# `UndefinedFunctionError` [🔗](https://github.com/elixir-lang/elixir/blob/v1.20.1/lib/elixir/lib/exception.ex#L1677) An exception raised when a function is invoked that is not defined. For example: # Let's use apply/3 as otherwise Elixir emits a compile-time warning iex> apply(String, :non_existing_fun, ["hello"]) ** (UndefinedFunctionError) function String.non_existing_fun/1 is undefined or private The following fields of this exception are public and can be accessed freely: * `:module` (`t:module/0`) - the module name * `:function` (`t:atom/0`) - the function name * `:arity` (`t:non_neg_integer/0`) - the arity of the function --- *Consult [api-reference.md](api-reference.md) for complete listing*