src/TraitBuilder.ts

@@ -1,7 +1,7 @@
 import { AbstractConstructor, Constructor, Simplify } from "type-fest"
 import { Trait } from "./Trait"
 import { TraitExpression } from "./TraitExpression"
-import { Extend, StaticMembers } from "./util"
+import { Extend, StaticMembers, type } from "./util"
 
 
 declare const implSuperSymbol: unique symbol
@@ -35,6 +35,17 @@ export class TraitBuilder<
         )
     }
 
+    abstractType<A extends Abstract>(
+        _: (Super: Abstract) => typeof type<A>
+    ) {
+        return new TraitBuilder(
+            this.traitSuperExpression,
+            {} as Simplify<A>,
+            this.traitStaticAbstract,
+            this.traitApply,
+        )
+    }
+
     staticAbstract<A extends StaticAbstract>(
         _: (Super: AbstractConstructor<StaticAbstract>) => AbstractConstructor<A, []>
     ) {
@@ -46,6 +57,17 @@ export class TraitBuilder<
         )
     }
 
+    staticAbstractType<A extends StaticAbstract>(
+        _: (Super: StaticAbstract) => typeof type<A>
+    ) {
+        return new TraitBuilder(
+            this.traitSuperExpression,
+            this.traitAbstract,
+            {} as Simplify<A>,
+            this.traitApply,
+        )
+    }
+
     implement<
         ImplClassWithAbstract extends TraitBuilder.ImplSuper<typeof this>  // TODO: find a way to set the constraint to concrete classes while keeping the Super arg as an abstract class
     >(

src/tests.ts

@@ -2,6 +2,7 @@ import { TraitClass } from "./Trait"
 import { trait } from "./TraitBuilder"
 import { Implements, StaticImplements, TraitExpressionClass } from "./TraitExpression"
 import { expression } from "./TraitExpressionBuilder"
+import { type } from "./util"
 
 
 const PrintsHelloOnNew = trait
@@ -18,9 +19,12 @@ const PrintsHelloOnNew = trait
 type PrintsHelloOnNewClass = TraitClass<typeof PrintsHelloOnNew>
 
 const Identifiable = <ID>() => trait
-    .abstract(Super => class extends Super {
-        declare readonly id: ID
-    })
+    // .abstract(Super => class extends Super {
+    //     declare readonly id: ID
+    // })
+    .abstractType(Super => type<typeof Super & {
+        readonly id: ID
+    }>)
     .implement(Super => class Identifiable extends Super {
         equals(el: Identifiable) {
             return this.id === el.id

src/util/misc.ts

@@ -15,3 +15,5 @@ export type CommonKeys<A, B> = Extract<keyof A, keyof B>
 export type StaticMembers<Class extends AbstractConstructor<any>> = (
     Omit<Class, "prototype">
 )
+
+export function type<T>() { return {} as T }