Moving on to the stream-element-type function.  We should first describe streams and their element types.  A stream in Lisp looks a little like an iostream in C++.  It is an opaque object that defines an interaction between the program and what a UNIX system would call a character device (as opposed to a block device).  A stream may take special action to distinguish between text and binary data, and has equivalents to tellp and seekp.

A Lisp stream has an associated element type which must be a subtype of character or of integer.  If it’s a subtype of character, it can be used with functions like format, read, and others.  If it’s a subtype of integer, it can be used with functions like read-byte, write-byte, read-sequence, and write-sequence.

The stream-element-type function allows the programmer to detect at runtime whether a stream is a character stream or an integer stream.  Appropriate behaviour can then take place based on the compatibility with that particular stream.

One thing I will note.  While I was testing behaviour in SBCL 1.1.14, I was a bit surprised to see that the write-byte and read-byte functions, while defined in the CLHS as writing individual bytes, actually write and read one entry from the stream.  If the stream element type is (integer 0 65535), the entries are 16 bits long, and read-byte reads a 16-bit word from the stream, not a single byte.  I was unable to find anything in the CLHS among the clarifying notes that would tell me whether this is required, allowed, or a bug.  If anybody knows, please pass on your knowledge in the comments.