Creating a TODO cli with Clojure

Recently I decided to learn clojure, a functional programming language. In this blog I've documented my experience with creating a standalone todo client using clojure.

I began by creating a new project using Leiningen.

Leiningen is the easiest way to use Clojure. With a focus on project automation and declarative configuration, it gets out of your way and lets you focus on your code.

$ lein new app todo

The above command asks Leiningen to create a new project based on application project template.

Folder Structure

Lets take a look at the project folder structure

.
├── CHANGELOG.md
├── doc
│   └── intro.md
├── example.txt
├── hello.txt
├── LICENSE
├── project.clj
├── README.md
├── resources
├── src
│   └── todo
│       └── core.clj
└── test
    └── todo
        └── core_test.clj

The main application logic lies under src/todo/core.clj

(ns todo.core
  (:gen-class)
  (:require [clojure.string :refer [split]])
  (:require [clojure.java.io :refer [writer reader]]))

(use 'clojure.pprint)

(def file-location (System/getenv "TODO_LOCATION"))

(defn now [] (new java.util.Date))

(defn add-content
  "appends content to todo file"
  [file-location text-content]
  (with-open [file (writer file-location :append true)]
    (.write file (str text-content "\t" (now) "\n"))))

(defn print-helper
  "Converts line content to a row obj"
  [line-content]
  (let [[todo created_at] (split line-content #"\t")]
    {:todo todo :created_at (or created_at "UNKNOWN")}))

(defn read-content
  "reads content from todo file"
  [file-location]
  (with-open [file (reader file-location)]
    (let [file-content (slurp file)]
      (print-table
       (map print-helper
            (split file-content #"\n"))))))

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (if (nil? file-location)
    (throw (AssertionError. "empty $TODO_LOCATION")))
  (case (first args)
    "a" (do
          (add-content file-location (second args))
          (read-content file-location))
    "ls" (read-content file-location)
    (println "Choose either a or ls")))

Before going into what each function means, lets see what the app can do

$ lein run a "practise clojure"

|            :todo |                  :created_at |
|------------------+------------------------------|
| practise clojure | Fri Jul 24 17:51:27 BST 2020 |

$ lein run ls

|            :todo |                  :created_at |
|------------------+------------------------------|
| practise clojure | Fri Jul 24 17:51:27 BST 2020 |

Running the application

Running lein run executes the -main function and passes the command line arguments to it.There are two actionable commands that you can pass to the main function

a : adds text content to to-do's

ls: Lists all the to-do's

Lets have a look at what each function does. I have two functions handling the main use cases of the application

add-content

Function accepts a file location and text content as arguments.It then writes the text content to a file along with the time at which the content was added. Contents are separated by tab and a newline is added at the end.

read-content

Function accepts a file location as argument.It then reads all the file contents using slurp . Splits every line in content then prints the results

I have a couple of helper functions too

now

Function spits out a new date object whenever it gets called

print-helper

Function helps converting text content into a map so that I can print my to-do content in a table format

Building the app

To package up the project files and dependencies into a jar file, you can run

$ lein uberjar
Compiling todo.core
Created /mnt/d/todo-cli/target/uberjar/todo-0.1.0-SNAPSHOT.jar
Created /mnt/d/todo-cli/target/uberjar/todo-0.1.0-SNAPSHOT-standalone.jar

You can then run the app using

$ java -jar target/uberjar/todo-0.1.0-SNAPSHOT-standalone.jar

I noticed it was pretty tedious to run the built app this way. It would be pretty cool if we could run our app stand alone without having to invoke java -jar every time.

I searched for a solution that could make this possible and found lein-bin.

Leiningen plugin for producing standalone console executables that work on OS X, Linux, and Windows.
It basically just takes your uberjar and stuffs it in another file with some fancy magical execution stuff.

I added lein-bin to my ~/.lein/profiles.clj.

{:user {:plugins [[lein-bin "0.3.5"]]}}

Modified my ./project.clj to look like below

(defproject todo "0.1.0-SNAPSHOT"
  :description "a todo cli"
  :url "https://github.com/PrasannaGnanaraj/todo-cli"
  :license {:name "EPL-2.0 OR GPL-2.0-or-later WITH Classpath-exception-2.0"
            :url "https://www.eclipse.org/legal/epl-2.0/"}
  :dependencies [[org.clojure/clojure "1.10.1"]]
  :main ^:skip-aot todo.core
  :target-path "target/%s"
  :profiles {:uberjar {:aot :all}}
  :bin {:name "todo"
        :bin-path "~/bin"
        :bootclasspath true})

with the :bin key you can specify the name of your standalone executable , the path in which you would like your executable to be placed

After which if you run

$ lein bin                                                                                                                                                                                  ok | at 18:24:21
Compiling todo.core
Created /mnt/d/todo-cli/target/uberjar/todo-0.1.0-SNAPSHOT.jar
Created /mnt/d/todo-cli/target/uberjar/todo-0.1.0-SNAPSHOT-standalone.jar
Creating standalone executable: /mnt/d/todo-cli/target/default/todo
Copying binary to #object[java.io.File 0x7c40ffef /home/user/bin]

you will then be able to use the application like a stand alone executable like

$ todo a "practise clojure"
|            :todo |                  :created_at |
|------------------+------------------------------|
| practise clojure | Fri Jul 24 17:51:27 BST 2020 |

Overall I found Clojure and its functional programming paradigms help you make software that has

Clear separation in terms of responsibilities
Easy to modify/refactor/build on top of
Easy to Understand / Debug

I would continue learning and exploring Clojure by trying to apply Clojure to real life software problems.

You can find the source code in Github here

Also , Ive found this book incredibly useful in my clojure learning path.
Learn to Program the World's Most Bodacious Language with Clojure for the Brave and True

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