Implementing shadow.remote API

Since version 0.8.0 of Chlorine, there’s a new way to evaluate ClojureScript code: that’s the Shadow-CLJS Remote API. It is basically a new REPL (not nREPL, no Socket REPL) over WebSockets to try to solve problems when translating other REPLs to ClojureScript. So, to understand why these problems exist, I’ll first introduce the difference between ClojureScript and Clojure.

On Clojure, you’re always inside a JVM. This means that compilation happens on the same JVM that your REPL, and your code is running. If you practice REPL-Driven Development, even your tests are running on the same JVM. In practical terms, it means that when you fire up your REPL, you already have everything ready to run code, compile code, and evaluate forms.

On ClojureScript, the compiler is written in Clojure – that means it’s running on the JVM. So, to produce Javascript code you don’t need a Javascript environment – and that’s when things become confusing, because when exactly will you run the REPL? Let’s try from another angle: if you start the REPL on compilation time, you can’t evaluate code (because there’s no Javascript generated, nor any Javascript engine running). If you start the REPL when you run the compiled code, this REPL can become unusable if you stop the Javascript environment, and also you have to coordinate lots of state and translations between formats.
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Reagent Mastermind

One of these days, a friend of mine posted about his experience writing the “Mastermind” game in React (in portuguese only). The game is quite simple:

  1. You have six possible colors
  2. A combination of 4 colors is chosen randomly (they can be repeated – for example, blue,blue,blue,blue is a valid combination) – you have to guess that number
  3. You have up to 10 guesses of 4 colors. For each color on the right position, you win a “black” point. For each color in the wrong position, you win a “white” point
  4. If you can’t guess on the 10th try, you loose.

So, first, we’ll create a shadow-cljs app – create a package.json file, fill it with {"name": "Mastermind"}, then run npm install shadow-cljs. Traditional stuff.

Then, we’ll create the shadow-cljs.edn file. It’ll only contain a single target (:browser), opening up a dev-http server so we can serve our code, and we’ll add reagent library dependency. I also added the material-ui dependency, but you don’t really need it for the code. Now, running npx shadow-cljs watch browser will start a webserver at port 3000, and we can start to develop things.
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Please, use the right terms for “typing”…

Names matter. That’s one of the things that I learned, working multiple years on programming software. When you have something called “integration”, for example, that means different things for different people, that’s a recipe for disaster.

That’s why I get quite triggered when people use Untyped / Typed, Weak / Strong typed, Uni / Multi typed, to describe programming languages – because it means different things to different people, or are simply wrong. So let’s dive into it:

Untyped languages or languages without types are quite rare: Assembly is the most interesting example (there’s literally no types in Assembly: everything is a memory address, byte, word, etc). Some virtual machines’ bytecodes are also untyped. In this case, most languages are typed. Ruby, for example, have types, and you can dispatch different behavior depending on the type of the caller (because it’s a class-based object-oriented language). Clojure is also typed, because you can do type-based polymorphism with protocols using defprotocol and defrecord for example. You can also use deftype, and if there’s a specific command to define types, how can we call it “untyped”?

Weak / Strong typed is a term that’s hard to define. Some people say that it’s when you have pointers, because you can bypass the typing at all; others, when the compiler erases typing at run-time; some say that’s when the language tries to coerce and implicitly convert from one type to another, to try to figure out what you want…

Now, a language that allows you to bypass typing because you have pointers is called “memory unsafe”; if the compiler erases typing information, is called “type erasure”; coercion of course is called “coercion”, and can be “automatic” or “manual” coercion (for example, most languages try to coerce integers to decimals at some point); even the ability to convert between types is controversial: on Ruby and Scala, operators are also methods of the class (and, on Ruby, you can rewrite methods on classes that already exist, so automatic conversion is a user-defined feature) – so, please use the right term.

Unityped means a language that only have one type. I believe only some markup languages enter on this classification. Calling a language that have multiple types as unityped because someone imagined that all these multiple types can somewhat collapse into a single one is, at minimum, strange; and also, again, we do have a term for languages like Javascript, Ruby, Python, Smalltalk, and Clojure.
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ClojureScript vs clojure.core.async

I’m going to make a somewhat bold statement: core.async does not work with ClojureScript. And, in this post, I’m going to show some examples why this is true, at least for the current versions of core.async.

So let’s start by understanding a little bit about the runtime: Javascript is a single-threaded runtime that implicitly runs an event-loop. So, for example, when you ask to read a file, you can do it synchronously or asynchronously. If you decide to run in that asynchronously, it means that as soon as you issue the fs.readFile command, you need to register a callback and the control is returned to the “main thread”. It’ll keep running until it runs out of commands to execute, then the runtime will wait the result from the callback; when it returns, the function that you registered will be called with the file contents. When the function ends, the JS runtime will await to see if there’s any other pending call, and it’ll exit if there’s nothing else to do.

The same thing happens in browser environment, but in this case the callbacks are events from the DOM: like clicking on buttons or listening for changes in some elements. The same rules apply here: the runtime is single threaded and when something happens it will first execute everything that needs to be executed, then it will be called back with the event that happened.

So maybe we can change these callbacks with core.async channels right? But the answer is no, because core.asyncs go blocks will not run in different threads (because, again, the runtime is single-threaded). Instead, it creates a state machine and it’ll control of when each of these go blocks will be called, at what time, eventually replacing the event-loop that Javascript environment already have.
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Ubiquitous interface – how to integrate things in Clojure

Inspired by this thread on Reddit, I decided to write a little bit about my experience integrating things in Clojure.

