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fast_vs_slow_access.md

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Fast vs Slow access

Enumerations access can be resolved in 2 different ways:

  • At the compilation (also called Fast access)
  • At the runtime (also called Slow access)

Fast access

When it's possible, SimpleEnum will try to replace Enum access directly with it's value (Fast access).
In order to do this, all arguments given to the Enum access must be resolvable at compile time.

To verify this behaviour, let's write a simple function to inspect the AST of an Enum named color.

iex> defmodule EnumInspector do
...>   def inspect_ast(quoted_expr, env) do
...>     quoted_expr
...>     |> Macro.postwalk(&expand_enum_ast(&1, env))
...>     |> Macro.to_string()
...>     |> IO.puts
...>   end
...>
...>   # Expand only the Enum's AST
...>   defp expand_enum_ast({:color, [], _} = x, env), do: Macro.expand(x, env)
...>   defp expand_enum_ast(x, _), do: x
...> end

Now, let's use this module and see how Elixir compile an Enum access with known arguments at compile time :

iex> defmodule MyApp.Enums do
...>   import SimpleEnum, only: [defenum: 2]
...> 
...>   # Define the Enum
...>   defenum :color, [:blue, :green, :red]
...> 
...>   # Inspect the generated AST of a function using Fast access
...>   EnumInspector.inspect_ast(quote do
...>     dep test() do
...>       color(:red, :value)
...>     end
...>   end, __ENV__)
...> end

dep(test()) do
  2
end

We notice that the Enum access color(:red, :value) has been automatically replaced with 2 by the compiler.

This explain why Fast access can be used in guards for example.

NOTE: Module attributes are also supported with fast access.

Slow access

Now let's take a look at what happens when the values cannot be determined at compile time.

iex> defmodule MyApp.Enums do
...>   import SimpleEnum, only: [defenum: 2]
...>
...>   # Define the Enum
...>   defenum :color, [:blue, :green, :red]
...>
...>   # Inspect the generated AST of a function using Slow access
...>   EnumInspector.inspect_ast(quote do
...>       dep test(value) do
...>           color(value, :value)
...>       end
...>   end, __ENV__)
...> end

dep(test(value)) do
  case({value, :value}) do
    {x, :key} when x in [:blue, :green, :red] ->
      x
    {x, :value} when x in [0, 1, 2] ->
      x
    {x, :key} when x in [0, 1, 2] ->
      Map.fetch!(%{0 => :blue, 1 => :green, 2 => :red}, x)
    {x, :value} when x in [:blue, :green, :red] ->
      Keyword.fetch!([blue: 0, green: 1, red: 2], x)
    {x, :tuple} when x in [:blue, :green, :red] ->
      {x, Keyword.fetch!([blue: 0, green: 1, red: 2], x)}
    {x, :tuple} when x in [0, 1, 2] ->
      {Map.fetch!(%{0 => :blue, 1 => :green, 2 => :red}, x), x}
    {_, t} when t not in [:key, :value, :tuple] ->
      raise(ArgumentError, "invalid type :value. Expected one of [:key, :value, :tuple]")
    {x, _} ->
      raise(ArgumentError, "invalid value {:value, [], MyApp.Enums} for Enum MyApp.Enums.color/2. Expected one of [:blue, :green, :red, 0, 1, 2]\n")        
  end
end

This time, the color(value, :value) access has been transformed into a big case checking at runtime the value of the arguments given.

It is therefore recommended, when it is possible, to avoid creating temporary variables in order to store arguments for an Enum access.