I was fiddling with the language for a while to implement bit diddling within a personal project, and liked it a lot. The story is the same for “niche” programming languages: there are not a lot of discussions nor tutorials and guides. I hope that my post will be one of them.

The post aimed at people who are not afraid of tinkering around the command line interfaces, and knew a bit of Vim (Emacs should be fine here, also). Experience with Lisp-family languages (Clojure is the closest one) can also make your life much easier here.

Developing Environment Setup

Janet can be downloaded and tested easily, but I still recommend people to download the source, do sudo make install and sudo make install-jpm-git if it is possible, since the other “ways” can caused some strange bugs from not having installed enough C header files (it feels ugly, but we have to bear with it as Janet was written in C).

For code editing, Janet is well-supported within three editors:

  • Emacs
  • Vim
  • Visual Studio Code

You can find more information at this repository.

As for me, I “engaged” to Vim (Neovim), so Conjure is the natural choice.

use 'olical/conjure'
use 'janet-lang/janet.vim'

A “Nontrivial” Program

I found it simplest to explain my Janet code that serve a “real” purpose, and then help people “picking” useful stuff along the way. Here are my “nontrivial” requirements:

Write a program that picks a random option from an option pool. The picking is random, but also depends on each option’s weight.

For example:

  • We have two options, one is weighted 1, and another is weighted 3.
  • The first one should be picked 1/(1 + 3) = 25% of all the times.
  • The second one should be picked 3/(1 + 3) = 75% of all the times.

For the sake of simplicity, here are the works that are going to be done:

  1. Create a “special” CSV file (how is “special” defined is going to be written later)
  2. Read the “special” CSV file
  3. Mold the CSV file into structured data
  4. Pick one option from the data

“Special” CSV format

We all are familiar with the “normal” CSV:


A “special” one that I invented looks like this:

column 1,                column 2,                column 3
value 1,                 value 2 which is longer, value 3
value 4 which is longer, value 5,                 value 6 filled

Basically, there are spaces padded before the start of each value/header. The spaces are going to be stripped in our reader.

“Special” CSV file

Let us have some sample content:

name,   weight
Vim,    8
Emacs,  3
VSCode, 1

Save the content to any path you want. I am going to save it to /tmp and my path then is /tmp/input.csv

Lisp/Clojure/Janet 101

To define a variable, we use this syntax:

(def ...)

To define a function, we use this syntax:

(defn function-name
  [argument-1 argument-2]

To “execute” a function, we wrap the function name within parenthesis:


To execute a function with arguments, we put them into the execution, next to the function name:

(function-name argument-1 argument-2)

There is only one “main” data structure within Lisp (the language is literraly LISt Processing): linked list. Clojure adds a bit more to that however: map/table/dictionary and set. Janet copies Go and does not implement a delicated set in the hope that people is going to use map instead. It also defaults to mutable data that are prefixed with an @.

# comments in Janet start with `#`

[1 2 3 4] # immutable
{:key value} # immutable

@[1 2 3 4] # mutable
@{:key value} # mutable 

File Reading

File reading in Janet is extremely simple:

(def path "/tmp/input.csv")
(def my-file (file/open path :r))
(def raw-text (file/read my-file :all))
# name,   weight
# Vim,    8
# Emacs,  3
# VSCode, 1

In which we can put into small function:

(defn read-raw-text
  (file/read (file/open path :r)

The code works, but kind of hard to read. We can use the threading macro -> like this:

(defn read-raw-text
  (-> path
      (file/open :r)
      (file/read :all)))

To be simply put, -> is the same as our direct call with much better readability:

(-> argument-1
    (function-1 argument-2)
    (function-2 argument-3))

# the code gets transformed into
# ```
# (function-2 (function-1 argument-1 argument-2) argument-3))
# ```
# look carefully at how `argument-1` gets put into `function-1`, and `(function-1
# ...)` gets put into `function-2`

Data Transformation

Having our raw text read, the natural progress is to transform the data to our needs:

(defn read-special-csv-lines
  (->> raw-text
       (|(string/split "\n" $))
       (map string/trim)
       (|(slice $ 1 -2))

(defn parse-special-csv-line
  (->> csv-line
       (string/split ",")
       (map string/trim)

(defn infere-option
  (def [name weight] values_)
  {:name name
   :weight (parse weight)})

read-special-csv-lines does nothing special:

  • Split the raw text by line
  • Trim white spaces at the beginning and the end of each line
  • Gets the lines, except the first and the last, since we

parse-special-csv-line also does nothing special:

  • Split each line by a comma (,)
  • Trim white spaces at the beginning and the end of each word

infere-option simply structure our data into a table/dictionary/map.