The first thing to understand is that Clojure have an ubiquitous interface: EDN. And it is important to understand what this means. In the beginning, I made this mistake of “Death By Specificity” on my now abandoned Relational project: to abstract things that don’t need to be abstracted.

But can we do even better? How about we de-abstract (concretize? Is this a real word?) things that are already abstracted?
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Strange decisions in Clojure – keyword inheritance

First, a disclaimer: the opinions on these posts are my own, and they reflect (for me) a design decision on the language that I don’t understand, specially considering other decisions that seems to contradict it. I also want to say that Clojure (and ClojureScript) is my favorite language, the one that I enjoy writing on my free time and professionally, so by no means this is a rant on the whole language!

Well, this is a new “series” on this blog: what is on the Clojure language that I don’t like, that I feel is out-of-place, and sometimes I can’t understand? In this first post, “keyword inheritance”. And what is that?

Clojure allows us to use derive to generate a “parent-child” inheritance against keywords. So, for example:

(isa? ::dog ::animal) ; => false
(derive ::dog ::animal)
(isa? ::dog ::animal) ; => true

This will change the way multimethods work too: so, for example, if derive is used and a multimethod expects an ::animal and you send a ::dog, it’ll use the implementation for ::animal:

(defmulti cry :type)
(defmethod cry ::animal [_] "Some animal crying")

(cry {:type ::wolf})
; Execution error (IllegalArgumentException) at user/eval152 (REPL:1).
; No method in multimethod 'cry' for dispatch value: :user/wolf
(cry {:type ::dog})
; => "Some animal crying"

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Introduction to Kafka with Clojure

Recently I was trying to study Kafka, but I didn’t find a single resource that would give me a quick introduction and hands-on experience with it and Clojure. So, I’m making my own here! Don’t expect a “too deep introduction” – this is just the quick-and-dirty introduction about the concepts, and then I’ll show some code examples in Clojure

Kafka is a messaging system similar to RabbitMQ and SQS. The great differential is that it’s faster than both solutions, and works very well in cluster mode. Installing Kafka locally is quite complicated so you probably will wants to use the docker-compose.yaml file below:

version: '2'
services:
  zookeeper:
    image: confluentinc/cp-zookeeper:5.3.1
    hostname: zookeeper
    container_name: zookeeper
    ports:
      - "2181:2181"
    environment:
      ZOOKEEPER_CLIENT_PORT: 2181
      ZOOKEEPER_TICK_TIME: 2000
  kafka:
    image: confluentinc/cp-enterprise-kafka:5.3.1
    hostname: broker
    container_name: broker
    depends_on:
      - zookeeper
    ports:
      - "9092:9092"
    environment:
      KAFKA_BROKER_ID: 1
      KAFKA_ZOOKEEPER_CONNECT: 'zookeeper:2181'
      KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT
      KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://broker:29092,PLAINTEXT_HOST://localhost:9092
      KAFKA_METRIC_REPORTERS: io.confluent.metrics.reporter.ConfluentMetricsReporter
      KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
      KAFKA_GROUP_INITIAL_REBALANCE_DELAY_MS: 0
      CONFLUENT_METRICS_REPORTER_BOOTSTRAP_SERVERS: broker:29092
      CONFLUENT_METRICS_REPORTER_ZOOKEEPER_CONNECT: zookeeper:2181
      CONFLUENT_METRICS_REPORTER_TOPIC_REPLICAS: 1
      CONFLUENT_METRICS_ENABLE: 'true'
      CONFLUENT_SUPPORT_CUSTOMER_ID: 'anonymous'

This file will create the Kafka broker (like a single node of the messaging) and will add Zookeeper (that will allow you to coordinate between different Kafkas, decide which node is the leader, and also participate on the node election when the leader goes down, and other things). You will connect into 9092 port, and then listen and send messages from there.
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Subtyping on functional languages

This is almost a new post on the Function SOLID series. It should be about the Liskov Substitution Princible, but before we talk about it it’s important to understand the concept of subtyping.

Subtyping in Dynamic languages like Clojure or in languages that do not have hierarchical typing like Haskell seems strange. But subtyping is not only a concept about object-oriented programming languages – it’s about restrictions, and the concept of variance. I’m going to try to explain both in this post

To begin with, we can say that a supertype is more generic than a subtype, but that’s not all. In truth, is all about properties that you can prove about a specific data. For example, if we use Clojure as a starting language you could say that the coll? is a property of maps and vectors, but vector? is a property that only applies to vectors. In practice, this means that coll? is a superclass of vector?.

(coll? []) ; => true
(coll? {}) ; => true

(vector? []) ; => true
(vector? {}) ; => false

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My last experience with Ruby

On my last job I tried to go back to Ruby programming language. Not really by choice – but just because it was a language and was familiar with, and my last two jobs in Clojure didn’t really work out for me (not because of the language, really). I even imagined that some things would be easier in Ruby, specially while working with relational databases (something I really did miss while I was working in the Clojure language).

At the time, in my opinion, Rails was still a great framework – maybe a little too complex. And I imagined that somehow there could be better ways of doing Ruby code that could use the best parts of Rails and avoid the worst (like ActiveRecord, for example, maybe even substitute it with Sequel or ROM).

No a year and a half later I’m back to Clojure – and I will probably never go back to Ruby. And the reason is quite simple: I was working with Ruby doing exactly the same code that I did about three years ago. Rails didn’t change that much, but still was a huge mess when you had to upgrade from one version to another. The same problems I had in the past kept happening again and again, and it felt like a huge step backwards for me.
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