[Openmcl-devel] Side effects when redefining dispatching reader macro?

[Gary Byers]

On 11/12/2015 11:37 AM, Ron Garret wrote:
> (MAKE-DISPATCH-MACRO-CHARACTER #\# NIL*readtable*)
CLHS says (of MAKE-DISPATCH-MACRO-CHARACTER) that,

"Initially, every /character/ 
<http://www.lispworks.com/documentation/lw50/CLHS/Body/26_glo_c.htm#character> 
in the dispatch table associated with the /char/ has an associated 
function that signals an error of /type/ 
<http://www.lispworks.com/documentation/lw50/CLHS/Body/26_glo_t.htm#type> *reader-error* 
<http://www.lispworks.com/documentation/lw50/CLHS/Body/e_rder_e.htm#reader-error>."

In the body of your message, you seemed to expect #' to have retained 
its original definition.

If the term "... makes char be a dispatching-macro-character ..." is 
read as "does nothing if the character is already defined as a 
dispatching-macro, else ...",
your expectation is reasonable. I am probably more accustomed to the 
behavior that that you said that you get in 1.10, but both expectations 
seem reasonable.

Portable code could avoid the issue by calling 
GET-DISPATCH-MACRO-CHARACTER and only calling 
MAKE-DISPATCH-MACRO-CHARACTER if the character is not already defined as 
a dispatching macro.  I don't know what the library that the original 
poster mentioned does. or what version of that library they are using.
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