feat(lang): include callable class object

includes a class object that produces objects that can be called to
execute custom functions without additional API methods

README.md

@@ -45,6 +45,7 @@ in a clean and consitent fashion. Nothing more, and nothing less.
     * [`slugify`(text: String [, separator: String = '-']): String](#slugifytext-string--separator-string----string)
     * [`typecast`(text: String): Object](#typecasttext-string-object)
   * [`typeOf`(element: `any`): String](#typeofelement-any-string)
+  * [`Callable`: Class](#callable-class)
 * [Authors](#authors)
 * [Contributors ✨](#contributors-)
 
@@ -376,6 +377,32 @@ typeOf(() => {}) // function
 typeOf(new Set()) // set
 ```
 
+### `Callable`: [Class][]
+
+A class object whose instances are derived from [Function][] and can be called.
+When exteded, a [Symbol][] function defined by `Symbol.for('call')` will be executed
+with any arguments that were passed
+
+##### Example
+
+```javascript
+const {Callable} = require('@logdna/stdlib')
+const __call__ = Symbol.for('call')
+class Hello extends Callable {
+  constructor(opts) {
+    this.yell = !!opts.yell
+  }
+  [__call__](name) {
+    const output = `Hello, ${name}`
+    console.log(this.yell ? `${output.toUpperCase()}!` : output)
+  }
+}
+
+const screamAt = new Hello({yell: true})
+
+screamAt('bill') // HELLO, BILL!
+```
+
 ## Authors
 
 * [**Eric Satterwhite**](mailto:eric.satterwhite@logdna.com) <eric.satterwhite@logdna.com>
@@ -388,6 +415,8 @@ typeOf(new Set()) // set
 [Number]: https://mdn.io/number
 [Object]: https://mdn.io/object
 [Function]: https://mdn.io/function
+[Class]: https://mdn.io/class
+[Symbol]: https://mdn.io/symbol
 [Generator]: https://mdn.io/generator
 [itertools]: https://docs.python.org/3.7/library/itertools.html
 

lib/callable.js

@@ -0,0 +1,42 @@
+'use strict'
+/**
+ * @module lib/callable
+ * @author Eric Satterwhite
+ **/
+
+/**
+ * A class object whose instances are also callable
+ * To define a callable object, extend this class and gif it a symbol function
+ * using Symbol.for("call")
+ * @constructor
+ * @alias module:lib/callable
+ * @extends Function
+ * @example
+ * class Message extends Callable {
+ *  constructor(name) {
+ *    super()
+ *    this.name = name
+ *  }
+ *
+ *  [__call__]() {
+ *    return `Hello ${this.name}`
+ *  }
+ * }
+ *
+ * const bob = new Message('bob')
+ * bob.name // bob
+ * bob() // Hello bob
+ **/
+class Callable extends Function {
+  get [Symbol.toStringTag]() {
+    return 'Callable'
+  }
+
+  constructor() {
+    super('...args', 'return this._bound[Symbol.for("call")](...args)')
+    this._bound = this.bind(this)
+    return this._bound
+  }
+}
+
+module.exports = Callable

test/unit/callable.js

@@ -0,0 +1,23 @@
+'use strict'
+
+const {test, threw} = require('tap')
+const typeOf = require('../../lib/type-of.js')
+const Callable = require('../../lib/callable.js')
+
+test('callable', async (t) => {
+  t.test('string representation', async (t) => {
+    t.equal(typeOf(new Callable()), 'callable', 'identifies as callable')
+  })
+
+  t.test('extensbility', async (t) => {
+    const __call__ = Symbol.for('call')
+    class Sum extends Callable {
+      [__call__](a, b = 1) {
+        return a + b
+      }
+    }
+
+    const sum = new Sum()
+    t.equal(sum(10, 3), 13, 'call function executed')
+  })
+}).catch(threw)