From 3f91e35790be6e3d7111295121441e492e03a263 Mon Sep 17 00:00:00 2001
From: Pablo Baleztena <pablo@ultharsoftware.com>
Date: Fri, 4 Oct 2024 11:07:57 -0300
Subject: [PATCH] [fabric/core] Add isVariant utility function

---
 .../core/src/variant/is-variant.spec.ts       | 40 +++++++++++++++++++
 .../fabric/core/src/variant/is-variant.ts     | 11 +++++
 2 files changed, 51 insertions(+)
 create mode 100644 packages/fabric/core/src/variant/is-variant.spec.ts
 create mode 100644 packages/fabric/core/src/variant/is-variant.ts

diff --git a/packages/fabric/core/src/variant/is-variant.spec.ts b/packages/fabric/core/src/variant/is-variant.spec.ts
new file mode 100644
index 0000000..7632352
--- /dev/null
+++ b/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);
+  });
+});
diff --git a/packages/fabric/core/src/variant/is-variant.ts b/packages/fabric/core/src/variant/is-variant.ts
new file mode 100644
index 0000000..c4f86e1
--- /dev/null
+++ b/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;
+}