allow default interface impl

src/algebra/index.js

@@ -1,6 +1,7 @@
 import { mix } from '../mixin.js'
 
 /** @import { MixinFunction } from '../mixin.js' */
+/** @import { Fn } from './types.js' */
 
 export const ProtectedConstructor = Symbol('ProtectedConstructor')
 export class ProtectedConstructorError extends Error {
@@ -76,6 +77,9 @@ class Method {
   /** @type {MethodType} */
   #type = MethodType.Instance
 
+  /** @type {Fn} */
+  #implementation
+
   /**
    * @param {string | Method} name 
    */
@@ -104,6 +108,23 @@ class Method {
     return this
   }
 
+  /** @param {Fn} f */
+  implementation(f) {
+    this.#implementation = f
+    return this
+  }
+
+  /**
+   * @param {string} interfaceName 
+   */
+  _defaultImplementation(interfaceName) {
+    const err = new NotImplementedError(
+      `${interfaceName}::${this.#name}`
+    )
+
+    return function() { throw err }
+  }
+
   /**
    * @param {Function} target 
    */
@@ -120,13 +141,10 @@ class Method {
    * @param {Function} target 
    */
   implement(builder, target) {
-    const err = new NotImplementedError(
-      `${builder.name}::${this.#name}`
-    )
 
-    this._getInstallationPoint(target)[this.#name] = function() {
-      throw err
-    }
+    const impl = this.#implementation || this._defaultImplementation(builder.name)
+
+    this._getInstallationPoint(target)[this.#name] = impl
   }
 }
 
@@ -196,7 +214,15 @@ class Interface {
   }
 }
 
-class BaseSet { }
+/** @template T*/
+class BaseSet {
+  _value
+
+  /** @param {T} value */
+  constructor(value) {
+    this._value = value
+  }
+}
 
 /**
  * @param {PropertyKey} key
@@ -301,7 +327,12 @@ export const Traversable = new Interface('Traversable')
 
 export const Chain = new Interface('Chain')
   .extends(Apply)
-  .specifies('chain')
+  .specifies(
+    Method.from('chain')
+      .implementation(function(f) {
+        return f(this._value)
+      }))
+  )
 
 export const ChainRef = new Interface('ChainRec')
   .extends(Chain)