File Output（文件输出）
To write data to a file, you need an output stream, which you obtain
by calling OPEN with a :direction keyword argument of :output. When
opening a file for output, OPEN assumes the file shouldn't already
exist and will signal an error if it does. However, you can change
that behavior with the :if-exists keyword argument. Passing the value
:supersede tells OPEN to replace the existing file. Passing :append
causes OPEN to open the existing file such that new data will be
written at the end of the file, while :overwrite returns a stream that
will overwrite existing data starting from the beginning of the
file. And passing NIL will cause OPEN to return NIL instead of a
stream if the file already exists. A typical use of OPEN for output
looks like this:
为了向一个文件中写数据，你需要一个输出流。你可以通过在调用 OPEN
时使用一个值为 :output 的 :direction
关键字参数来获取它。当你打开一个用于输出的文件时，OPEN
会假设该文件不该存在并会在文件存在时报错。但你可以使用 :if-exists
关键字参数来改变该行为。传递值 :supersede 可以告诉 OPEN
来替换已有文件。传递 :append 将导致 OPEN
打开已有的文件并保证新数据被写到文件结尾处，而 :overwrite
返回一个从文件开始处开始的流从而覆盖已有数据。而传递 NIL
将导致 OPEN 在文件已存在时返回 NIL
而不是流。一个典型的使用 OPEN 来输出的例子如下所示：
(open "/some/file/name.txt" :direction :output :if-exists :supersede)Common Lisp also provides several functions for writing data: WRITE-CHAR writes a single character to the stream. WRITE-LINE writes a string followed by a newline, which will be output as the appropriate end-of-line character or characters for the platform. Another function, WRITE-STRING, writes a string without adding any end-of-line characters. Two different functions can print just a newline: TERPRI--short for "terminate print"--unconditionally prints a newline character, and FRESH-LINE prints a newline character unless the stream is at the beginning of a line. FRESH-LINE is handy when you want to avoid spurious blank lines in textual output generated by different functions called in sequence. For example, suppose you have one function that generates output that should always be followed by a line break and another that should start on a new line. But assume that if the functions are called one after the other, you don't want a blank line between the two bits of output. If you use FRESH-LINE at the beginning of the second function, its output will always start on a new line, but if it's called right after the first, it won't emit an extra line break.
Common Lisp 也提供了几个用于写数据的函数：WRITE-CHAR 会向流中写入一个单一字符；WRITE-LINE 写一个字符串并紧跟一个换行，其将被输出成用于当前平台的适当行结束字符或字符序列。另一个函数 WRITE-STRING 写一个字符串而不会添加任何行结束符。有两个不同的函数 可以只打印一个换行：TERPRI 是 “终止打印（terminate print）” 的简称，即无条件地打印一个换行字符；FRESH-LINE 打印一个换行字符，除非该流已经在一行的开始处。在想要避免由按顺序调用的不同函数所生成的文本输出中的额外换行时，FRESH-LINE 很有用。例如，假设一个函数在其生成输出时总是带有一个换行，而另一个函数应当每次从一个新行开始输出。但假设这两个函数被依次调用，而你又不希望在两个输出操作之间产生一个空行，那么如果你在第二个函数开始处使用 FRESH-LINE，那么它的输出将总是从一个新行开始。但如果它刚好在前一个函数之后调用，则它将不会产生一个额外换行。
Several functions output Lisp data as s-expressions: PRINT prints an s-expression preceded by an end-of-line and followed by a space. PRIN1 prints just the s-expression. And the function PPRINT prints s-expressions like PRINT and PRIN1 but using the "pretty printer," which tries to print its output in an aesthetically pleasing way.
有几个函数会将 Lisp 数据输出成 S-表达式：PRINT 打印一个 S-表达式，前缀一个换行及一个空格；PRIN1 只打印 S-表达式；而函数 PPRINT 会像 PRINT 和 PRIN1 那样打印 S-表达式，但使用的是美化打印器（pretty printer），它试图将输出打印得赏心悦目。
However, not all objects can be printed in a form that READ will
understand. The variable *PRINT-READABLY* controls what happens if you
try to print such an object with PRINT, PRIN1, or PPRINT. When it's
NIL, these functions will print the object in a special syntax that's
guaranteed to cause READ to signal an error if it tries to read it;
otherwise they will signal an error rather than print the object.
但并非所有对象都能以一种 READ 可理解的形式打印出来。当试图使用
PRINT、PRIN1 或 PPRINT 来打印这样一种对象时，变量
*PRINT-READABLY* 将会予以控制。当它是 NIL
时，这些函数将以一种导致 READ
在试图读取时肯定会报错的特殊语法来打印该对象；否则它们将直接报错而不打印该对象。
Another function, PRINC, also prints Lisp objects, but in a way designed for human consumption. For instance, PRINC prints strings without quotation marks. You can generate more elaborate text output with the incredibly flexible if somewhat arcane FORMAT function. I'll discuss some of the more important details of FORMAT, which essentially defines a mini-language for emitting formatted output, in Chapter 18.
另一个函数 PRINC 也会打印 Lisp 对象，但其工作方式却很适合人们使用。例如，PRINC 在打印字符串时不带有引号。你可以使用极其灵活但有时略显神秘的 FORMAT 函数来生成更加复杂的文本输出。我将在第 18 章里讨论一些关于 FORMAT 的更重要的细节，它从本质上定义了一种用于产生格式化输出的微型语言。
To write binary data to a file, you have to OPEN the file with the
same :element-type argument as you did to read it: '(unsigned-byte 8).
You can then write individual bytes to the stream with WRITE-BYTE.
为了向一个文件中写入二进制数据，你需要在使用 OPEN
打开文件时带有与读取该文件时相同的 :element-type 实参，其值为
'(unsigned-byte 8)，然后就可以使用 WRITE-BYTE 向流中写入单独的字节。
The bulk output function WRITE-SEQUENCE accepts both binary and character streams as long as all the elements of the sequence are of an appropriate type for the stream, either characters or bytes. As with READ-SEQUENCE, this function is likely to be quite a bit more efficient than writing the elements of the sequence one at a time.
批量输出函数 WRITE-SEQUENCE 可同时接受二进制和字符流，只要序列中的所有元素都是用于该流的适当类型即可，无论其是字符还是字节。和 READ-SEQUENCE 一样，该函数会比每次输出一个序列元素更为高效。