structural equality, booleans, relational barriers for propagation

src/lang/eval/error.ts

@@ -14,6 +14,8 @@ export type RuntimeError =
   | { tag: "TypeMismatch", expected: ValueTag, received: Value }
   | { tag: "DuplicateVariableNamesInPattern", pattern: Pattern, duplicates: VariableName[] }
   // | { tag: "DuplicateVariableNamesInProductPattern", pattern: ProductPattern, duplicates: VariableName[] }
+  | { tag: "ClosureEqualityComparison", value0: Closure, value1: Value } // Closures cannot be compared for equality
+  | { tag: "NotABoolean", value: Value } // Attempt to use a non-boolean when a boolean is expected
 
 export type Result<T> =
   | { tag: "ok", value: T }

src/lang/eval/signal.ts

@@ -230,7 +230,6 @@ function tupleThen<A, B>(Xs: Signal<A>[], f: (values: A[]) => B): Signal<B> {
 
 
 // === Primitive Signals ===
-
 export function makeTickSignal(intervalMs: number): Signal<Value> {
   const s = signal(Value.number(0));
 

src/lang/eval/signalValue.ts

@@ -1,9 +1,9 @@
-import { Expr, ProductPattern, SignalExpr, SignalName } from "../expr";
+import { Expr, ProductPattern, SignalExpr, SignalName, VariableName } from "../expr";
 import { Program } from "../program";
 import { Result, RuntimeError, ThrownRuntimeError } from "./error";
 import { eval_expr } from "./evaluator";
 import { match_product_pattern } from "./pattern_match";
-import { Env, Value } from "./value";
+import { Env, equals, forceBool, Value } from "./value";
 import { PriorityQueue } from "./priority_queue"
 
 // === Reactive DAG ===
@@ -62,14 +62,6 @@ export function getNode(program: Program, signalName: SignalName): SignalRuntime
   }
 }
 
-// TODO: This feels wrong. We shouldn't allow setting of arbitrary signals... only special signals that are "cells",
-// and are meant to be modified by the user...
-export function setSignal(program: Program, name: SignalName, value: Value) {
-  const node = getNode(program, name); 
-  node.currentValue = value;
-  propagate(program, node);
-}
-
 export function spawnSignal(program: Program, signalName: SignalName, expr: SignalExpr): SignalRuntime.DAGNode {
   const maybeNode = program.signal_runtimeNew.store.get(signalName);
   if (maybeNode !== undefined) {
@@ -82,6 +74,33 @@ export function spawnSignal(program: Program, signalName: SignalName, expr: Sign
   return node;
 }
 
+export function spawnSource(program: Program, signalName: SignalName, initValue: Value): [SignalRuntime.DAGNode, (value: Value) => void] {
+  const maybeNode = program.signal_runtimeNew.store.get(signalName);
+  if (maybeNode !== undefined) {
+    // TODO: Make this into a proper error
+    throw Error(`Attempt to spawn a signal '${signalName}' that already exists`);
+  }
+
+  const node: SignalRuntime.DAGNode = {
+    signal: SignalValue.const_(initValue),
+    signalName,
+    rank: 0,
+    internalInputs: [],
+    internalOutputs: [],
+    externalOutputs: [],
+    currentValue: initValue,
+  };
+  program.signal_runtimeNew.store.set(signalName, node);
+
+
+  function setValue(value: Value) {
+    node.currentValue = value;
+    propagate(program, node);
+  }
+
+  return [node, setValue];
+}
+
 // TODO: Should take in a `SignalName`, and find `oldNode` first
 export function hotSwapSignal(program: Program, oldNode: SignalRuntime.DAGNode, expr: SignalExpr) {
   reeval_signal_expression(program, Env.nil(), oldNode, expr)
@@ -166,13 +185,26 @@ function wouldCreateCycle(node: SignalRuntime.DAGNode, newParents: SignalRuntime
 
 type SignalValue =
   | { tag: "closure", closure: SignalClosure }
-  | { tag: "const", value: Value } // Is this really necessary? Yes... its not a special case of SignalClosure - it has a value as a body, not an expression...
+  // "source" is either a constant signal, or an external-input signal. The point is that these have no internal-inputs, i.e. rank-0 nodes.
+  | { tag: "source", value: Value } // Is this really necessary? Yes... its not a special case of SignalClosure - it has a value as a body, not an expression...
   | { tag: "proxy", parent: SignalRuntime.DAGNode }
 
 type SignalClosure = {
   bindings: SignalBinding[],
   env: Env,
-  body: Expr
+  body: Expr,
+  barrier?: Barrier, // Affects how propagation is done
+}
+
+type Barrier =
+  | { tag: "eq" }
+  // User-defined predicate (old, new) -> bool. What value is considered to be the canonical bool?
+  | { tag: "relation", relation: Relation }
+
+type Relation = {
+  oldVar: VariableName,
+  newVar: VariableName,
+  body: Expr,
 }
 
 type SignalBinding = {
@@ -363,7 +395,7 @@ function eval_signal_closure(program: Program, closure: SignalClosure): Value {
 }
 
