74 lines
1.7 KiB
ReStructuredText
74 lines
1.7 KiB
ReStructuredText
.. _reader-macros:
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.. highlight:: clj
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=============
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Reader Macros
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=============
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Reader macros gives LISP the power to modify and alter syntax on the fly.
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You don't want polish notation? A reader macro can easily do just that. Want
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Clojure's way of having a regex? Reader macros can also do this easily.
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Syntax
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======
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::
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=> (defreader ^ [expr] (print expr))
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=> #^(1 2 3 4)
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(1 2 3 4)
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=> #^"Hello"
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"Hello"
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=> #^1+2+3+4+3+2
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1+2+3+4+3+2
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Hy has no literal for tuples. Lets say you dislike `(, ...)` and want something
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else. This is a problem reader macros are able to solve in a neat way.
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::
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=> (defreader t [expr] `(, ~@expr))
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=> #t(1 2 3)
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(1, 2, 3)
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You could even do like clojure, and have a literal for regular expressions!
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::
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=> (import re)
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=> (defreader r [expr] `(re.compile ~expr))
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=> #r".*"
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<_sre.SRE_Pattern object at 0xcv7713ph15#>
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Implementation
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==============
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``defreader`` takes a single character as symbol name for the reader macro,
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anything longer will return an error. Implementation wise, ``defreader``
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expands into a lambda covered with a decorator, this decorater saves the
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lambda in a dict with its module name and symbol.
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::
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=> (defreader ^ [expr] (print expr))
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;=> (with_decorator (hy.macros.reader ^) (fn [expr] (print expr)))
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``#`` expands into ``(dispatch_reader_macro ...)`` where the symbol
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and expression is passed to the correct function.
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::
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=> #^()
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;=> (dispatch_reader_macro ^ ())
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=> #^"Hello"
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"Hello"
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.. warning::
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Because of a limitation in Hy's lexer and parser, reader macros can't
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redefine defined syntax such as ``()[]{}``. This will most likely be
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addressed in the future.
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