Karen Rustard's Cuddles

Hy's mascot, Cuddles the cuttlefish.

This chapter provides a quick introduction to Hy. It assumes a basic background in programming, but no specific prior knowledge of Python or Lisp.

Lisp-stick on a Python

Let's start with the classic:

(print "Hy, world!")

This program calls the print() function, which, like all of Python's built-in functions, is available in Hy.

All of Python's binary and unary operators are available, too, although == is spelled = in deference to Lisp tradition. Here's how we'd use the addition operator +:

(+ 1 3)

This code returns 4. It's equivalent to 1 + 3 in Python and many other languages. Languages in the Lisp family, including Hy, use a prefix syntax: +, just like print or sqrt, appears before all of its arguments. The call is delimited by parentheses, but the opening parenthesis appears before the operator being called instead of after it, so instead of sqrt(2), we write (sqrt 2). Multiple arguments, such as the two integers in (+ 1 3), are separated by whitespace. Many operators, including +, allow more than two arguments: (+ 1 2 3) is equivalent to 1 + 2 + 3.

Here's a more complex example:

(- (* (+ 1 3 88) 2) 8)

This code returns 176. Why? You can see the infix equivalent with the command echo "(- (* (+ 1 3 88) 2) 8)" | hy2py, which returns the Python code corresponding to the given Hy code. Or you can pass the --spy option to Hy when starting the interactive read-eval-print loop (REPL), which shows the Python equivalent of each input line before the result. The infix equivalent in this case is:

((1 + 3 + 88) * 2) - 8

To evaluate this infix expression, you'd of course evaluate the innermost parenthesized expression first and work your way outwards. The same goes for Lisp. Here's what we'd get by evaluating the above Hy code one step at a time:

(- (* (+ 1 3 88) 2) 8)
(- (* 92 2) 8)
(- 184 8)

The basic unit of Lisp syntax, which is similar to a C or Python expression, is the form. 92, *, and (* 92 2) are all forms. A Lisp program consists of a sequence of forms nested within forms. Forms are typically separated from each other by whitespace, but some forms, such as string literals ("Hy, world!"), can contain whitespace themselves. An expression is a form enclosed in parentheses; its first child form, called the head, determines what the expression does, and should generally be a function or macro. Functions, the most ordinary sort of head, constitute reusable pieces of code that can take in arguments and return a value. Macros (described in more detail below) are a special kind of function that's executed at compile-time and returns code to be executed at run-time.

Comments start with a ; character and continue till the end of the line. A comment is functionally equivalent to whitespace.

(setv password "susan")   ; My daughter's name

Although # isn't a comment character in Hy, a Hy program can begin with a shebang line, which Hy itself will ignore:

#!/usr/bin/env hy
(print "Make me executable, and run me!")


Hy has literal syntax for all of the same data types that Python does. Here's an example of Hy code for each type and the Python equivalent.

























#(1 2 3)

(1, 2, 3)


[1 2 3]

[1, 2, 3]


#{1 2 3}

{1, 2, 3}


{1 2  3 4}

{1: 2, 3: 4}


The Hy REPL prints output in Hy syntax by default, with the function hy.repr:

=> [1 2 3]
[1 2 3]

But if you start Hy like this:

$ hy --repl-output-fn=repr

the REPL will use Python's native repr() function instead, so you'll see values in Python syntax:

=> [1 2 3]
[1, 2, 3]

Basic operations

Set variables with setv:

(setv zone-plane 8)

Access the elements of a list, dictionary, or other data structure with get:

(setv fruit ["apple" "banana" "cantaloupe"])
(print (get fruit 0))  ; => apple
(setv (get fruit 1) "durian")
(print (get fruit 1))  ; => durian

Access a range of elements in an ordered structure with cut:

(print (cut "abcdef" 1 4))  ; => bcd

Conditional logic can be built with if:

(if (= 1 1)
  (print "Math works. The universe is safe.")
  (print "Math has failed. The universe is doomed."))

As in this example, if is called like (if CONDITION THEN ELSE). It executes and returns the form THEN if CONDITION is true (according to bool) and ELSE otherwise.

What if you want to use more than form in place of the THEN or ELSE clauses, or in place of CONDITION, for that matter? Use the macro do (known more traditionally in Lisp as progn), which combines several forms into one, returning the last:

(if (do (print "Let's check.") (= 1 1))
    (print "Math works.")
    (print "The universe is safe."))
    (print "Math has failed.")
    (print "The universe is doomed.")))

For branching on more than one case, try cond:

(setv somevar 33)
  (> somevar 50)
    (print "That variable is too big!")
  (< somevar 10)
    (print "That variable is too small!")
    (print "That variable is jussssst right!"))

The macro (when CONDITION THEN-1 THEN-2 …) is shorthand for (if CONDITION (do THEN-1 THEN-2 …) None).

Hy's basic loops are while and for:

(setv x 3)
(while (> x 0)
  (print x)
  (setv x (- x 1)))  ; => 3 2 1

(for [x [1 2 3]]
  (print x))         ; => 1 2 3

A more functional way to iterate is provided by the comprehension forms such as lfor. Whereas for always returns None, lfor returns a list with one element per iteration.

