balance-platform / ex_pression

Evaluate user input expressions

ExPression

Evaluate user input expressions.

Installation

The package can be installed by adding ex_pression to your list of dependencies in mix.exs:

def deps do
  [
    {:ex_pression, "~> 0.5.0"}
  ]
end

Key features

1. Safe evaluation without access to other Elixir modules.

iex> ExPression.eval("exit(self())")
{:error, %ExPression.Error{name: "UndefinedFunctionError", message: "Function 'self/0' was referenced, but was not defined", data: %{function: :self}}}

2. Support for JSON syntax and data types.

iex> ExPression.eval("""
{
  "name": "ex_pression",
  "deps": ["xpeg"]
}
""")
{:ok, %{"name" => "ex_pression", "deps" => ["xpeg"]}}

3. Familiar python-like operators and standard functions.

iex> ExPression.eval(~s/{"1": "en", "2": "fr"}[str(int_code)]/, bindings: %{"int_code" => 1})
{:ok, "en"}

4. Extend expressions by providing Elixir module with functions that you want to use.

defmodule MyFunctions do
  # use $ special symbol in expressions
  def handle_special("$", date_str), do: Date.from_iso8601!(date_str)
  # Use diff function in expressions
  def diff(date_1, date_2), do: Date.diff(date_1, date_2)
end

iex> ExPression.eval(~s/diff($"2023-02-02", $"2022-02-02")/, functions_module: MyFunctions)
{:ok, 365}

Full language description can be found in FULL_DESCRIPTION.md

Implementation

String representation of expression is parsed into AST form. Parsing is done with PEG grammar parser xpeg. Grammar is defined in module ExPression.Parser.Grammar. AST interpretation logic is written in plain Elixir in module ExPression.Interpreter.

Contribution

Feel free to make a pull request. All contributions are appreciated!