 export namespace SignalValue {
-  export const const_ = (value: Value): SignalValue => ({ tag: "const", value });
+  export const const_ = (value: Value): SignalValue => ({ tag: "source", value });
   export const closure = (closure: SignalClosure): SignalValue => ({ tag: "closure", closure });
   export const proxy = (parent: SignalRuntime.DAGNode): SignalValue => ({ tag: "proxy", parent });
 }
@@ -385,10 +417,9 @@ function propagate(program: Program, rootNode: SignalRuntime.DAGNode) {
   while (!heap.isEmpty()) {
     const node = heap.pop()!;
     switch (node.signal.tag) {
-      case "const":
+      case "source":
         break;
       case "proxy": {
-        // TODO: new PROXY code: Is this correct?
         const value = node.internalInputs[0].currentValue;
         node.currentValue = value;
         descendantsWithExternalEffects.push(node);
@@ -398,8 +429,34 @@ function propagate(program: Program, rootNode: SignalRuntime.DAGNode) {
         break;
       }
       case "closure": {
-      // TODO: This may throw an exception!
-        const value = eval_signal_closure(program, node.signal.closure);
+        const closure = node.signal.closure;
+        // TODO: This may throw an exception!
+        const value = eval_signal_closure(program, closure);
+        if (closure.barrier !== undefined) {
+          switch (closure.barrier.tag) {
+            case "eq":
+              // TODO: This may throw an exception!
+              if (equals(value, node.currentValue)) {
+                continue
+              } else {
+                break;
+              }
+            case "relation":
+              // TODO: This may throw an exception!
+              const should_propagate = eval_relation(
+                program, 
+                closure.env, 
+                node.currentValue, 
+                value, 
+                closure.barrier.relation
+              );
+              if (!should_propagate) {
+                continue
+              } else {
+                break;
+              }
+          }
+        }
         node.currentValue = value;
         descendantsWithExternalEffects.push(node);
         for (const childNode of node.internalOutputs) {
@@ -419,3 +476,38 @@ function propagate(program: Program, rootNode: SignalRuntime.DAGNode) {
 
 }
 
+function eval_relation(
+  program: Program, 
+  env: Env, 
+  old_value: Value, 
+  new_value: Value, 
+  relation: Relation
+): boolean {
+  const frame = Env.nil_frame();
+  Env.frame_insert_mut(frame, relation.oldVar, old_value);
+  Env.frame_insert_mut(frame, relation.newVar, new_value);
+  
+  const extendedEnv = Env.push_frame(env, frame);
+  // TODO: This may throw an exception!
+  const result = eval_expr(program, extendedEnv, relation.body);
+  
+  // TODO: This may throw an exception!
+  return forceBool(result);
+}
+
+// ===External Input Sources===
+
+export function spawnTick(program: Program, signalName: SignalName, intervalMs: number): SignalRuntime.DAGNode {
+  const [node, setValue] = spawnSource(program, signalName, Value.number(0));
+
+  setInterval(() => {
+    const state = node.currentValue;
+    if (state.tag === "number"){ 
+      setValue(Value.number(state.value + 1))
+    } else {
+      console.log(`Something is really wrong with the state of tick-signal '${signalName}'`, state);
+    }
+  }, intervalMs);
+  return node;
+}
+

src/lang/eval/value.ts

@@ -75,3 +75,59 @@ export namespace Env {
   }
 }
 
+export function equals(v1: Value, v2: Value): boolean {
+  if (v1 === v2) return true; // Reference equality optimization
+  if (v1.tag !== v2.tag) return false;
+  switch (v1.tag) {
+    case "number":
+      return v1.value === (v2 as Extract<Value, { tag: "number" }>).value;
+    case "string":
+      return v1.value === (v2 as Extract<Value, { tag: "string" }>).value;
+    case "tag":
+      return v1.tag_name === (v2 as Extract<Value, { tag: "tag" }>).tag_name;
+    case "tagged": {
+      const other = v2 as Extract<Value, { tag: "tagged" }>;
+      return v1.tag_name === other.tag_name && equals(v1.value, other.value);
+    }
+    case "tuple": {
+      const other = v2 as Extract<Value, { tag: "tuple" }>;
+      if (v1.values.length !== other.values.length) return false;
+      for (let i = 0; i < v1.values.length; i++) {
+        if (!equals(v1.values[i], other.values[i])) return false;
+      }
+      return true;
+    }
+    case "record": {
+      const other = v2 as Extract<Value, { tag: "record" }>;
+      if (v1.fields.size !== other.fields.size) return false;
+      for (const [key, val1] of v1.fields) {
+        const val2 = other.fields.get(key);
+        if (val2 === undefined || !equals(val1, val2)) return false;
+      }
+      return true;
+    }
+    case "closure":
+      // Philosophical/Mathematical barrier: throw error as requested
+      throw ThrownRuntimeError.error({
+        tag: "ClosureEqualityComparison",
+        value0: v1.closure,
+        value1: v2,
+      });
+  }
+}
+
+// Canonical bools are:
+// - True is `#T`
+// - False is `#F`
+// TODO: This is not a great design. Probably introducing completely new values would be better.
+export function forceBool(value: Value): boolean {
+  if (value.tag === "tag") {
+    if (value.tag_name === "T") return true;
+    if (value.tag_name === "F") return false;
+  }
+  throw ThrownRuntimeError.error({
+    tag: "NotABoolean",
+    value
+  });
+}
+