Elex.Function behaviour (Elex v0.2.1)

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Behaviour for implementing custom Elex functions.

Functions are registered on a Elex.Context via Elex.Context.add_function/2 and invoked during evaluation by Elex.Evaluator.

Implementing a function

defmodule MyApp.Functions.Double do
  @behaviour Elex.Function

  @impl Elex.Function
  def signature, do: %{name: :double, arity: 1}

  @impl Elex.Function
  def validate([arg_ast], context) do
    case Elex.Validator.validate(arg_ast, context) do
      {:ok, :decimal} -> {:ok, :decimal}
      {:ok, type} -> {:error, "double expects a number, got " <> inspect(type)}
      {:error, reason} -> {:error, reason}
    end
  end

  @impl Elex.Function
  def call([arg]), do: {:ok, Decimal.mult(arg, Decimal.new(2))}

  @impl Elex.Function
  def documentation do
    %{signature: "double(x)", description: "returns x multiplied by 2"}
  end
end

Summary

Types

error_reason()
value()

Callbacks

call(args)

Evaluates the function with already-evaluated argument values.

documentation()

Returns human-readable documentation for the function.

signature()

Returns the function name and arity.

validate(args_ast, context)

Validates function arguments at parse time and returns the result type.

Types

error_reason()

@type error_reason() :: term()

value()

@type value() :: String.t() | boolean() | Decimal.t() | nil

Callbacks

call(args)

@callback call(args :: [value()]) :: {:ok, value()} | {:error, error_reason()}

Evaluates the function with already-evaluated argument values.

Must return {:ok, value} on success or {:error, reason} on failure.

documentation()

@callback documentation() :: %{signature: String.t(), description: String.t()}

Returns human-readable documentation for the function.

Used for introspection and documentation generation.

signature()

@callback signature() ::
  %{name: atom() | String.t(), arity: non_neg_integer()}
  | %{name: atom() | String.t(), variadic: true, min_arity: non_neg_integer()}

Returns the function name and arity.

The name is used in expression syntax (e.g. my_func(1, 2)).

validate(args_ast, context)

@callback validate(args_ast :: [term()], context :: Elex.Context.t()) ::
  {:ok, return_type :: atom()} | {:error, reason :: term()}

Validates function arguments at parse time and returns the result type.

Receives unevaluated AST nodes for each argument.