Rodrigo Flores's Corner Code, Cats, Books, Coffee

Clojure Ecosystem

One of the things I dislike the most when learning a Programming Language is how frequent you get stuck with something. When dealing with something different than what we’re used to deal with every day we’re pushed out of our confort zone, which is good, but unconfortable. This blog post will give you a few tips on tools that usually help me when I’m writing Clojure code.


REPL is an acronym for Read Eval Print Loop: which means a prompt where you send expressions and it prints the result of that expression. Rubysts are probably familiar with irb or pry and Pythonists are familiar with Python interactive interpreter that comes bundled with the language (just run python without arguments to see it).

To run it, execute lein repl on a Leiningen project directory and you shall see an output like this one:

nREPL server started on port 57755 on host - nrepl://
REPL-y 0.3.7, nREPL 0.2.10
Clojure 1.7.0
Java HotSpot(TM) 64-Bit Server VM 1.8.0_65-b17
    Docs: (doc function-name-here)
          (find-doc "part-of-name-here")
  Source: (source function-name-here)
 Javadoc: (javadoc java-object-or-class-here)
    Exit: Control+D or (exit) or (quit)
 Results: Stored in vars *1, *2, *3, an exception in *e


Just like a shell, it is waiting for your command: you can type:

(+ 2 3)

And it will simply output:


And then it will wait for yout next command. So it read the expression (+ 2 3), evaluated the expression to 5 and then output the result: 5.

An useful function is called doc, it will print the docs for a function:

test.core=> (doc +)
([] [x] [x y] [x y & more])
  Returns the sum of nums. (+) returns 0. Does not auto-promote
  longs, will throw on overflow. See also: +'

What happened here? doc is a function with a side-effect: it prints the documentation for a given function. When you call it with + it prints the documentation for the + function and outputs the return value for the function which is nil.


As you have noticed, the first way to find docs about a function is through the doc command. It is a succint description, going straight to the point on what it does along with the different arities (an arity is a possible number of arguments that a function accepts, naturally you can sum 0 up to infinite number of integers, but a function may only allow a limited number of arities, like the doc function that only accepts one arity). However, sometimes you don’t remember the function name, or an usage example could be useful, on these cases you can use other alternatives.

On there is an online version of what the doc function prints. It may be more visually attactive as the arities are on different lines, but it is essentially the same thing. also provides a really useful cheatsheet, that may also be useful as it provides a quick reference for the most common used functions. If you have already played with another language, you will be familiar with the meaning of most of the functions here. Each entry links to the documentation of that function.

Clojure docs is similar to the documentation on the Clojure website, but also contains examples: you can check here how the arities work: you can call + with an arbitrary number of arguments and it will work.

There is also Grimoire, that is similar to clojure docs, but also contains a link to the function source.



I had my first contact with Clojure through Bruce Tate’s book: Seven Languages in Seven Weeks. It contained a short chapter (to be read and explored within a week) about Clojure. I really liked the reading, but it couldn’t of course go deeply into Clojure. So, my first book to learn Clojure was Stuart Halloway’s and Aaron Bedra Programming Clojure: a really nice book that teaches you not only the Clojure language, but also the Clojure Way. And speaking about Clojure Way, my next reading was Michael Fogus Joy of Clojure a book maybe more targeted to someone with already some knowledge on the language as it goes deeply into concepts and more subtle things.

There is also good options that were recommended by some friends: Clojure for the Brave and the True, Clojure Programming, Clojure Programming and Living Clojure: this last one deserves a special highlight, it contains a training guide to learn Clojure (similar to a Running program like Couch to 5k but for Clojure).


There are a few websites or projects to learn Clojure online. Back on my first days I tried Clojure Koans and I really liked: is some kind of filling the gaps exercise to make some expressions to be true. Now I see that there are other cool online resources: Try Clj: to try Clojure with an interactive tutorial of the language and 4Clojure with a similar experience compared to Clojure Koans but you do both without even installing anything on your computer.

Lastly, there is Wonderland Clojure Katas, a series of proposed puzzles to be solved with Clojure: the exercises come with a failing test suite and you should build code to make it pass.


After some years of Ruby, I discovered that the best thing you can do with your code is to write tests for it. It works like a safety net: no matter what you do with your code, if the tests are passing you can be sure that it behaves the way it should (or at least the way it was before the changes). And as learning a programming language requires experimentation, testing is a fast way to see if that new thing works the same way than that not-so idiomatic thing you did before.

