From d132941acba63ec82ee197455b22c631b2a83c4b Mon Sep 17 00:00:00 2001
From: Lexus Drumgold <unicornware@flexdevelopment.llc>
Date: Sat, 13 May 2023 05:00:02 -0400
Subject: [PATCH] feat(types): `IsNil`, `IfNil`

Signed-off-by: Lexus Drumgold <unicornware@flexdevelopment.llc>
---
 src/types/__tests__/if-nil.spec-d.ts | 20 ++++++++++++++++++++
 src/types/__tests__/is-nil.spec-d.ts | 17 +++++++++++++++++
 src/types/if-nil.ts                  | 19 +++++++++++++++++++
 src/types/index.ts                   |  2 ++
 src/types/is-nil.ts                  | 15 +++++++++++++++
 5 files changed, 73 insertions(+)
 create mode 100644 src/types/__tests__/if-nil.spec-d.ts
 create mode 100644 src/types/__tests__/is-nil.spec-d.ts
 create mode 100644 src/types/if-nil.ts
 create mode 100644 src/types/is-nil.ts

diff --git a/src/types/__tests__/if-nil.spec-d.ts b/src/types/__tests__/if-nil.spec-d.ts
new file mode 100644
index 00000000..139225ac
--- /dev/null
+++ b/src/types/__tests__/if-nil.spec-d.ts
@@ -0,0 +1,20 @@
+/**
+ * @file Type Tests - IfNil
+ * @module tutils/types/tests/unit-d/IfNil
+ */
+
+import type TestSubject from '../if-nil'
+import type NIL from '../nil'
+
+describe('unit-d:types/IfNil', () => {
+  type False = false
+  type True = true
+
+  it('should equal False if IsNil<T> extends false', () => {
+    expectTypeOf<TestSubject<NIL[], True, False>>().toEqualTypeOf<False>()
+  })
+
+  it('should equal True if IsNil<T> extends true', () => {
+    expectTypeOf<TestSubject<NIL, True, False>>().toEqualTypeOf<True>()
+  })
+})
diff --git a/src/types/__tests__/is-nil.spec-d.ts b/src/types/__tests__/is-nil.spec-d.ts
new file mode 100644
index 00000000..ccaaa1fe
--- /dev/null
+++ b/src/types/__tests__/is-nil.spec-d.ts
@@ -0,0 +1,17 @@
+/**
+ * @file Type Tests - IsNil
+ * @module tutils/types/tests/unit-d/IsNil
+ */
+
+import type TestSubject from '../is-nil'
+import type NIL from '../nil'
+
+describe('unit-d:types/IsNil', () => {
+  it('should equal false if T does not extend NIL', () => {
+    expectTypeOf<TestSubject<bigint>>().toEqualTypeOf<false>()
+  })
+
+  it('should equal true if T extends NIL', () => {
+    expectTypeOf<TestSubject<NIL>>().toEqualTypeOf<true>()
+  })
+})
diff --git a/src/types/if-nil.ts b/src/types/if-nil.ts
new file mode 100644
index 00000000..1b99ff3f
--- /dev/null
+++ b/src/types/if-nil.ts
@@ -0,0 +1,19 @@
+/**
+ * @file Type Definitions - IfNil
+ * @module tutils/types/IfNil
+ */
+
+import type IsNil from './is-nil'
+
+/**
+ * Conditional type that resolves depending on whether or not `T` extends `NIL`.
+ *
+ * @see {@linkcode IsNil}
+ *
+ * @template T - Type to evaluate
+ * @template True - Type if `T` is `nil`
+ * @template False - Type if `T` is not `nil`
+ */
+type IfNil<T, True, False> = IsNil<T> extends true ? True : False
+
+export type { IfNil as default }
diff --git a/src/types/index.ts b/src/types/index.ts
index a6cb681c..406db3ee 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -29,6 +29,7 @@ export type { default as IfBoolean } from './if-boolean'
 export type { default as IfFunction } from './if-function'
 export type { default as IfJsonPrimitive } from './if-json-primitive'
 export type { default as IfNever } from './if-never'
+export type { default as IfNil } from './if-nil'
 export type { default as IndexSignature } from './index-signature'
 export type { default as IsAny } from './is-any'
 export type { default as IsArray } from './is-array'
@@ -37,6 +38,7 @@ export type { default as IsBoolean } from './is-boolean'
 export type { default as IsFunction } from './is-function'
 export type { default as IsJsonPrimitive } from './is-json-primitive'
 export type { default as IsNever } from './is-never'
+export type { default as IsNil } from './is-nil'
 export type { default as IsTuple } from './is-tuple'
 export type { default as Join } from './join'
 export type { default as JsonArray } from './json-array'
diff --git a/src/types/is-nil.ts b/src/types/is-nil.ts
new file mode 100644
index 00000000..29e40504
--- /dev/null
+++ b/src/types/is-nil.ts
@@ -0,0 +1,15 @@
+/**
+ * @file Type Definitions - IsNil
+ * @module tutils/types/IsNil
+ */
+
+import type NIL from './nil'
+
+/**
+ * Returns a boolean indicating if `T` extends {@linkcode NIL}.
+ *
+ * @template T - Type to evaluate
+ */
+type IsNil<T> = T extends NIL ? true : false
+
+export type { IsNil as default }