hy/docs/language/readermacros.rst

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.. _reader-macros:
.. highlight:: clj
=============
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.
Syntax
======
::
=> (defreader ^ [expr] (print expr))
=> #^(1 2 3 4)
(1 2 3 4)
=> #^"Hello"
"Hello"
=> #^1+2+3+4+3+2
1+2+3+4+3+2
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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)
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You could even do like clojure, and have a literal for regular expressions!
::
=> (import re)
=> (defreader r [expr] `(re.compile ~expr))
=> #r".*"
<_sre.SRE_Pattern object at 0xcv7713ph15#>
Implementation
==============
``defreader`` takes a single character as symbol name for the reader macro,
anything longer will return an error. Implementation wise, ``defreader``
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expands into a lambda covered with a decorator, this decorator saves the
lambda in a dict 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"
"Hello"
.. warning::
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.