Steven M. Haflich <firstname.lastname@example.org> wrote:
| Harald Hanche-Olsen wrote:
| > | What happens when the file is compiled? Is there a need to wrap
| > | eval-when forms around the defconstant forms?
| > Um, after some further reading of the DEFCONSTANT section in the CLHS
| > I guess not. Sorry 'bout the noise.
| I'm glad that, after all this noise about naming of gensyms etc., someone
| has resturned to the initial queation. But I don't agree with the above
| ...the ANS says:
| If a defconstant form appears as a top level form, the compiler must
| recognize that name names a constant variable. An implementation may
| choose to evaluate the value-form at compile time, load time, or both.
| If the implementation chooses _not_ to evaluate the value form, that
| value form can hardly be available to the reader. Even if the
| implementation +does+ choose to evaluate the value form at compile
| time, I can still find no proof within the ANS that the _reader_ will
| have that definition available to it.
And in fact, in CMUCL it will *NOT*!!
This led to an actual portability problem in Tim Bradshaw's HTOUT macro
package a while back, which he & I hashed out at great length before
reaching the same conclusion you reach above. [ISTR he was using Lispworks,
while I was using CMUCL.] CMUCL obeys the letter of the ANS, but both
Tim & I agreed that the above-quoted section of the ANS makes CONSTANTP
practically *useless* [or at least non-portable] at compile time for
doing any kind of constant folding, since it is possible in a conforming
implementation that CONSTANTP [at compile time, say, in a macro] might
return "T" for every sub-form of a "constant" expression, yet the value(s)
of some sub-form(s) might not be available at compile time. (*sigh*)
 The CLHS says:
...symbols declared as constant by the user in the indicated
environment using DEFCONSTANT are always considered constant forms
and must be recognized as such by CONSTANTP.
But what is *NOT* required by DEFCONSTANT is that the *value* of a
"symbol declared as constant by the user" be available at compile time!
Rob Warnock <email@example.com>
627 26th Avenue <URL:http://rpw3.org/>
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