fare-quasiquote ? a pattern-matching friendly implementation of Quasiquote
Copyright ? 2002-2016 Fahree Reedaw
Now, developing this implementation was also a challenge in understanding the ins and outs of quasiquotation, and in exploring the way it interacts with extended support for constant-building syntactic constructs. And this, at least to me was as much fun as it was an intellectual challenge.
How to use it
The recommended way to use
fare-quasiquote is to have your
defsystem depend on
fare-quasiquote-extras, and at the start your file, use:
Then you can match expressions using
optima:match, such as:
(optima:match '(1 (2 3) 4 5) (`(a (b ,c) ,@d) (list a b c d))) ; => (1 2 3 (4 5))
You can also at the SLIME REPL use:
(asdf:load-system "fare-quasiquote-extras") (named-readtables:in-readtable :fare-quasiquote)
However, beware to not leak
fare-quasiquote into systems you load that do not
:depends-on ("fare-quasiquote"); and so until ASDF is fixed to do that for you (hopefully by ASDF 3.3), before you call
(asdf:load-system ...), you need to restore the default
*readtable* with, e.g.:
fare-quasiquote system is a reimplementation of quasiquote with two advantages:
first it has all the bugs straightened out, including dark corner cases (some implementations still get the simple
second, it expands into a stable format that allows for pattern matching, by privileging
nconc(and by the way, you should never, ever, use
fare-quasiquote-optima, expressions parsed by
fare-quasiquote can be used as pattern matching patterns with
trivia also supports
We recommend you use
named-readtables to enable at the beginning and end of your Lisp files, or around their compilation (e.g. using ASDF around-compile hooks). Note however that it is important not to let such
readtables leak into the compilation of files that do not depend on
Note that since pattern matchers like
trivia do not support backtracking (unlike say an embedded logic programming language), they cannot match
append patterns, and those quasiquote templates that expand into something using
append can't be used as patterns to match. This means that the use of
,. is restricted to the end of a list when used as a pattern.
fare-quasiquote was originally written to work with
fare-matcher, and legacy support for
fare-matcher is available in
fare-quasiquote-matcher, now distributed with
fare-matcher itself. But
fare-matcher is deprecated.
Essential documents consulted while implementing this file:
- Alan Bawden's PEPM 99 paper: Quasiquotation in Lisp
- The CLtL2 Appendix C: Backquote
- The CLHS section 2.4.6: Backquote
- Slate reference manual section 2.6.2 on quoting and unquoting
- Common Lisp backquote implementation, written in Common Lisp. (public domain) Author: Guy L. Steele Jr. Date: 27 December 1985. To be used with 2010 patch by Alex Plotnick regarding the simplification pass.
- SBCL backquote implementation (derived from CMUCL, used the October 2010 version).
If you for whatever reason ever feel like reimplementing backquote, you should probably leverage the many tests in my test file quasiquote-test.lisp --- in addition to my own test cases, I notably included the tests from SBCL; they include regression tests for many actual bugs in subtle cases that you might otherwise miss.
Read-time vs Macro-expansion-time
In conformance with the CLHS,
fare-quasiquote expands its patterns at read-time, at least unless you enable a feature
In both cases,
fare-quasiquote handles unquoting in
#n() syntax by re-defining the syntax for hash-left-paren as well as for backquote.
If you enable feature
fare-quasiquote will expands its patterns at macro-expansion time, using the same convention as Scheme, with symbols quasiquote, unquote, unquote-splicing and unquote-nsplicing defined in package
fare-quasiquote (but not exported from it).
Either at read-time or at macro-expansion-time, the implementation of quasiquote is fragile in case the user explicitly uses its internal syntax marker inside the expression being quasiquoted. Such expressions may lead to confusion between the body of expressions being quasiquoted and the internal quasiquote infrastructure. If you use these kinds of tricks, you're on your own. In
fare-quasiquote, the syntax markers are the
unquote-nsplicingsymbols present in the
fare-quasiquotepackage and not exported. Any objects that would be used as markers instead of these interned symbols would lead to the same "bug" if somehow used inside quasiquoted expressions; at least, non-interned symbols or gensyms would allow to avoid this bug in expressions being
read. The "perfect" solution would be to use objects
gensymed at the moment a given
quasiquoteis read, and kept in a lexical variable to prevent introspective cheat by
#.syntax. Then, after simplifying expressions, we could pass the read expression through functions that would
substitute proper symbols for the markers, from the
fare-quasiquotepackage if pretty-printing is to be supported, or otherwise from the CL package. Note that this would have to be interleaved with support for working inside vectors and other syntax. This is all very tricky and until further notice is left as an exercise to the intrepid reader. Thus, while the behaviour of our implementation isn't strictly correct, we don't go through the hassle of modifying it into something much less readable just for the sake of preventing code that would deliberately confuse the quasiquote engine. Now, if we imagine that some code were dynamically generated based on system introspection, that could contain some of our magic markers, then this code would have to be made safe for interaction with
quasiquote; this might (or might not?) require making
This implementation allows for simplifying quasiquoting of literals and other self-evaluating objects into the object itself, instead of a
`(quote ,object)expression, if you enable the
#+quasiquote-passes-literalsfeature. This is the behaviour of the simplifier in CMUCL and SBCL, but some people have expressed concerns that it might not be strictly allowed in letter or spirit by the Common Lisp standards, and it can make the pretty-printer trickier.
