Internal Hy Documentation


These bits are mostly useful for folks who hack on Hy itself, but can also be used for those delving deeper in macro programming.

Hy Models

Introduction to Hy Models

Hy models are a very thin layer on top of regular Python objects, representing Hy source code as data. Models only add source position information, and a handful of methods to support clean manipulation of Hy source code, for instance in macros. To achieve that goal, Hy models are mixins of a base Python class and Object Protocol.


hy.models.Object is the base class of Hy models. It only implements one method, replace, which replaces the source position of the current object with the one passed as argument. This allows us to keep track of the original position of expressions that get modified by macros, be that in the compiler or in pure hy macros.

Object is not intended to be used directly to instantiate Hy models, but only as a mixin for other classes.

Compound Models

Parenthesized and bracketed lists are parsed as compound models by the Hy parser.

Hy uses pretty-printing reprs for its compound models by default. If this is causing issues, it can be turned off globally by setting hy.models.PRETTY to False, or temporarily by using the hy.models.pretty context manager.

Hy also attempts to color pretty reprs and errors using colorama. These can be turned off globally by setting hy.models.COLORED and hy.errors.COLORED, respectively, to False.


hy.models.Sequence is the abstract base class of “iterable” Hy models, such as hy.models.Expression and hy.models.List.

Adding a Sequence to another iterable object reuses the class of the left-hand-side object, a useful behavior when you want to concatenate Hy objects in a macro, for instance.

Sequences are (mostly) immutable: you can’t add, modify, or remove elements. You can still append to a variable containing a Sequence with += and otherwise construct new Sequences out of old ones.


hy.models.List is a Sequence Protocol for bracketed [] lists, which, when used as a top-level expression, translate to Python list literals in the compilation phase.


hy.models.Expression inherits Sequence Protocol for parenthesized () expressions. The compilation result of those expressions depends on the first element of the list: the compiler dispatches expressions between macros and and regular Python function calls.


hy.models.Dict inherits Sequence Protocol for curly-bracketed {} expressions, which compile down to a Python dictionary literal.

Atomic Models

In the input stream, double-quoted strings, respecting the Python notation for strings, are parsed as a single token, which is directly parsed as a String.

An uninterrupted string of characters, excluding spaces, brackets, quotes, double-quotes and comments, is parsed as an identifier.

Identifiers are resolved to atomic models during the parsing phase in the following order:

  • Integer

  • Float

  • Complex (if the atom isn’t a bare j)

  • Keyword (if the atom starts with :)

  • Symbol


hy.models.String represents string literals (including bracket strings), which compile down to unicode string literals (str) in Python.

Strings are immutable.

Hy literal strings can span multiple lines, and are considered by the parser as a single unit, respecting the Python escapes for unicode strings.

Strings have an attribute brackets that stores the custom delimiter used for a bracket string (e.g., "==" for #[==[hello world]==] and the empty string for #[[hello world]]). Strings that are not produced by bracket strings have their brackets set to None.


hy.models.Bytes is like String, but for sequences of bytes. It inherits from bytes.

Numeric Models

hy.models.Integer represents integer literals, using the int type.

hy.models.Float represents floating-point literals.

hy.models.Complex represents complex literals.

Numeric models are parsed using the corresponding Python routine, and valid numeric python literals will be turned into their Hy counterpart.


hy.models.Symbol is the model used to represent symbols in the Hy language. Like String, it inherits from str (or unicode on Python 2).

Symbols are mangled when they are compiled to Python variable names.


hy.models.Keyword represents keywords in Hy. Keywords are symbols starting with a :. See keywords.

The .name attribute of a hy.models.Keyword provides the (unmangled) string representation of the keyword without the initial :. For example:

=> (setv x :foo-bar)
=> (print

If needed, you can get the mangled name by calling mangle.

Hy Macros

Using gensym for Safer Macros

When writing macros, one must be careful to avoid capturing external variables or using variable names that might conflict with user code.

We will use an example macro nif (see for a more complete description.) nif is an example, something like a numeric if, where based on the expression, one of the 3 forms is called depending on if the expression is positive, zero or negative.

A first pass might be something like:

(defmacro nif [expr pos-form zero-form neg-form]
    (setv obscure-name ~expr)
    (cond [(> obscure-name 0) ~pos-form]
          [(= obscure-name 0) ~zero-form]
          [(< obscure-name 0) ~neg-form])))

where obscure-name is an attempt to pick some variable name as not to conflict with other code. But of course, while well-intentioned, this is no guarantee.

The method gensym is designed to generate a new, unique symbol for just such an occasion. A much better version of nif would be:

(defmacro nif [expr pos-form zero-form neg-form]
  (setv g (hy.gensym))
     (setv ~g ~expr)
     (cond [(> ~g 0) ~pos-form]
           [(= ~g 0) ~zero-form]
           [(< ~g 0) ~neg-form])))

This is an easy case, since there is only one symbol. But if there is a need for several gensym’s there is a second macro with-gensyms that basically expands to a setv form:

(with-gensyms [a b c]

expands to:

  (setv a (hy.gensym)
        b (hy.gensym)
        c (hy.gensym))

so our re-written nif would look like:

(defmacro nif [expr pos-form zero-form neg-form]
  (with-gensyms [g]
       (setv ~g ~expr)
       (cond [(> ~g 0) ~pos-form]
             [(= ~g 0) ~zero-form]
             [(< ~g 0) ~neg-form]))))

Finally, though we can make a new macro that does all this for us. defmacro/g! will take all symbols that begin with g! and automatically call gensym with the remainder of the symbol. So g!a would become (hy.gensym "a").

Our final version of nif, built with defmacro/g! becomes:

(defmacro/g! nif [expr pos-form zero-form neg-form]
     (setv ~g!res ~expr)
     (cond [(> ~g!res 0) ~pos-form]
           [(= ~g!res 0) ~zero-form]
           [(< ~g!res 0) ~neg-form])))