2021-02-28

Author

William Yao <williamyaoh@gmail.com>

Maintainer

William Yao <williamyaoh@gmail.com>

BSD-3

Description

Read macros are pretty handy when Lispy sexps just don't cut it. Unfortunately, having to hunt around in the dispatch table or in the read macro table to avoid read macro collisions doesn't leave a whole bunch of usable characters. So instead, `NAMED-READ-MACROS` lets you attach a read macro to any symbol, and call it like any other function or macro.

``````(named-readtables:in-readtable named-read-macros:readtable)

(with-output-to-string (output)
(loop for char = (read-char *standard-input* nil nil t)
while char
do (write-char char output))))

(escapify
Any characters I put in here will be put into the output string, even
things that would normally require escapes, like backslashes!

\ \ \ \
end-escapify)
=> "Any characters I put in here will be put into the output string, even
things that would normally require escapes, like backslashes!

\\ \\ \\ \\"
``````

You can hijack the Lisp reader without having to carefully avoid read macro conflicts! And since `NAMED-READ-MACROS` attaches read macros to symbols, you don't have to fuss about with readtables either, and you can export and import read macros the same way you do with functions and macros and all the other symbol-based stuff.

Usage

For any file which needs to use your named read macros, make sure to either switch your readtable to `NAMED-READ-MACROS:READTABLE`, or fuse a read table you're using with `NAMED-READ-MACROS:READTABLE-MIXIN`.

Contains the standard readtable, but with the macro character for `#\(` rewritten to check for a named read macro in the head position.

Just contains the rewritten macro for `#\(`.

Then define a named read macro with `NAMED-READ-MACROS:DEFINE`.

• DEFINE NAME &BODY BODY

Creates a named read macro, which executes `BODY` in a context where `*STANDARD-INPUT*` is bound to a stream containing just the contents which the read macro was called with, and associates this macro with `NAME`.

Just like a normal macro, `BODY` should return a Lisp form to then get evaluated where the named read macro was called.

Because `NAMED-READ-MACROS` hijacks the Lisp reader, we can't rely on matching parentheses to know when the newly-defined read macro ends, so we look for the sequence of characters `"END-\${NAME}"`, immediately followed by a close parenthesis, in order to know when the read macro ends. Case is checked against `NAME` by transforming the ending string according to the case of the current readtable; if `(SYMBOL-NAME NAME)` matches the transformed ending string exactly, we've found the end tag. In particular, this means that using `DEFINE` with a pipe-enclosed symbol with lowercase characters will make such a read macro impossible to end under the standard readtable (though why one would define such a macro is another question entirely!)

Leading whitespace after opening the read macro will not be passed to `BODY`, but trailing whitespace before the ending tag will.

Note that `DEFINE` has no compile-time effects by default; the rationale is that doing so would also require any functions used by a read macro defined by `DEFINE` to be available at compile-time. Since this is not the usual, `DEFINE` explicitly has no compile-time effects to avoid this problem. If you want a named read macro to be available at compile-time, wrap `DEFINE` and any necessary functions in an explicit `EVAL-WHEN`.

Going forward

An interesting usage would be to write an LALR(1) parser generator for Common Lisp, but instead of generating a function or a standalone executable, it instead generates read macros. Any parsers generated could then be embedded directly within Lisp code, allowing for very complex read macros to be easily written, and compile directly to Lisp forms.

Another use would be something like Perl's `__DATA__` directive, embedding files and data directly in Lisp code.

• fiveam