Clojure's backtick, tilde and tilde-at rather than Common Lisp's comma forms:
is_delimiter already treats a comma as whitespace and every binding vector in
the corpus assumes it, so freeing the comma would rewrite more of the language
than macros are worth. They read as (quasiquote x), (unquote x) and
(unquote-splicing x), the way 'x already reads as (quote x) - the reader stays
dumb and the meaning is resolved later.
The backtick previously read as an ordinary symbol character, which is exactly
the failure the reader's own header warns about for the apostrophe. Both sigils
are delimiters now, so a~b is two things and can never be one name.
defmacro validates its shape before refusing, because a malformed one and a
well-formed one are different mistakes and deserve different sentences. The
three new reader names are refused by name too, or they would fall through to
Call and come back as unknown name quasiquote. unquote outside a quasiquote is
refused as a mistake rather than as a milestone, since the reader cannot know
where it is.
Nothing is stored: no Ast.Defmacro and no macro table. A new decl variant would
have forced edits to four files other agents hold this session, and a
process-global registry spanning the prelude parse, the package parses and
hundreds of test snippets would make results order-dependent. The storage shape
is the expander author's first decision anyway.
The design note records what the expander needs, and the blocker worth knowing:
a macro is [Form] -> Form, so Form has to be a Flan union whose layout the
compiler and the loaded macro agree on exactly, and union values are milestone
6.
Joseph Ferano
99e59dba9f