The Clojure language itself has a test framework called Clojure Test. It comes bundled with the language, so the only thing you need to do is require the namespace and write your tests. To try Clojure Test, you can do this on a REPL:

(require '[clojure.test :refer [deftest is run-all-tests]])

(deftest one-is-one (is (= 1 1)))
(deftest two-is-one (is (= 2 1)))


The first line requires the test namespace and the :refer [deftest ...] part is just a way to avoid having to call the namespace (or an alias) everytime you call something of that namespace. Then we define the tests: the deftest registers your test expressions as tests and run-all-tests looks up for this definitions and calls it. To assert the value of something you can use the is macro (for now, think on a macro as a special type of function that gets evaluated before the code is run: we will get on macros soon): it will show a summary of the test run and also output all failed tests (the falsy expressions).

FAIL in (two-is-one) (form-init7127606434032677106.clj:1)
expected: (= 2 1)
  actual: (not (= 2 1))

Ran 2 tests containing 2 assertions.
1 failures, 0 errors.
{:test 2, :pass 1, :fail 1, :error 0, :type :summary}

Another popular testing framework is Midje. It has a quite simple syntax fn-call => expected result. After setting up midje (as it isn’t bundled in the language you will need to include it on your project), you can write tests this way:

(facts "sum function"
       (fact "one + one is two"
             (+ 1 1) => 2)
       (fact "one + two is four"
             (+ 1 2) => 4))

And the error display is really nice (and colourful):

> (require '[midje.repl :refer :all])
> (autotest)
Loading (test-app-midje.core test-app-midje.core-test)

FAIL "sum function - one + two is four" at (core_test.clj:9)
    Expected: 4
    Actual: 3
FAILURE: 1 check failed.  (But 1 succeeded.)

There is also plenty of other cool features: mocking, loose checkers, exception thrower checker and even the possibility to write custom checking functions. The project wiki on Github also contains some documentation on how to get started with Midje.

Editors & IDEs

As Clojure is becoming more popular each day, I’m pretty sure that almost all popular editors and IDEs may have support for Clojure syntax (or at least a plugin for it). I’ve already written Clojure code in Vim, Lighttable, Emacs, Sublime Text, Atom and IntelliJ with Cursive. I also know that Eclipse also have a plugin for Clojure called Counterclockwise.

But as being a dialect of LISP, you may need more things on an editor/IDE than simply syntax highlighting and REPL integration: you will need a smarter way to deal with open and close parenthesis/brackets/quotes. One of the things that make me more productive in Clojure, is the ability to deal with open and close parenthesis in a more structured way that is called Paredit or Structural Editing. This feature automatically adds a closing parenthesis/bracket/quote when you add a opening parenthesis/bracket/quote. Also, it has a few more commands to manipulate those expresions in order to be able to edit your code more efficiently. You can see a few examples of commands on this blog post.

Of course, this is not mandatory: you can write Clojure without using this techniquebut if you want to be really productive with Clojure, learning this technique is important. But feel free to skip it for now: I would only ask one thing: try to see if it exists on your favourite editor: if it does, that’s fantastic, if it does not, another editor might be a good choice for your Clojure adventure.


The best place to find a library is Clojars: it is an equivalente of RubyGems/CPAN for Clojure: there is a search where you can search for the library name or for a term like http, soap or aws, and it will return all the libraries on clojars that match that term. Also, due to Clojure interoperabilty with Java, you can also include and use libraries from Maven.

After finding the library, you can add it to your project through the project.clj dependencies (I explained how to do so on First Steps with Clojure post.


Just like all programming language communities, the Clojure community has some mailing lists, a Planet, an IRC channel and some wikis. You can check a list of these resources on the community page of the Clojure website. Another source for Clojure news and discussions is the Clojure Subreddit where people usually post links, ask questions and discuss. From time to time, there is a thread called “Ask Anything”, where new Clojurists can ask any questions.


You can also check some Video resources:


The goal of this blog post is to talk about some useful resources when learning Clojure. So, I hope you found your way through the REPL, was able to write tests, found a good editor, learned where to search for libraries and where to ask questions and discuss about the language.

Next blog post I will go deeper into Leiningen: the idea is to create a boilerplate for Clojure apps.

First steps in Clojure

I remember the first time I had to deal with Clojure. I just came back after a job interview and I was asked to deliver a solution on a functional language to an exercise. Naturally, I opened a terminal and typed:

$ brew install clojure

brew is the package manager that install open source stuff on OS X. But instead of installing Clojure, homebrew returned this error:

Error: No available formula with the name "clojure"
Clojure isn't really a program but a library managed as part of a
project and Leiningen is the user interface to that library.

To install Clojure you should install Leiningen:
  brew install leiningen
and then follow the tutorial:

Unlike Ruby or Python, Clojure does not have a native compiler for OSX or Linux: it runs on the Java Virtual Machine (also called JVM). So, you don’t need to install a compiler, you just need to install Java, download a JAR and you should be ready to run Clojure.

