geolog-zeta-fork/examples/geolog/petri_net_full.geolog

// Full Petri Net Reachability - Type-Theoretic Encoding
//
// This demonstrates the complete type-theoretic encoding of Petri net
// reachability from the original geolog design vision.
//
// Original design: loose_thoughts/2025-12-12_12:10_VanillaPetriNetRechability.md
//
// Key concepts:
// - PetriNet: places, transitions, input/output arcs (with proper arc semantics)
// - Marking: tokens in a net (parameterized by net)
// - ReachabilityProblem: initial and target markings (nested instances)
// - Trace: sequence of firings with wires connecting arcs
// - Iso: isomorphism between two sorts (used for bijections)
// - Solution: a trace with isomorphisms to markings
//
// This encoding is more type-theoretically precise than the simple
// PlaceReachability: it tracks individual tokens and arc multiplicities,
// enabling correct handling of "multi-token" transitions.

// ============================================================
// THEORY: PetriNet
// Basic structure: places, transitions, and arcs
// ============================================================

theory PetriNet {
  P : Sort;      // Places
  T : Sort;      // Transitions

  in : Sort;     // Input arcs (place -> transition)
  out : Sort;    // Output arcs (transition -> place)

  // Each arc knows which place/transition it connects
  in/src : in -> P;    // Input arc source place
  in/tgt : in -> T;    // Input arc target transition
  out/src : out -> T;  // Output arc source transition
  out/tgt : out -> P;  // Output arc target place
}

// ============================================================
// THEORY: Marking (parameterized by N : PetriNet)
// A marking assigns tokens to places
// ============================================================

theory (N : PetriNet instance) Marking {
  token : Sort;
  token/of : token -> N/P;
}

// ============================================================
// THEORY: ReachabilityProblem (parameterized by N : PetriNet)
// Defines initial and target markings as nested instances
// ============================================================

theory (N : PetriNet instance) ReachabilityProblem {
  initial_marking : N Marking instance;
  target_marking : N Marking instance;
}

// ============================================================
// THEORY: Trace (parameterized by N : PetriNet)
// A trace is a sequence of transition firings with "wires"
// connecting input and output arcs
// ============================================================

theory (N : PetriNet instance) Trace {
  // Firings
  F : Sort;
  F/of : F -> N/T;

  // Wires connect output of one firing to input of another
  W : Sort;
  W/src : W -> [firing : F, arc : N/out];  // Wire source (firing, output arc)
  W/tgt : W -> [firing : F, arc : N/in];   // Wire target (firing, input arc)

  // Wire coherence: output arc must belong to source firing's transition
  ax1 : forall w : W. |- w W/src .arc N/out/src = w W/src .firing F/of;
  // Wire coherence: input arc must belong to target firing's transition
  ax2 : forall w : W. |- w W/tgt .arc N/in/tgt = w W/tgt .firing F/of;

  // Wire uniqueness: each (firing, out-arc) pair has at most one wire
  ax3 : forall w1, w2 : W. w1 W/src = w2 W/src |- w1 = w2;
  // Wire uniqueness: each (firing, in-arc) pair has at most one wire
  ax4 : forall w1, w2 : W. w1 W/tgt = w2 W/tgt |- w1 = w2;

  // Terminals: for initial marking tokens (input) and final marking tokens (output)
  input_terminal : Sort;
  output_terminal : Sort;
  input_terminal/of : input_terminal -> N/P;
  output_terminal/of : output_terminal -> N/P;
  input_terminal/tgt : input_terminal -> [firing : F, arc : N/in];
  output_terminal/src : output_terminal -> [firing : F, arc : N/out];

  // Every output arc of every firing must be wired OR be an output terminal
  ax5 : forall f : F, arc : N/out. arc N/out/src = f F/of |-
    (exists w : W. w W/src = [firing: f, arc: arc]) \/
    (exists o : output_terminal. o output_terminal/src = [firing: f, arc: arc]);

  // Every input arc of every firing must be wired OR be an input terminal
  ax6 : forall f : F, arc : N/in. arc N/in/tgt = f F/of |-
    (exists w : W. w W/tgt = [firing: f, arc: arc]) \/
    (exists i : input_terminal. i input_terminal/tgt = [firing: f, arc: arc]);
}

// ============================================================
// THEORY: Iso (parameterized by two sorts)
// An isomorphism between two sorts
// ============================================================

theory (X : Sort) (Y : Sort) Iso {
  fwd : X -> Y;
  bwd : Y -> X;
  fb : forall x : X. |- x fwd bwd = x;
  bf : forall y : Y. |- y bwd fwd = y;
}

// ============================================================
// THEORY: Solution (parameterized by N and RP)
// A solution to a reachability problem
// ============================================================

theory (N : PetriNet instance) (RP : N ReachabilityProblem instance) Solution {
  // The witnessing trace
  trace : N Trace instance;

  // Bijection between input terminals and initial marking tokens
  initial_marking_iso : (trace/input_terminal) (RP/initial_marking/token) Iso instance;

  // Bijection between output terminals and target marking tokens
  target_marking_iso : (trace/output_terminal) (RP/target_marking/token) Iso instance;

  // Initial marking commutes: token placement matches terminal placement
  initial_marking_P_comm : forall i : trace/input_terminal.
    |- i trace/input_terminal/of = i initial_marking_iso/fwd RP/initial_marking/token/of;

  // Target marking commutes: token placement matches terminal placement
  target_marking_P_comm : forall o : trace/output_terminal.
    |- o trace/output_terminal/of = o target_marking_iso/fwd RP/target_marking/token/of;
}

// ============================================================
// INSTANCE: ExampleNet - a small Petri net
//
//   (A) --[ab]--> (B) --[bc]--> (C)
//    ^             |
//    +---[ba]------+
// ============================================================

instance ExampleNet : PetriNet = {
  // Places
  A : P;
  B : P;
  C : P;

  // Transitions
  ab : T;
  ba : T;
  bc : T;

  // A -> B (via ab)
  ab_in : in;
  ab_in in/src = A;
  ab_in in/tgt = ab;
  ab_out : out;
  ab_out out/src = ab;
  ab_out out/tgt = B;

  // B -> A (via ba)
  ba_in : in;
  ba_in in/src = B;
  ba_in in/tgt = ba;
  ba_out : out;
  ba_out out/src = ba;
  ba_out out/tgt = A;

  // B -> C (via bc)
  bc_in : in;
  bc_in in/src = B;
  bc_in in/tgt = bc;
  bc_out : out;
  bc_out out/src = bc;
  bc_out out/tgt = C;
}

// ============================================================
// Example queries (once query solving is implemented):
//
// query can_reach_B_from_A {
//   ? : ExampleNet problem0 Solution instance;
// }
//
// where problem0 : ExampleNet ReachabilityProblem = {
//   initial_marking = { tok : token; tok token/of = ExampleNet/A; };
//   target_marking = { tok : token; tok token/of = ExampleNet/B; };
// }
// ============================================================