5 ms·

Apropos. Is there reader macro definitions for "(" and ")" ? Or is Gödel rolling in his grave for such insolent heresy?

sph 4y ago
It would be a noop. A reader macro that takes a s-expr bounded by parentheses, and returns a s-expr, literally is just the identity function^Hmacro.
- stassats 4y ago
  It's not. Reader macros don't take s-expressions, they take characters.
stassats 4y ago
Sure, what's so special about it? (get-macro-character #\() => SB-IMPL::READ-LIST (get-macro-character #\)) => SB-IMPL::READ-RIGHT-PAREN (that one just signals an error, the read-list one picks up the closing #\))
- timonoko 4y ago
  In other words : You cannot redefine ")" because it cannot be undefined while reading the new definition.
  - aidenn0 4y ago
    No, it can totally be redefined. Reading happens before evaluation
  - creepycrawler 4y ago
    Sure you can. CL-USER> (set-macro-character #\) (get-macro-character #\()) T CL-USER> )format t "Hello") Hello NIL
  - thelopa 4y ago
    That’s completely incorrect. The parent comment has it right. In normal usage, the meaning of #\) 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.
    - timonoko 4y ago
      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".

sph 4y ago

It would be a noop. A reader macro that takes a s-expr bounded by parentheses, and returns a s-expr, literally is just the identity function^Hmacro.

stassats 4y ago
It's not. Reader macros don't take s-expressions, they take characters.

stassats 4y ago

Sure, what's so special about it? (get-macro-character #\() => SB-IMPL::READ-LIST (get-macro-character #\)) => SB-IMPL::READ-RIGHT-PAREN (that one just signals an error, the read-list one picks up the closing #\))

timonoko 4y ago
In other words : You cannot redefine ")" because it cannot be undefined while reading the new definition.
- aidenn0 4y ago
  No, it can totally be redefined. Reading happens before evaluation
- creepycrawler 4y ago
  Sure you can. CL-USER> (set-macro-character #\) (get-macro-character #\()) T CL-USER> )format t "Hello") Hello NIL
- thelopa 4y ago
  That’s completely incorrect. The parent comment has it right. In normal usage, the meaning of #\) 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.
  - timonoko 4y ago
    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".