7.1 transcoder

如上所述， transcoder 封装了3个值：一个 codec ，一个 eol-style，以及 error-handling-mode。本节介绍创建或操作 transcoder 的过程以及 transcoder 封装的值。

procedure: (make-transcoder codec)
procedure: (make-transcoder codec eol-style)
procedure: (make-transcoder codec eol-style error-handling-mode)
返回：一个 transcoder
库路径：(rnrs io ports), (rnrs)

codec需要由下面的过程调用生成，比如 (utf-8-codec) 返回一个 UTF-8 codec。

eol-style必须是合法的 eol-style 符号（lf, cr, nel, ls, crlf, crnel或者none），它默认为当前平台的本地风格（windows 下是 crlf, 而 Linux 下是 lf）。

error-handling-mode必须是合法的错误处理模式(ignore, raise, replace), 它的默认值为 replace。

procedure: (transcoder-codec transcoder)
returns: the codec encapsulated in transcoder
procedure: (transcoder-eol-style transcoder)
returns: the eol-style symbol encapsulated in transcoder
procedure: (transcoder-error-handling-mode transcoder)
returns: the error-handling-mode symbol encapsulated in transcoder
libraries: (rnrs io ports), (rnrs)

procedure: (native-transcoder)
returns: 返回本地 transcoder
libraries: (rnrs io ports), (rnrs)

The native transcoder is implementation-dependent and may vary by platform or locale.

procedure: (latin-1-codec)
returns: a codec for ISO 8859-1 (Latin 1) character encodings
procedure: (utf-8-codec)
returns: a codec for Unicode UTF-8 character encodings
procedure: (utf-16-codec)
returns: a codec for Unicode UTF-16 character encodings
libraries: (rnrs io ports), (rnrs)

syntax: (eol-style symbol)
returns: symbol
libraries: (rnrs io ports), (rnrs)

symbol必须是 lf, cr, nel, ls, crlf, crnel, none之一。(eol-style symbol)表达式与 (quote symbol)是等价的，除了前者在宏展开期会检查 symbol 是否是合法的符号。另外，eol-style宏提供了有用的文档。

(eol-style crlf) => crlf
(eol-style lfcr) => syntax violation

procedure: (native-eol-style)
returns: the native eol style
libraries: (rnrs io ports), (rnrs)

The native eol style is implementation-dependent and may vary by platform or locale.

syntax: (error-handling-mode symbol)
returns: symbol
libraries: (rnrs io ports), (rnrs)

symbol must be one of the symbols ignore, raise, or replace. The expression (error-handling-mode symbol) is equivalent to the expression (quote symbol) except that the former checks at expansion time that symbol is one of the error-handling-mode symbols. The error-handling-mode syntax provides useful documentation as well.

(error-handling-mode replace) => replace
(error-handling-mode relpace) => syntax violation