76 lines
4.0 KiB
Racket
76 lines
4.0 KiB
Racket
#reader(lib "docreader.ss" "scribble")
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@require["mz.ss"]
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@title[#:tag "mz:encodings"]{Encodings and Locales}
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When a port is provided to a character-based operation, such as
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@scheme[read-char] or @scheme[read], the port's bytes are read and
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interpreted as a UTF-8 encoding of characters. Thus, reading a single
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character may require reading multiple bytes, and a procedure like
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@scheme[char-ready?] may need to peek several bytes into the stream to
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determine whether a character is available. In the case of a byte
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stream that does not correspond to a valid UTF-8 encoding, functions
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such as @scheme[read-char] may need to peek one byte ahead in the
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stream to discover that the stream is not a valid encoding.
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When an input port produces a sequence of bytes that is not a valid
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UTF-8 encoding in a character-reading context, then bytes that
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constitute an invalid sequence are converted to the character
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@litchar{?}. Specifically, bytes 255 and 254 are always converted to
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@litchar{?}, bytes in the range 192 to 253 produce @litchar{?} when
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they are not followed by bytes that form a valid UTF-8 encoding, and
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bytes in the range 128 to 191 are converted to @litchar{?} when they
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are not part of a valid encoding that was started by a preceding byte
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in the range 192 to 253. To put it another way, when reading a
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sequence of bytes as characters, a minimal set of bytes are changed to
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63 (which is the value that @scheme[(char->integer #\?)] produces) so
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that the entire sequence of bytes is a valid UTF-8 encoding.
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See @secref["mz:bytestrings"] for procedures that facilitate
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conversions using UTF-8 or other encodings. See also
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@scheme[reencode-input-port] and @scheme[reencode-output-port] for
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obtaining a UTF-8-based port from one that uses a different encoding
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of characters.
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A @deftech{locale} captures information about a user's
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culture-specific interpretation of character sequences. In particular,
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a locale determines how strings are ``alphabetized,'' how a lowercase
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character is converted to an uppercase character, and how strings are
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compared without regard to case. String operations such as
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@scheme[string-ci?] are @italic{not} sensitive to the current locale,
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but operations such as @scheme[string-locale-ci?] (see
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@secref["mz:strings"]) produce results consistent with the current
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locale.
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A locale also designates a particular encoding of code-point sequences
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into byte sequences. Scheme generally ignores this aspect of the
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locale, with a few notable exceptions: command-line arguments passed
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to Scheme as byte strings are converted to character strings using the
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locale's encoding; command-line strings passed as byte strings to
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other processes (through @scheme[subprocess]) are converted to byte
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strings using the locale's encoding; environment variables are
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converted to and from strings using the locale's encoding; filesystem
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paths are converted to and from strings (for display purposes) using
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the locale's encoding; and, finally, Scheme provides functions such as
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@scheme[string->bytes/locale] to specifically invoke a locale-specific
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encoding.
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A Unix user selects a locale by setting environment variables, such as
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@envvar{LC_ALL}. Under Windows and Mac OS X, the operating system
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provides other mechanisms for setting the locale. Within Scheme, the
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current locale can be changed by setting the @scheme[current-locale]
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parameter. The locale name within Scheme is a string, and the
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available locale names depend on the platform and its configuration,
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but the @scheme[""] locale means the current user's default locale;
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under Windows and Mac OS X, the encoding for @scheme[""] is always
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UTF-8, and locale-sensitive operations use the operating system's
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native interface. (In particular, setting the @envvar{LC_ALL} and
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@envvar{LC_CTYPE} environment variables do not affect the locale
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@scheme[""] under Mac OS X. Use @scheme[getenv] and
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@scheme[current-locale] to explicitly install the
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environment-specified locale, if desired.) Setting the current locale
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to @scheme[#f] makes locale-sensitive operations locale-insensitive,
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which means using the Unicode mapping for case operations and using
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UTF-8 for encoding.
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