There are a few interesting syntaxes of Janet that can be explained here:

  • ->> is the same as ->, but with a small twist: ->> put the argument into the last position, while -> put the argument into the second position.
  • |(...) is Janet’s syntax for short anonymous function. $ within |(...) is the function’s argument.
  • slice create a copy of the array from a beginning position to an end position. (slice ... 1 -2) means we are skipping the first and the last element of the original array.

Data Pipeline

Our data pipeline then can be seen like this:

(def path "/tmp/input.csv")
(->> path
# @"name,   weight\nVim,    8\nEmacs,  3\nVSCode, 1\n"

We can put everything else into the pipeline naturally:

(->> path
     read-raw-text                # @"name,   weight\nVim,    8\nEmacs,  3\nVSCode, 1\n"
     read-special-csv-lines       # ("Vim,    8" "Emacs,  3" "VSCode, 1")
     (map parse-special-csv-line) # @[@["Vim" "8"] @["Emacs" "3"] @["VSCode" "1"]]
     (map infere-option)          # @[{:name "Vim" :weight 8} {:name "Emacs" :weight 3} {:name "VSCode" :weight 1}]

Functional Processing

Our function to pick an option from the above options looks like this:

(defn pick-an-option
  (let [weights (map |(get $ :weight) options)
        weights-sum (sum weights)
        weights-accumulated (accumulate2 + weights)
        random-threshold (* weights-sum (math/random))
        picked-option-index (find-index
                              (fn [weight]
                                (>= weight random-threshold))
        picked-option (options picked-option-index)]

map and sum may not need to be explained, but accumulate2 surely needs. It takes an operator, and a collection, and assure that the elements of the result are the accumulated values.

(accumulate + [1 2 3 4])
# [1 (+ 1 2) (+ 1 2 3) (+ 1 2 3 4)]

find-index takes a predicate, and a collection, and return the index of the first element that satisfies the predicate.

(find-index |(> 3 $) [1 2 3 4 5])
# 4

A number that gets into a form with an array is the 0-indexed “number-th” element of that array. That was what we have done with picked-option.

([1 2 3] 0)
# 1
([1 2 3] 1)
# 2

Let us slap a quick printf-picked-option

(defn printf-picked-option
  (string "The picked option: "
          (get option :name)))

Into the current pipeline:

(->> path
     (map parse-special-csv-line)
     (map infere-option)

Evaluate it a few times and we immediately see the result:

# --------------------------------------------------------------------------------
# eval (root-form): (->> path read-raw-text read-special-csv-lines (map pars...
"The picked option: Emacs"
# --------------------------------------------------------------------------------
# eval (root-form): (->> path read-raw-text read-special-csv-lines (map pars...
"The picked option: Emacs"
# --------------------------------------------------------------------------------
# eval (root-form): (->> path read-raw-text read-special-csv-lines (map pars...
"The picked option: Vim"
# --------------------------------------------------------------------------------
# eval (root-form): (->> path read-raw-text read-special-csv-lines (map pars...
"The picked option: Vim"
# --------------------------------------------------------------------------------
# eval (root-form): (->> path read-raw-text read-special-csv-lines (map pars...
"The picked option: Vim"
# --------------------------------------------------------------------------------
# eval (root-form): (->> path read-raw-text read-special-csv-lines (map pars...
"The picked option: VSCode"

We have done a lot with just less than 100 lines of code! You can see the full code on this GitHub gist.


I covered the basic of Janet within my post. Reading the input argument and then compiling (yes; it is possible to compile your script into a binary with Janet) is left as exercises for the reader.