> 'read' can occur strictly before interpreting any of S-expressions, and reading in incorrect encoding can > cause an I/O error so you may not have a chance to interpret those forms. > The source file encoding should be a property of the port (as is the > case-folding property). It could be set with a "#!" directive (at the > top of the file). The 'read' procedure that looks for the encoding declaration should be a special reader that's much simpler than the normal Scheme reader and handles encoding errors gracefully (perhaps it should simply read raw bytes from a binary port, treat 0..127 as ASCII characters, and ignore all other characters or treat them as whitespace or symbol/string constituent). Skipping/parsing the Unix shebang line (#!) at the start of a script is in many Schemes/Lisps a similar magic job that needs its own reader (or a hack to the standard reader). > BTW, the "magic encoding comment" is supported in a few languages: > > Python: https://www.python.org/dev/peps/pep-0263/ > Ruby: https://idiosyncratic-ruby.com/26-file-encoding-magic.html The fact that it's widespread is nice, but the syntax is not really well specified. Ruby uses this regexp to look for it: ENCODING_SPEC_RE = %r"coding\s*[=:]\s*([[:alnum:]\-_]+)" Python uses this regexp: ^[ \t\f]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+) Since the Scheme/Lisp is traditionally to resist the temptation of quick hacks and do things in a principled way, I would really like to avoid this approach even though other languages and editors are doing it. If a magic comment syntax is to way to go, I would at least like to have a principled and well specified grammar that can also be used other kinds of magic comments (I have been collecting samples from many languages in the hopes of specifying such a grammar, but I don't know if anyone would adopt it). > Technically the encoding info should be a metadata of a file, not in the content of the file, so the "coding" comment > is certainly a kluge. What I thought is that it might be useful to codify the current practice. Correct, but it's certainly good to have the info somewhere since the whole world is not Unicode yet.
