A model of an imperative interpreter of a pure language composing overlays

overlays.nix

let
  f = final: previous: { a = "fa"; b = "fb"; };
  g = final: previous: { a = previous.a + final.b; };
  h = final: previous: { b = "hb"; };
in let
  current0 = { };
  current1 = current0 // f final current0;
  current2 = current1 // g final current1;
  current3 = current2 // h final current2;
  final = current3;
in
  final.a

overlays.py

# This models an imperative interpreter of a pure language composing overlays.

def f(final, previous):
    return {
        'a': lambda: 'fa',
        'b': lambda: 'fb',
    }


def g(final, previous):
    return {
        'a': lambda: previous['a']() + final['b'](),
    }


def h(final, previous):
    return {
        'b': lambda: 'hb',
    }


final = {}
current = {}

for func in f, g, h:
    own = func(final, current)
    current = current.copy()
    current.update(own)

final.update(current)
print(final['a']())