[fabric/core] Add isVariant utility function

packages/fabric/core/src/variant/is-variant.spec.ts

@@ -0,0 +1,40 @@
+import { describe, expect, expectTypeOf, it } from "vitest";
+import { isVariant } from "./is-variant.js";
+import { TaggedVariant, VariantTag } from "./variant.js";
+
+interface SuccessVariant extends TaggedVariant<"success"> {
+  [VariantTag]: "success";
+  data: string;
+}
+
+interface ErrorVariant extends TaggedVariant<"error"> {
+  [VariantTag]: "error";
+  message: string;
+}
+
+describe("isVariant", () => {
+  const successVariant = {
+    [VariantTag]: "success",
+    data: "Operation successful",
+  } as SuccessVariant | ErrorVariant;
+
+  const errorVariant = {
+    [VariantTag]: "error",
+    message: "Operation failed",
+  } as SuccessVariant | ErrorVariant;
+
+  it("should return true for a matching tag and correctly infer it", () => {
+    if (isVariant(successVariant, "success")) {
+      expectTypeOf(successVariant).toEqualTypeOf<SuccessVariant>();
+    }
+
+    if (isVariant(errorVariant, "error")) {
+      expectTypeOf(errorVariant).toEqualTypeOf<ErrorVariant>();
+    }
+  });
+
+  it("should return false for a non-matching tag", () => {
+    expect(isVariant(successVariant, "error")).toBe(false);
+    expect(isVariant(errorVariant, "success")).toBe(false);
+  });
+});

packages/fabric/core/src/variant/is-variant.ts

@@ -0,0 +1,11 @@
+import { TaggedVariant, VariantTag } from "./variant.js";
+
+export function isVariant<
+  TVariant extends TaggedVariant<string>,
+  TTag extends TVariant[VariantTag],
+>(
+  variant: TVariant,
+  tag: TTag,
+): variant is Extract<TVariant, { [VariantTag]: TTag }> {
+  return variant[VariantTag] === tag;
+}