(print (lfor  x [1 2 3]  (* x 2)))  ; => [2, 4, 6]

Functions, classes, and modules

Define named functions with defn:

(defn fib [n]
  (if (< n 2)
    (+ (fib (- n 1)) (fib (- n 2)))))
(print (fib 8))  ; => 21

Define anonymous functions with fn:

(print (list (filter (fn [x] (% x 2)) (range 10))))
  ; => [1, 3, 5, 7, 9]

Special symbols in the parameter list of defn or fn allow you to indicate optional arguments, provide default values, and collect unlisted arguments:

(defn test [a b [c None] [d "x"] #* e]
  [a b c d e])
(print (test 1 2))            ; => [1, 2, None, 'x', ()]
(print (test 1 2 3 4 5 6 7))  ; => [1, 2, 3, 4, (5, 6, 7)]

Set a function parameter by name with a :keyword:

(test 1 2 :d "y")             ; => [1, 2, None, 'y', ()]

Note that unlike Python, Hy doesn't always evaluate function arguments (or the items in a literal list, or the items in a literal dictionary, etc.) in the order they appear in the code. But you can always force a particular evaluation order with do, or with other macros that provide an implicit do, like when or fn.

Define classes with defclass:

(defclass FooBar []
  (defn __init__ [self x]
    (setv self.x x))
  (defn get-x [self]

Here we create a new instance fb of FooBar and access its attributes by various means:

(setv fb (FooBar 15))
(print fb.x)         ; => 15
(print (. fb x))     ; => 15
(print (.get-x fb))  ; => 15
(print (fb.get-x))   ; => 15

Note that syntax like fb.x and fb.get-x only works when the object being invoked (fb, in this case) is a simple variable name. To get an attribute or call a method of an arbitrary form FORM, you must use the syntax (. FORM x) or (.get-x FORM), or call getattr().

Access an external module, whether written in Python or Hy, with import:

(import math)
(print (math.sqrt 2))  ; => 1.4142135623730951

Or use the one-shot import syntax hy.I:

(print (hy.I.math.sqrt 2))

Python can import a Hy module like any other module so long as Hy itself has been imported first, which, of course, must have already happened if you're running a Hy program.


Macros are the basic metaprogramming tool of Lisp. A macro is a function that is called at compile time (i.e., when a Hy program is being translated to Python ast objects) and returns code, which becomes part of the final program. Here's a simple example:

(print "Executing")
(defmacro m []
  (print "Now for a slow computation")
  (setv x (% (** 10 10 7) 3))
  (print "Done computing")
(print "Value:" (m))
(print "Done executing")

If you run this program twice in a row, you'll see this:

$ hy example.hy
Now for a slow computation
Done computing
Value: 1
Done executing
$ hy example.hy
Value: 1
Done executing

The slow computation is performed while compiling the program on its first invocation. Only after the whole program is compiled does normal execution begin from the top, printing "Executing". When the program is called a second time, it is run from the previously compiled bytecode, which is equivalent to simply:

(print "Executing")
(print "Value:" 1)
(print "Done executing")

Our macro m has an especially simple return value, an integer (int), which at compile-time is converted to an integer model (hy.models.Integer). In general, macros can return arbitrary Hy models to be executed as code. There are several helper macros that make it easy to construct forms programmatically, such as quote ('), quasiquote (`), unquote (~), unquote-splice (~@), and defmacro!. The previous chapter has a simple example of using ` and ~@ to define a new control construct do-while.

What if you want to use a macro that's defined in a different module? import won't help, because it merely translates to a Python import statement that's executed at run-time, and macros are expanded at compile-time, that is, during the translation from Hy to Python. Instead, use require, which imports the module and makes macros available at compile-time. require uses the same syntax as import.

(require some-module.macros)
(some-module.macros.rev (1 2 3 +))  ; => 6

Hy also supports reader macros, which are similar to ordinary macros, but operate on raw source text rather than pre-parsed Hy forms. They can choose how much of the source code to consume after the point they are called, and return any code. Thus, reader macros can add entirely new syntax to Hy. For example, you could add a literal notation for Python's decimal.Decimal class like so:

(defreader d
   (.slurp-space &reader)
   `(hy.I.decimal.Decimal ~(.read-ident &reader)))
(print (repr #d .1))          ; => Decimal('0.1')
(import fractions [Fraction])
(print (Fraction #d .1))      ; => 1/10
;; Contrast with the normal floating-point .1:
(print (Fraction .1))         ; => 3602879701896397/36028797018963968

require can pull in a reader macro defined in a different module with syntax like (require mymodule :readers [d]).

Next steps

You now know enough to be dangerous with Hy. You may now smile villainously and sneak off to your Hydeaway to do unspeakable things.

Refer to Python's documentation for the details of Python semantics. In particular, the Python tutorial can be helpful even if you have no interest in writing your own Python code, because it will introduce you to the semantics, and you'll need a reading knowledge of Python syntax to understand example code for Python libraries.

Refer to the rest of this manual for Hy-specific features. Like Hy itself, the manual is incomplete, but contributions are always welcome. See the wiki for tips on getting Hy to work with other software. For an official full-blown example Hy program, see Infinitesimal Quest 2 + ε.

Bear in mind that Hy is still unstable, and with each release along the way to Hy 1.0, there are new breaking changes. Refer to the NEWS file for how to update your code when you upgrade Hy, and be sure you're reading the version of this manual (shown at the top of each page) that matches the version of Hy you're running.