the Term “Hash Notation“ in the Elisp Manual

Question

I'm reading GNU Emacs Lisp Reference Manual, and I see the phrase "hash notation".
Two places in the document seem to have different interpretations, so I have 2 questions.

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1. 2.1 Printed Representation and Read Syntax:

   ...... some types have no read syntax, ....... These objects are printed in hash notation, which consists of the characters ‘#<’, ..., and a closing ‘>’. For example:

   (current-buffer)
        ⇒ #<buffer objects.texi>
   

   OK, doc says that hash notation is something like "#<...>".

2. 2.2 Special Read Syntax:

   Emacs Lisp represents many special objects and constructs via special hash notations.

   ‘#<…>’ ......

   ‘##’ ......

   ......

   ‘?C’

• But here it also implies that some of the above notations (#<...>, ##, ?C, ...) are also hash notation. Doesn't this contradict the last one?

• Why does the name "hash notation" include "hash"?

Answer 1

I’m not really sure that there is a widely–recognized name for these things in Emacs Lisp. The language has grown and changed over time, and things have been added to the language as needed. You could look into the version control history of the Elisp manual to find out more about how it has changed over time. I am sure that they would consider improvements to it as well. You could fix it by simply inserting another sentence after the #<…> example saying something like “The # character also introduces these other syntactic elements recognized by the reader”. Also, I have no idea why the character syntax (?C) is even on that page.

The use of # in Lisp syntax is influenced by other Lisps such as Common Lisp, where the # character indicates the use of a reader macro. The Common Lisp Specification calls it a non–terminating dispatching macro character, which is a long–winded way of saying that the character after the # tells the reader which macro to invoke. But notice that it never actually names the syntactic elements themselves; it just says that they all dispatch to reader macros.

Answer 2

Just for the second question:

The “Hash Notation” is just a kind of notation using “#”.
The mark “#” is named “hash mark”, so there is nothing to do with “hash function”.

Answer 3

It seems that the GNU Emacs Lisp Reference Manual has actually cleared up this misunderstanding.

See Chapter 8 Hash Tables:

Hash notation, the initial ‘#’ character used in the printed representations of objects with no read representation, has nothing to do with hash tables.

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This is a belated explanation, IMHO, since the term “Hash Notation” has been covered since Chapter 2.