |> parse_response()
  |> handle_result()
end

defp parse_response({:ok, result}), do: Jason.decode(result)
defp parse_response(error), do: error

# Prefer - caller controls error handling
def main do
  with {:ok, response} <- call_service(data),
       {:ok, decoded} <- Jason.decode(response) do
    decoded
  end
end

Rationale: Each function shouldn't need to know how it's called or what order it's composed in. The caller has enough context to decide error handling strategy.

Keep critical error handling in the calling function

When errors are vital to control flow (circuit breakers, fallbacks, caching), use explicit case statements:

elixir

def main(id) do
  case :fuse.check(:service) do
    :ok ->
      case call_service(id) do
        {:ok, result} ->
          :ok = Cache.put(id, result)
          {:ok, result}

        {:error, error} ->
          :fuse.melt(:service)
          {:error, error}
      end

    :blown ->
      case Cache.get(id) do
        nil -> {:error, :service_unavailable}
        cached -> {:ok, cached}
      end
  end
end

This increases function size but makes every control path explicit and readable.

Don't pipe into case statements

Assign intermediate values to variables instead:

elixir

# Avoid
build_post(attrs)
|> store_post()
|> case do
  {:ok, post} -> # ...
  {:error, _} -> # ...
end

# Prefer
changeset = build_post(attrs)

case store_post(changeset) do
  {:ok, post} -> # ...
  {:error, _} -> # ...
end

Avoid else in with blocks

Use else only for truly generic error handling. If you need to handle specific errors differently, use case instead:

elixir

# Avoid - tagging just to differentiate errors
with {:service, {:ok, resp}} <- {:service, call_service(data)},
     {:decode, {:ok, decoded}} <- {:decode, Jason.decode(resp)} do
  :ok
else
  {:service, {:error, error}} -> # ...
  {:decode, {:error, error}} -> # ...
end

# Prefer - use case when errors matter
case call_service(data) do
  {:ok, resp} ->
    case Jason.decode(resp) do
      {:ok, decoded} -> decoded
      {:error, error} -> # handle decode error
    end

  {:error, error} -> # handle service error
end

Alternative: Create a unified error type for consistent error handling across your app:

elixir

defmodule MyApp.Error do
  defexception [:code, :msg, :meta]

  def not_found(msg, meta \\ %{}), do: %__MODULE__{code: :not_found, msg: msg, meta: meta}
  def internal(msg, meta \\ %{}), do: %__MODULE__{code: :internal, msg: msg, meta: meta}
end

# Wrap external errors in your unified type
defp decode(resp) do
  case Jason.decode(resp) do
    {:ok, decoded} -> {:ok, decoded}
    {:error, _} -> {:error, Error.internal("could not decode: #{inspect(resp)}")}
  end
end

Higher-Order Functions

Expose single-item operations and use Enum/Stream at call sites:

elixir

# Avoid - hides higher-order functions
def main do
  collection
  |> parse_items()
  |> add_items()
end

def parse_items(list), do: Enum.map(list, &String.to_integer/1)
def add_items(list), do: Enum.reduce(list, 0, & &1 + &2)

# Prefer - makes functions reusable
def main do
  collection
  |> Enum.map(&parse_item/1)
  |> Enum.sum()
end

defp parse_item(item), do: String.to_integer(item)

Benefits: Functions become reusable with Enum, Stream, Task. Better solutions often emerge (like using Enum.sum/1 instead of manual reduce).

Guards and Expectations

State what you want, not what you don't

elixir

# Avoid
def call_service(%{req: req}) when not is_nil(req) do
  # ...
end

# Prefer - be explicit about requirements
def call_service(%{req: req}) when is_binary(req) do
  # ...
end

Only return error tuples when caller can act on them

If the caller can't do anything about an error, raise or throw instead:

elixir

# Avoid - forces caller to handle unrecoverable errors
def get(table \\ __MODULE__, id) do
  try do
    :ets.lookup(table, id)
  catch
    _, _ -> {:error, "Table is not available"}
  end
end

# Prefer - let it fail if table doesn't exist
def get(table \\ __MODULE__, id) do
  :ets.lookup(table, id)
end

Raise on invalid data from external sources

Use bang functions (!) when downstream services should always return expected formats:

elixir

# Avoid - unnecessary error handling for impossible cases
def main do
  {:ok, resp} = call_service(id)
  case Jason.decode(resp) do
    {:ok, decoded} -> decoded
    {:error, e} -> # What can we even do here?
  end
end

# Prefer - let it crash and restart
def main do
  {:ok, resp} = call_service(id)
  Jason.decode!(resp)
end

Testing

Use for comprehensions in assertions to get better failure messages:

elixir

# Avoid - generic failure message
assert Enum.all?(posts, fn post -> %Post{} = post end)

# Prefer - shows which specific post failed
for post <- posts, do: assert %Post{} = post