That’s completely incorrect. The parent comment has it right. In normal usage, the meaning of #\) is entirely determined by the implementation for the #\( reader macro. This happens because the reader macro for #\( keeps consuming characters until it has located AND consumed the matching #\). The reader would only run the #\) reader macro if it encountered a #\) that didn’t have a corresponding #\(. That’s what they meant when they said “it signals an error”. Specifically, it signals that there are unbalanced parentheses.
If you redefine the #\) reader macro, you could use it in a top-level context. That’s probably not a good idea since it’s easy to accidentally have too many #\) when closing a deeply nested expression and thus accidentally invoke the macro.
Aside from signaling an error, the #\) reader macro is also useful because it changes the rules when reading symbols. Basically, if you write (+ foo bar), the existence of the #\) macro helps the reader know that you’re referencing the “bar” symbol rather than the “bar)” symbol.
Generally, when people define new balanced-pair syntax for Common Lisp, (such as a #\{ macro for hash tables) they will follow the same pattern and define a corresponding reader macro for the closing side that always signals an error for all the same reasons.
Edit: also, as others have pointed out, you seem to be mistakenly assuming that the redefinition takes affect mid-way through reading the expression. That’s not how CL works. CL cleanly separates the process of executing code into a few distinct phases. First, the reader reads an entire expression (“form” in lisp terms). Then, that form is macroexpanded (traditional macros, not reader macros!) as needed before (optionally) being compiled and then executed.
The change to the read table would happen during the execution phase — well ordered after the original characters for that form are out of the picture.
You COULD force a change to the readtable mid-way through reading a form using the #. reader macro, but that definitely gets into chainsaw-juggling territory.
Ok. I understood my error in about 5.3 second but let it stay, because it was funny, and would trigger somebody to generate a wall-chart of explanations.
Anyways already in 1982 I was diabolically opposed to this kind of shit. Read and Print should be as simple as possible and always one-to-one. When you print something to disk, that is what you get when reading, no additional adjustment needed.
If you want reading macros, for example, you make your own Read. Better Read could even be in standard package "Additional-macros-for-common-lisp".
> If you want reading macros, for example, you make your own Read. Better Read could even be in standard package "Additional-macros-for-common-lisp".
It makes little sense to reimplement (and maintain over time) the whole Read if you only care about changing some small aspect of it. Instead, you can consider standard Read mechanism from CL to be extensible - reader macros are plugins/hooks/customization points/whatever you want to call it. You can maintain 1:1 Read/Print compatibility easily by defining/overriding a matching printer method, which is the Print side plugin/hook/customization point/whatever.
The only thing that could be simpler than this would be some magic that lets you automatically derive a read macro and a printing method from a simple declaration, but for that you'd have to sacrifice Turing completeness.
In my experience, reader macros are just freaking people out for unconscious and irrational reasons. I can tell because they still freak me out a little, even though I'm conceptually fine with them.
> A macro character is either terminating or non-terminating. The difference between terminating and non-terminating macro characters lies in what happens when such characters occur in the middle of a token. If a non-terminating macro character occurs in the middle of a token, the function associated with the non-terminating macro character is not called, and the non-terminating macro character does not terminate the token's name; it becomes part of the name as if the macro character were really a constituent character. A terminating macro character terminates any token, and its associated reader macro function is called no matter where the character appears. The only non-terminating macro character in standard syntax is sharpsign.
Or is Gödel rolling in his grave for such insolent heresy?