Joseph Ferano
d6fc15474b
The author: "I think I prefer length over len, because then I'll use len as the variable name". One arm in check.ml, one row in the table beside it, and every (len x) in lib, test, examples, vendor, spike, docs, web, emacs, plan.org and NEXT.md rewritten. Shadowing and builtin/ had already taken most of the sting out: a (defn len ...) was legal and won in its own file, and builtin/len reached past it. What was left is that len was still a builtin — the defn earned a warning, and a wrapper had to say builtin/ at every inner call. Now there is nothing under the short name: len is an ordinary identifier in every position, which is what (let [len (length xs)] ...) wants. length takes over as shadowing's worked example rather than the feature losing one. shadow-builtin.flan, builtin-qualified.flan, pkgs/shadowed and the builtin/ rows in test_flan move to it and go on testing shadowing. A call to a len nothing defines is answered where an unknown function is, after every table and after the shadowing guard, so a program with its own len never reaches it. The sentence is said rather than guessed at — len and length are three edits apart and the did-you-mean's net is one — and the call is written back out through spell_arg, as-slice's spelling lifted out of it and now shared, so what is printed compiles. sand.flan:33 still calls the old name and is the author's to change; until it does, test_acceptance and test_session abort there. Both were run green against a copy with that one line changed. FIX.org says so.
86 lines
3.5 KiB
Plaintext
86 lines
3.5 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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;;;; Every macro below is written [& args] and picks its arguments apart by
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;;;; hand, which is what a macro looked like before parameter lists: & binds
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;;;; the whole call as a slice of forms, and (length args) is how many were
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;;;; written. macro-params.flan is the other spelling of the same grammar —
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;;;; named parameters, [ ] patterns, & only for the tail — and the two files
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;;;; are kept apart on purpose, so that each is a whole program in one style.
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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 (= (length 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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