Project setup

src/expresses.ts

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+import { AbstractClass, Opaque } from "type-fest"
+import { Trait, TraitApplierSuperTag } from "."
+import { ClassesInstances, ClassesStaticMembers, MergeInheritanceTree, MergeInheritanceTreeWithoutOverriding, StaticMembers, TraitsClasses } from "./util"
+
+
+/**
+ * Extends a class with the given traits and expresses their combined functionality.
+ * @template C - The abstract class type.
+ * @template Traits - An array of traits.
+ * @param extend - The class to extend.
+ * @param traits - An array of traits to apply.
+ * @returns A new class type expressing the combined functionality of the base class and traits.
+ * @example
+ * Extends a superclass and applies traits:
+ * ```ts
+ * class User extends extendsAndExpresses(Entity,
+ *     Identifiable<bigint>(),
+ *     Permissible,
+ * ) {
+ *     readonly id: bigint
+ *
+ *     constructor(id: bigint) {
+ *         super()
+ *         this.id = id
+ *     }
+ * }
+ * ```
+ */
+export function extendsAndExpresses<
+    C      extends AbstractClass<any>,
+    Traits extends readonly Trait<any>[],
+>(
+    extend: C,
+    ...traits: Traits
+) {
+    return traits.reduce(
+        (previous, trait) => trait(previous),
+        extend as Opaque<C, TraitApplierSuperTag>,
+    ) as unknown as (
+        AbstractClass<
+            MergeInheritanceTreeWithoutOverriding<[
+                InstanceType<C>,
+                ...ClassesInstances<
+                    TraitsClasses<Traits>
+                >,
+            ]>,
+
+            ConstructorParameters<C>
+        > &
+
+        MergeInheritanceTree<[
+            StaticMembers<C>,
+            ...ClassesStaticMembers<
+                TraitsClasses<Traits>
+            >,
+        ]>
+    )
+}
+
+/**
+ * Expresses the combined functionality of multiple traits.
+ * @template Traits - An array of trait.
+ * @param traits - An array of trait to apply.
+ * @returns A new class type expressing the combined functionality of the traits.
+ * @example
+ * Applies traits to a class:
+ * ```ts
+ * class User extends expresses(Identifiable<bigint>(), Permissible) {
+ *     readonly id: bigint
+ *
+ *     constructor(id: bigint) {
+ *         super()
+ *         this.id = id
+ *     }
+ * }
+ * ```
+ */
+export function expresses<
+    Traits extends readonly Trait<any>[],
+>(
+    ...traits: Traits
+) {
+    return extendsAndExpresses(Object, ...traits)
+}