Tutorial

Karen Rustard's Cuddles

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? We 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 by passing the --spy option to Hy when starting the 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)
176

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, macro, or special operator. Functions are the most ordinary sort of head, whereas macros (described in more detail below) are functions executed at compile-time instead and return code to be executed at run-time. Special operators are one of a fixed set of names that are hard-coded into the compiler, and used to implement everything else.

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

(print (** 2 64))   ; Max 64-bit unsigned integer value

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!")

Literals

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.

Hy Python Type
1 1 int
1.2 1.2 float
4j 4j complex
True True bool
None None NoneType
"hy" 'hy' str
b"hy" b'hy' bytes
(, 1 2 3) (1, 2, 3) tuple
[1 2 3] [1, 2, 3] list
#{1 2 3} {1, 2, 3} set
{1 2  3 4} {1: 2, 3: 4} dict

In addition, Hy has a Clojure-style literal syntax for fractions.Fraction: 1/3 is equivalent to fractions.Fraction(1, 3).

The Hy REPL prints output in Python syntax by default:

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

But if you start Hy like this (a shell alias might be helpful):

$ hy --repl-output-fn=hy.contrib.hy-repr.hy-repr

the interactive mode will use hy-repr instead of Python’s native repr function to print out values, so you’ll see values in Hy 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* / if-not:

(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. If ELSE is omitted, None is used in its place.

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 special operator 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))
  (do
    (print "Math works.")
    (print "The universe is safe."))
  (do
    (print "Math has failed.")
    (print "The universe is doomed.")))

For branching on more than one case, try cond:

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

The macro (when CONDITION THEN-1 THEN-2 …) is shorthand for (if CONDITION (do THEN-1 THEN-2 …)). unless works the same as when, but inverts the condition with not.

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)
    n
    (+ (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 &optional c [d "x"] &rest 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', ()]

Define classes with defclass:

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

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).

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

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

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

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")
  x)
(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
Executing
Value: 1
Done executing
$ hy example.hy
Executing
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, which at compile-time is converted to an integer literal. In general, macros can return arbitrary Hy forms to be executed as code. There are several special operators and macros that make it easy to construct forms programmatically, such as quote ('), quasiquote (`), unquote (~), and defmacro!. The previous chapter has a simple example of using ` and ~ to define a new control construct do-while.

Sometimes it’s nice to be able to call a one-parameter macro without parentheses. Tag macros allow this. The name of a tag macro is often just one character long, but since Hy allows most Unicode characters in the name of a macro (or ordinary variable), you won’t out of characters soon.

=> (deftag ↻ [code]
...  (setv op (last code) params (list (butlast code)))
...  `(~op ~@params))
=> #↻(1 2 3 +)
6

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 tutorial.macros)
=> (tutorial.macros.rev (1 2 3 +))
6

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 documention for the details of Python semantics, and the rest of this manual for Hy-specific features. Like Hy itself, the manual is incomplete, but contributions are always welcome.