Joseph Ferano
69646e534e
(defmacro do-grid [[r rows c cols] & body] ...) — positional names, a [ ] pattern wherever an argument is a vector, and & for the tail. The reading of the list lives in Expand, below both sides that need it: Parse turns it into the bindings a macro body opens with, and Macro checks a call against the same reading before expanding it, so arity and shape are refused with the call's own location rather than with the Loc.from_macro stamp every node of an expansion carries. The breaking half: [args] used to bind the whole argument list and now binds the first argument. The whole list is [& args], and every defmacro in the tree — prelude, vendor, tests, the elisp fixtures — was migrated to it. One grammar, not a legacy mode.
83 lines
3.3 KiB
Plaintext
83 lines
3.3 KiB
Plaintext
;;;; Macros: a defmacro in the file, called from the file.
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;;;;
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;;;; There is no interpreter, so every macro below was compiled into a shared
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;;;; object and dlopened into the compiler before this file's first line was
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;;;; parsed. What arrives here is the expansion; nothing at run time knows a
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;;;; macro was involved.
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;;;;
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;;;; A macro takes one parameter, the slice of forms written at its call site,
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;;;; and answers one form. That is where variadics come from in a language with
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;;;; no &rest: (len args) is how many were written.
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;; The simplest one there is: two forms, in order. It proves the call site's
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;; arguments arrive as forms and come back as code.
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(defmacro both [& args]
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`(do ~(at args 0) ~(at args 1)))
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;; Splicing, which is the only reason ~@ exists: the body is however many forms
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;; were written, and they go where a list is expected.
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(defmacro when2 [& args]
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`(if ~(at args 0) (do ~@(form-rest args 1))))
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;; Expansion is not hygienic -- Common Lisp's rule and Clojure's, settled in
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;; plan.org's open decision 2 -- so a macro that needs a name of its own asks
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;; for one. gensym is a prelude function the loaded module runs while it runs,
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;; and the name it answers starts with ~, which is a delimiter, so no symbol
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;; the reader can produce is able to collide with it.
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;;
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;; Without this, `twice` would bind `tmp` and the caller's own `tmp` would be
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;; shadowed inside it. The two calls below are the difference.
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(defmacro twice [& args]
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(let [v (gensym)]
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`(let [~v ~(at args 0)]
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(+ ~v ~v))))
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;; A macro that answers a call to another macro. This costs the pre-pass
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;; nothing: `both` is inside the quasiquote, so it is part of what this macro
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;; *returns* and is expanded again after it returns, and `announce` can be
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;; compiled without `both` existing.
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(defmacro announce [& args]
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`(both (print "-> ") ~(at args 0)))
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;; This is the one that makes the pre-pass a fixpoint rather than a sweep. The
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;; call to `id` is not inside a quasiquote, so it runs while *this macro is
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;; being compiled* -- which means `id` has to be compiled and dlopened first,
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;; and until it is, `id` is a name nothing defines and this body will not
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;; compile at all. So round 0 takes `id`, round 1 expands this against it, and
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;; the module that finally answers a call holds both.
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(defmacro id [& args]
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(at args 0))
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(defmacro quiet [& args]
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(id `(println "a macro that called a macro")))
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;; And a macro that expands into a call to itself, which is what every
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;; conditional macro in every Lisp is. It gets smaller each time and stops at
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;; the empty case, so the expander's fuel never comes into it.
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(defmacro all-of [& args]
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(if (= (len args) 0)
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`true
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`(if ~(at args 0) (all-of ~@(form-rest args 1)) false)))
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(defn main [] i32
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(both (print "a") (println "b"))
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(when2 true (print "c") (println "d"))
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(when2 false (println "not printed"))
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;; 21 + 21. The argument is evaluated once, into the gensym'd binding.
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(print (twice 21)) (println "")
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;; The caller's own `tmp` is untouched by the one the macro bound, because
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;; the macro did not bind `tmp`.
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(let [tmp 5]
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(print (twice tmp)) (print " ") (print tmp) (println ""))
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(announce (println "announced"))
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(quiet)
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(print (all-of)) (println "")
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(print (all-of true true true)) (println "")
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(print (all-of true false true)) (println "")
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0)
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