An external function (function(a, b) external {java: {...}}, DMN 1.6 10.3.2.13), lowered to a call of the function that an extension adds for its kind (D-211).

FEEL does not say how an external function is run: the body names a kind (java, pmml) and describes the function in that kind's terms. DMN 1.6 has no way for the library to run one, so the analyzer refuses it, and Feel.Extension is where a host adds one. For the kind java an extension adds a function named external java, which takes the description and the list of the arguments:

function(n1, n2) external {java: {class: "java.lang.Math", method signature: "max(double,double)"}}

is read as

function(n1, n2) external java({class: "java.lang.Math", method signature: "max(double,double)"}, [n1, n2])

and is analyzed as that, so the function is an ordinary function of FEEL (it has the declared parameters, its closure, its type), the call is an ordinary call of a function of an extension (it is not folded when the function is not pure, and its result is typed by its signature), and nothing in the library knows what java means.

The body has to be a context of one entry, whose key (a name or a string) is the kind, and an extension has to add the function of that kind. Otherwise there is nothing to lower to, and the function stays external, which the analyzer reports as :not_implemented as it does without extensions. This module only builds the node; lower/2 is the whole of it.

iex> {:ok, ast} = Feel.parse(~S|function(a) external {demo: {x: 1}}|)
iex> Feel.Analysis.External.lower(ast, Feel.Extensions.none())
:error