Running Clojure from the JAR

But wait, the error said that I need to install Leiningen? Yes, we will get on Leiningen on a moment. But, for now let’s run Clojure directly from a JAR. You can download the most recent version of the Clojure JAR from this website: and unzip it. Then, you can run on a terminal:

java -cp clojure-1.7.0.jar clojure.main

This -cp option allows you to specify a list of directories, JAR archives and ZIP archives to the Java application launcher, the other argument clojure.main will be the namespace whose function -main will be called. You can also run a Clojure file like a script by passing the Clojure source file as another argument. To make it happen, I’m adding this content to a file called hello_world.clj:

(println "Hello world")

And I will run:

java -cp clojure-1.7.0.jar clojure.main hello.clj

And now you have run your first Clojure program. It is a simple program, the only dependency is on Clojure language. What if I want to write a program that needs to do an HTTP request? HTTP Kit is probably the most popular HTTP client in Clojure (and also can be used as a Server). Let’s write a code to do a HTTP request:

(require '[org.httpkit.client :as http])
(println @(http/get ""))

For now, no need to focus on the code, just trust me that it will work: I will explain what is this @ later. To run it, you will need something more: you will need to specify the http-kit JAR.

java -cp clojure-1.7.0.jar:http-kit-2.1.19.jar clojure.main hello.clj

And voilá, it should work.


Definitely this is not a straightforward way to run code. You will need to manually download JARs, and remember to insert all of them on your command to run your program. Happily, Leiningen can help us to make it really easy to bootstrap a Clojure application, to manage the dependency and to create different profiles (think of a profile as a “different” way to run your application). If you’re familiar with Ruby on Rails, it does basically what Bundler (dependency management), Rake (make-like tool to run different profiles of your application) and Thor (code bootstrap generator). It also compiles your Clojure code into JVM bytecode and also can create a JAR file for you to export it. If you wabt to a comprehensive explanation on everything that Leiningen does, you can check this excellent article about it.

So, to bootstrap a Clojure application with Leiningen you first need to install Leinigen: you can do so by downloading a script, through a package manager or if you’re a M$ Windows user, you can use a installer.

After installing it, you should be able to run

lein new app blog-example

lein is the command to invoke Leinigen, new tells Leinigen to bootstrap a new project, app is the template (this argument is optional and it defaults to a library project) and finally blog-example is the name of the application we’re bootstrapping.

It should have generated something similar to this:	project.clj	src
LICENSE		doc		resources	test







To build our hello world application, let’s for now focus on two files project.clj and src/blog_example/core.clj. project.clj is the project declaration: it contains the configuration that will be used to run your project, the name, description, the license information, etc.

(defproject blog-example "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url ""
  :license {:name "Eclipse Public License"
            :url ""}
  :dependencies [[org.clojure/clojure "1.7.0"]]
  :main ^:skip-aot blog-example.core
  :target-path "target/%s"
  :profiles {:uberjar {:aot :all}})

Let’s focus on two things on this file: :dependencies and :main: the first one is a vector (for now, think of vectors as the same as what is called as an array: I will explain more about sequences later) of all the dependencies along with the versions, and :main contains the namespace that will be run when you run lein run: the function that will actually run is the one called -main. Let’s take a look at the file core.clj in the directory src/blog_example:

(ns blog-example.core

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (println "Hello, World!"))

The -main only contains a println call that outputs "Hello World", and this function will be called when you call lein run.

The coolest thing about Leiningen is that you don’t even need to install Clojure to run the project: just install the java, Leiningen and the first time you run the program, it will install all the dependencies (including Clojure). Also, you don’t even need Leiningen to run your program: you can easily pack it with lein uberjar and run with java -jar jarfile.jar and it will run your Clojure program. So, to summarize Leiningen helps a lot.

If you want to add the http-kit you can add it to the dependencies vector:

lein deps only install the dependencies: but if you run lein run and there is a missing dependency it will install it anyway.

Last, you can add the same code inside the -main function:

(ns blog-example.core
  (:require [org.httpkit.client :as http])

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (println "Hello, World!")
  (println @(http/get "")))

And then running lein run will actually do the request.

Missing Parts

I know: I’ve skipped some parts to make your understanding of this basic tutorial easier. I will make a quickly explanation here of those parts:

  • @ signal on http call: this is related to the asynchronous nature of HTTP Kit: a call without to the http/get function will be asynchonous and a callback can be passed as an argument and the code inside the callback will be run once the request returns something. Using @ actually turns it synchronously: it will do the request and wait the result. This construction is called promise and I will cover it later.

