3.5 Bytes and Byte Strings

A byte is an exact integer between 0 and 255, inclusive. The byte? predicate recognizes numbers that represent bytes.

Examples:

> (byte? 0)

#t

> (byte? 256)

#f

A byte string is similar to a string – see Strings (Unicode) – but its content is a sequence of bytes instead of characters. Byte strings can be used in applications that process pure ASCII instead of Unicode text. The printed form of a byte string supports such uses in particular, because a byte string prints like the ASCII decoding of the byte string, but prefixed with a #. Unprintable ASCII characters or non-ASCII bytes in the byte string are written with octal notation.

Reading Strings in Reference: PLT Scheme documents the fine points of the syntax of byte strings.

Examples:

> #"Apple"

#"Apple"

> (bytes-ref #"Apple" 0)

65

> (make-bytes 3 65)

#"AAA"

> (define b (make-bytes 2 0))

> b

#"\0\0"

> (bytes-set! b 0 1)

> (bytes-set! b 1 255)

> b

#"\1\377"

The display form of a byte string writes its raw bytes to the current output port (see Input and Output). Technically, display of a normal (i.e,. character) string prints the UTF-8 encoding of the string to the current output port, since output is ultimately defined in terms of bytes; display of a byte string, however, writes the raw bytes with no encoding. Along the same lines, when this documentation shows output, it technically shows the UTF-8-decoded form of the output.

Examples:

> (display #"Apple")

Apple

> (display "\316\273")  ; same as "λ"

λ

> (display #"\316\273") ; UTF-8 encoding of λ

λ

For explicitly converting between strings and byte strings, Scheme supports three kinds of encodings directly: UTF-8, Latin-1, and the current locale’s encoding. General facilities for byte-to-byte conversions (especially to and from UTF-8) fill the gap to support arbitrary string encodings.

Examples:

> (bytes->string/utf-8 #"\316\273")

"λ"

> (bytes->string/latin-1 #"\316\273")

"λ"

> (parameterize ([current-locale "C"])  ; C locale supports ASCII,       (bytes->string/locale #"\316\273")) ; only, so...

bytes->string/locale: byte string is not a valid encoding

for the current locale: #"\316\273"

> (let ([cvt (bytes-open-converter "cp1253" ; Greek code page                                      "UTF-8")]           [dest (make-bytes 2)])       (bytes-convert cvt #"\353" 0 1 dest)       (bytes-close-converter cvt)       (bytes->string/utf-8 dest))

"λ"

Byte Strings in Reference: PLT Scheme provides more on byte strings and byte-string procedures.