Defines a behaviour for implementing custom ExCellerate functions.

Any module implementing this behaviour can be registered in an ExCellerate.Registry and called from within an expression.

Example

defmodule MyApp.Functions.Greet do
  @behaviour ExCellerate.Function

  def name, do: "greet"
  def arity, do: 1

  def call([name]) do
    "Hello, #{name}!"
  end
end

Arity

The arity/0 callback controls compile-time argument count validation:

• Fixed: def arity, do: 2 — requires exactly 2 arguments.
• Range: def arity, do: 1..2 — accepts 1 or 2 arguments (optional args).
• Variadic: def arity, do: :any — accepts any number of arguments.

Argument Validation

Use ExCellerate.Functions.Guards for consistent runtime type checking in your call/1 implementations. The guards produce descriptive ExCellerate.Error messages and reduce boilerplate:

defmodule MyApp.Functions.Double do
  @behaviour ExCellerate.Function
  import ExCellerate.Functions.Guards

  def name, do: "double"
  def arity, do: 1

  def call([n]) when is_number(n), do: n * 2
  def call([other]), do: ensure_number!(other, name())
end

See ExCellerate.Functions.Guards for the full list of available helpers.