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_posts/en/javascript/2017-04-11-protocols-for-the-brave.md

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+---
+layout: post
+
+title: Protocols for the Brave 
+tip-number: 73
+tip-username: loverajoel
+tip-username-profile: https://github.com/loverajoel
+tip-tldr: You might have heard about the old ways gaining hype recently, and we don't mean praying to the gods of the north. Today we're introducing a feature found in Clojure which allows you to define
+interfaces for your classes.
+
+categories:
+    - en
+    - javascript
+---
+
+
+You might have heard about the old ways gaining hype recently, and we don't
+mean praying to the gods of the north.
+
+Functional programming is the rediscovered toy which is bringing some sanity
+to the world of mutable state and global bindings.
+
+Today we're introducing a feature found in Clojure which allows you to define
+interfaces for your classes. Let's look at one-off implementation:
+
+``` javascript
+const protocols = (...ps) => ps.reduce((c, p) => p(c), Object);
+
+const Mappable = (klass) => {
+    return class extends klass {
+        map() {
+            throw 'Not implemented';
+        }
+    };
+};
+
+const Foldable = (klass) => {
+    return class extends klass {
+        fold() {
+            throw 'Not implemented';
+        }
+    };
+};
+
+class NaturalNumbers extends protocols(Mappable, Foldable) {
+    constructor() {
+        super();
+        this.elements = [1,2,3,4,5,6,7,8,9];
+    }
+
+    map (f) {
+        return this.elements.map(f);
+    }
+
+    fold (f) {
+        return this.elements.reduce(f, this.elements, 0);
+    }
+}
+```
+
+Yes, we're building a chain of class inheritance up there with that reduce boy.
+It's pretty cool. We're doing it dynamically! You see, each protocol receives
+a base class (Object) and extends it somehow returning the new class. The idea
+is similar to that of interfaces.
+
+We supply method signatures for the protocol and make sure we provide
+implementations for it on our base classes.
+
+What's so cool about it? We get to write things like these:
+
+``` javascript
+const map = f => o => o.map(f);
+const fold = f => o => o.fold(f);
+const compose = (... fns) => fns.reduce((acc, f) => (x) => acc(f(x)), id);
+```
+
+Ok, maybe we could have written those two functions without the above fuzz but,
+now that we know `NaturalNumbers` are `Mappable`, we can call `map` on them
+and trust it will return the right result. Furthermore, with our third function,
+we can *compose* any number of operations defined in protocols cleanly:
+
+``` javascript
+const plus1 = x => x + 1;
+const div5 = x => x / 5;
+const plus_then_div = compose(map(div5), map(plus1));
+console.log(plus_then_div(new NaturalNumbers));
+// => [ 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2 ]
+```
+
+More important, if we know that an object of ours is `Mappable`, we know `map`
+will work on it. Protocols gives us an idea of what we can do with an object and
+help us abstract common operations between data types, thus reducing the
+overhead of dealing with a hundred functions.
+
+What is easier? To have a hundred functions for every different object or  ten
+functions that work on a hundred objects?