A Framework for Dynamic Software Composition

# Illustrative sketch (not from the paper)
import copy

# Effect: a transformation with its inverse
def apply_effect(state, transform):
    """Apply a reversible transform to state and return new state and its inverse."""
    new_state = transform(state)
    # Inverse is simply the function that undoes the transform
    inverse = lambda s: state  # captures original state
    return new_state, inverse

# Coeffect: react to context changes according to a spec
def check_coeffect(context, spec):
    """Activate or deactivate component based on spec comparison."""
    return spec(context)  # spec is a predicate returning True/False

# Unified context that carries both effect and coeffect info
def unify_context(effect_ctx, coeffect_ctx):
    unified = copy.deepcopy(effect_ctx)
    unified.update(coeffect_ctx)
    return unified

# Dynamic composition calculus – interleave components safely
def compose_dynamic(components, init_context):
    ctx = init_context
    inverses = []
    for comp in components:
        # each component provides a transform and a spec
        ctx, inv = apply_effect(ctx, comp['transform'])
        inverses.append(inv)
        active = check_coeffect(ctx, comp['spec'])
        if not active:
            # revert if component should not stay active
            ctx = inverses.pop() (ctx)  # revert using last inverse
    return ctx

// Illustrative sketch (not from the paper)
const _ = require('lodash');

// Effect: a transformation with its inverse
function applyEffect(state, transform) {
  const newState = transform(state);
  // Inverse captures original state
  const inverse = (s) => state;
  return { newState, inverse };
}

// Coeffect: react to context changes based on a spec predicate
function checkCoeffect(context, spec) {
  return spec(context); // true => active, false => deactivate
}

// Unified context merging effect and coeffect information
function unifyContext(effectCtx, coeffectCtx) {
  return _.assign({}, effectCtx, coeffectCtx);
}

// Dynamic composition calculus – interleaves components safely
function composeDynamic(components, initContext) {
  let ctx = initContext;
  const inverses = [];
  for (const comp of components) {
    const { newState, inverse } = applyEffect(ctx, comp.transform);
    ctx = newState;
    inverses.push(inverse);
    const active = checkCoeffect(ctx, comp.spec);
    if (!active) {
      // revert using last inverse if component should not stay active
      const rev = inverses.pop();
      ctx = rev(ctx);
    }
  }
  return ctx;
}