  • project.clj: besides your app metadata like description and license, it also includes the dependencies, where your compiled code will be generated target-path, the main function (:main) that will be called on lein run and the profiles: that are different possible switches that you can enable or disable: like testing libraries, debug libraries and so on: by default the only profile tells that when generating an jar through the uberjar command it should do the ahead of time compilation. Remember this is just a subset of all possible options for a a Project.clj: it has lot of possible options and configurations and all of them are explained here.

  • core.clj: ns is the namespace declaration: it contains what namespaces should also be required and may also contain a gen-class that will generated a named Java class: this is useful here because the JVM requires a public main method and this gen-class directive will generate this method. Last, the -main function contain an & args as argument: it will contain the arguments given on the command line this & is actually a way to tell a clojure function that the function may receive an arbitrary number of arguments (besides the ones declared before the &) and will be stored on a vector.


So I hope you learned a few things here:

  • How to “install” Clojure
  • How to run Clojure code only with Java
  • How to install Leiningen
  • How to run code inside Leiningen
  • How to add a new dependency
  • How to require it on your library

Next time I will speak more about the Clojure ecosystem: what they are and how they work:

  • Where to look for Libraries;
  • Editor recommendations;
  • Project.clj overview;
  • Core.clj overview;

8 Small git tips

As git is one of my daily tools, I’ve compiled 8 useful (and short) tips that I use almost everyday.

Selectively add with -p

When you want to commit something, you can either: select all the files through git commit -am or add a specific file through git add file. However, you may want to only add only a part of a file to the commit. You can use git add -p and select interactively what parts you want to add to your commit.

After selecting interactively all the chunks you want to commit, just do a git commit (without -a) and you will only commit the selected parts. There is also the git checkout -p to select changes to be reverted. After adding, you can see what you have selected using git diff --cached.

Interactive Rebase

If you’re working on a branch and you made some WIP commits that you want to squash or you want to remove a commit plus the reversion of this commit, you can do a interactive rebase to reorganize your commits.

To do it, you just need to run git rebase -i <commit> where the <commit> is the sha1 of a commit before the one you want begin rewriting. Then it will open on your editor (the one specified on $EDITOR env var or the one specified on your git config) some instructions to change your commits history where you can pick, squash (merge two commits on a single one), reword (change the commit message), edit or even remove a commit.

Be aware that this change the history, so if you have pushed this commits, you will have to force to push it again, so never do this on master branches or branches that you’re not the only one using.


If you in the middle of something and you have to change a context to fix something else, you can git stash the current changes. However, you may end up forgetting about this stashed changes some time later. So I try to keep a stash zero (just like inbox zero but for stashes) policy. Every time I have a stash a dollar sign appears on my prompt and then I check it through git stash show -p and then I can pop it through git stash pop or discard it through git stash clear.

Global gitignore

You can specify what files git should ignore through a .gitignore file on the root of a project. But if you have files that git should ignore but you’re the only one that generates that files (like vim’s bkp files or something that your editor or OS generates like .DS_Store files for OSX), you can specify on your configure a global gitignore file that uses the same syntax of a project .gitignore that will .

git config --global core.excludesfile=/Users/flores/.gitignore

Warn whitespace

I have to admit: sometimes I forget some trailing whitespaces on my code. But I normally don’t commit them because I use this option: apply.whitespace=warn. Every time I’m adding a chunk of a file through git add -p, and this chunk contains trailing whitespaces, git warns me of it so I can fix it before committing.

Auto setup rebase

Another cool tip is to auto setup rebase: if you have a branch with some commits that aren’t pushed and someone else also commits and pushes on that branch, when you pull, git will create a commit merging your commits to the commits upstream. As this commit is meaningless, I prefer to setup auto rebase on pull through the configuration: branch.autosetuprebase=always. Doing so, on every pull, git will try to reapply your commits with the current version of the upstream branches.

Better logging

Have you tried to find an specific commit merged from a branch ? git log provides some basic information, but you can use git log with a more useful message:

git log --graph --decorate --pretty=oneline --abbrev-commit

--graph will generate lines between commits and will expose branches, --decorate shows where branches are located in this commits, --pretty=oneline will show only the sha1 and the title line and --abbrev-commit will reduce the sha1 to the first 7 chars (which is normally unique in your repository). You can check a more detailed explanation of these options (and a whole lot more) on explain shell.

Rewrite a commit message

So you’ve commited something but did a poor job describing it. Or else, you just made a typo. You can rewrite the message with git commit --amend. You can use the -m to set a message through the command line or it will open the default editor with the commit message so you can change. You can also include new things with git add and add it to the previous commit. Remember that just like Interactive Rebase, it changes the history, so if you have pushed that commit, you will need to force push this changes.

If you use git on your job, you probably know a git trick that increases your productivity. Can you share it on the comments ?