structural equality, booleans, relational barriers for propagation

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
+  });
+}
+