Hy at a Glance — A Language a Day, Advent Calendar Day 18/24

About this ‘A Language a Day’ Advent Calendar

This article became a part of my book ‘A Language a Day’, which you can get in both electronic and paper format. Languages covered in the book: C++, Clojure, Crystal, D, Dart, Elixir, Factor, Go, Hack, Hy, Io, Julia, Kotlin, Lua, Mercury, Nim, OCaml, Raku, Rust, Scala, and TypeScript.

Learn more where to get the book

Welcome to Day 18 of this year’s A Language a Day Advent Calendar. Today’s topic is introduction to the Hy programming language.

Facts about the language

• A dialect of List
• Basically, it is Python written with the syntax of Lisp
• Appeared in 2013
• Website: hylang.org

Installing and running Hy

To install Hy, you need to have Python installed first. Then, you simply install a package using pip:

$ pip install git+https://github.com/hylang/hy.git

To run a program, use the hy command-line tool:

$ hy hello-world.hy 

Hy is an interesting language that can help in introducing you to the world of Lisp-like languages. If you are familiar with Python, you can concentrate on re-thinking Python programs as if they would be written in Lisp with all that ‘inverted’ syntax and lots of parentheses.

(print "Hello, World!")

Looking at the Python code

The --spy command-line option allows you to see how your program is being transformed:

$ hy --spy < hello-world.hy 

In the output, you can see the equivalent Python code:

=> print('Hello, World!')

Variables

To create (or to bind) a variable, use the setv built-in feature.

(setv username "John")
(print (+ "Hello, " username "!"))

In this example, you can also see how to concatenate strings, and on a broader scale, how to use operators: instead of a + b, you type (+ a b).

Functions

Functions are created with the defn keyword. It is followed by the function name and the function arguments in square brackets:

(defn greet [username]
    (print (+ "Hello, " username "!")))

(greet "John")

Calling a function is done in a separate pair of parentheses, similarly to how we already invoked print.

Calling a method

There are two ways of calling a method on an object. For example, in the following code, the Python’s str.upper() method is called on a variable. You can use either syntax:

(setv greeting "hello")

(print (greeting.upper))
(print (.upper greeting))

(defn factorial [n]    
    (if (< n 2)
        1
        (* n (factorial (- n 1)))))

(print (factorial 1)) ; 1
(print (factorial 5)) ; 120
(print (factorial 7)) ; 5040

Classes

Let us see how you define classes in Hy with the defclass keyword. Here is a class for keeping the name and the age of a person:

(defclass Person [object]  
  (defn --init-- [self name age]
    (setv self.name name)
    (setv self.age age))

  (defn info [self]
    (+ self.name " is " (str self.age) " y.o.")))

(setv john (Person "John" 30))
(print (.info john)) ; John is 30 y.o.

(setv alla (Person "Alla" 20))
(print (.info alla)) ; Alla is 20 y.o.

Notice how Hy transforms hyphens in identifier names to underscores. In a similar manner, you could, for example, name a method info-string and it will be trans-compiled to info_string.

(defclass Animal [object]
    (defn what [self]
        "it is an animal"))

(defclass Dog [Animal]
    (defn info [self]
        (+ "This is a dog and " (.what self))))

(defclass Cat [Animal]
    (defn info [self]
        (+ "This is a cat and " (.what self))))

(setv zoo [(Cat) (Dog) (Cat) (Dog)])

(for [x zoo]
  (print (.info x)))

This is a cat and it is an animal
This is a dog and it is an animal
This is a cat and it is an animal
This is a dog and it is an animal

Using modules

To implement the next program we need to import a module, so let us see how you do it with Hy:

(import datetime)

(print (.now datetime.datetime))

import time
import threading
 
data = [10, 4, 2, 6, 2, 7, 1, 3]
for n in data:
    delay = n / 10
    timer = threading.Timer(delay, print, [n])
    timer.start()

time.sleep(2)

(import time)
(import threading)
 
(setv data [10, 4, 2, 6, 2, 7, 1, 3])

(for [n data]
    (setv delay (/ n 10))
    (setv timer (threading.Timer delay print [n]))
    (.start timer))

(.sleep time 2)

Resourses

There are not that many resources outside of the Hy website and the corresponding GitHub repository. Nevertheless, this language is a great thing to try before learning any other Lisp-like language.

• Tutorial
• Documentation index
• Hy on GitHub

You can find source codes of the examples shown in this article in my repository on GitHub.

Next: Day 19. Red