This version works inside simple vectors, so as to support unquoting inside the
#(...)syntax as the standard mandates. To do that, it replaces the hash-left-paren reader-macro as well as the backquote reader-macro. Note that this does not work in
#1A(...)syntax. This phenomenon has been documented before in the following message.
Interestingly, I haven't seen the following problem stated, to know which is correct of
`#5(1 ,@'(2 3)). In other words, does the read argument to
#()mean something at read-time or at vector-creation-time? Of course, in the original intended usage, outside of any quasiquoting, read-time and vector-creation-time are one and the same. But what when quasiquote breaks up that identity? Our answer is that it means something at vector-creation-time.
The CLHS section 2.4.6 says that
(x1 x2 ... xn . atom)is same as
(append [x1] [x2] [x3] ... [xn] (quote atom))--- mind that the atom is preserved. This means that you can't conformantly simplify
`(a b ,@c)to
`(a b . ,c)unless you know that
cis a proper list (if it isn't, that's an error). Yet, the CLHS itself suggests the simplification, and all implementations tested agree that a final
(quote nil)can be elided:
for l in sbcl ccl clisp cmucl ecl abcl \ scl allegro lispworks gcl xcl ; do cl -l $l -i \ '(format t "'$l': ~S~%" `(,@`(a b) ,@`c)))' \ 2>&1 | grep "^$l:" # LW, GCL are verbose done
yet at the same time, SBCL still complains about
fare-quasiquote follows the consensus, unless
#+quasiquote-strict-append is enabled.
- The current implementation of fare-quasiquote tends to simplify away things like
c. These simplifications probably need to be somehow prevented by default. Maybe with various kinds of (identity) wrappers to indicate quoting-unquoting?
Meta Unquote Protocol (not implemented)
The CLHS specifies that quasiquoting works with simple-vector, but not with other arrays (multi-dimensional or not simple), and not with arbitrary structures. However, a fully general quasiquote facility would support quasiquoting within arbitrary syntax, using a MUP: Meta Unquote Protocol.
The MUP would allow to extend the quasiquote mechanism with support for new constant-building syntactic constructs as such constructs are defined. Maybe we will end up with a full-fledge declarative infrastructure for a Parser-Preprocessor-Pretty-Printer, like
camlp4 only more declarative.
The MUP would have an abstract API for arbitrary readers; existing syntax for vectors would implemented using the MUP. For compliance reasons, further MUP extensions would be disabled by default, but could be made available with a suitable function call, e.g. for
The MUP might also implement tagged (and multiple-valued?) quasiquotes, unquotes and quotes.
Note that copying and modifying read-tables is expensive, that dynamically modifying and restoring read-tables might interfere with
#. syntax, and that caching modified read-tables will interfere with any subsequent modification of a cached read-table, comparison not being possible. This means that if we wanted the MUP to adapt to existing extensions without modifying existing code, we would have to intercept the definition of syntax reading functions before they are fed to either
set-dispatch-macro-character, or intercept the entire reader protocol. Spooky. Now, this also requires that the current depth of quasiquoting be consulted any time any of the MUP-enabled constructors is read.
The principle of the MUP is that:
structure readers that don't want to support unquote MUST be wrapped into something that dynamically binds
#c(args)that do want to support unquote MUST accumulate formal arguments to a structure constructor into a list
args, then, if
*quasiquote-level*is 0, behave like
#.(apply `c `args)otherwise, behave like
`,(apply `c `args)where
cis the name of the constructor for given structure, and
argsis whichever arguments have been deduced from the syntax, which may include as many levels of unquotations as
*quasiquote-level*says. Note that in a strong sense,
,assuming an infinite tower of read-time evaluators ? la 3-LISP.
Note that the above is obscured because we're trying to discuss the behaviour of quasiquote-containing programs without having a meta-level quasiquoting facility that could distinguish between what is constant or variable at the meta-level independently from what is constant or variable at the base level:
args would be better specified through a special meta-level unquotation, the above expressions being in a corresponding special quasiquotation. A feature that would allow for clear separation of levels of meta-language would be a tagged quasiquote feature, as in Slate.
The idea of making circular data-structures work within quasiquotation makes my head ache with overarching pain. You're crazier than I am if you do it and do it right.
PS: If you're able to follow this discussion, you impress me. Come join the TUNES project!