Reader Macros

Reader macros gives Lisp the power to modify and alter syntax on the fly. You don’t want Polish notation? A reader macro can easily do just that. Want Clojure’s way of having a regex? Reader macros can also do this easily.


=> (defreader ^ [expr] (print expr))
=> #^(1 2 3 4)
(1 2 3 4)
=> #^"Hello"
=> #^1+2+3+4+3+2

Hy has no literal for tuples. Lets say you dislike (, ...) and want something else. This is a problem reader macros are able to solve in a neat way.

=> (defreader t [expr] `(, ~@expr))
=> #t(1 2 3)
(1, 2, 3)

You could even do it like Clojure and have a literal for regular expressions!

=> (import re)
=> (defreader r [expr] `(re.compile ~expr))
=> #r".*"
<_sre.SRE_Pattern object at 0xcv7713ph15#>


defreader takes a single character as symbol name for the reader macro; anything longer will return an error. Implementation-wise, defreader expands into a lambda covered with a decorator. This decorator saves the lambda in a dictionary with its module name and symbol.

=> (defreader ^ [expr] (print expr))
;=> (with_decorator (hy.macros.reader ^) (fn [expr] (print expr)))

# expands into (dispatch_reader_macro ...) where the symbol and expression is passed to the correct function.

=> #^()
;=> (dispatch_reader_macro ^ ())
=> #^"Hello"


Because of a limitation in Hy’s lexer and parser, reader macros can’t redefine defined syntax such as ()[]{}. This will most likely be addressed in the future.