[Openmcl-devel] Why is remhash being called here?

Ron Garret ron at flownet.com
Mon Jul 1 05:30:19 UTC 2013


Thanks, Gary!

On Jun 30, 2013, at 8:46 PM, Gary Byers wrote:

> 
> 
> On Sun, 30 Jun 2013, Ron Garret wrote:
> 
>> On Jun 30, 2013, at 2:08 PM, Gary Byers wrote:
>> 
>>> My guess is that REMHASH is being called to remove something from a hash table.
>> 
>> Yes.  That much I was actually able to figure out on my own.
>> 
>>> If your next question is going to be something like "what hash table, and why
>>> is a hash table apparently being maintained when forms are being read bv the
>>> REPL (in this case) ?", a short answer is that this is involved in making
>>> M-. (the editor command) work in more cases for more people and in allowing
>>> DISASSEMBLE to annotate its output with the corresponding source code.
>> 
>> That would make sense except for two things:
>> 
>> 1.  This happens in the command-line version too where there is no meta-.
> 
> But you might want to call DISASSEMBLE someday (assuming that what was read
> is compiled into something that you might want to call DISASSEMBLE on someday ...)
> Slightly less snarkily, you can use (for instance) REBUILD-CCL to create a
> CCL image from the command-line, then run that image in an environment where
> M-. works; the source-location information associated with compiled functions
> will be available to guide M-., because that information was generated at read-time.
> 
> 
> 
>> 
>> 2.  REMHASH is (apparently) only called when I evaluate a form that invokes a reader macro.
>> 
> 
> The hash table is maintained by the reader (or at least by that variant of the
> reader that's used by the REPL and by COMPILE-FILE.)   It's ultimately doing
> this so that it if the form being read is passed to the compiler then information
> about the source location of the form and its subforms can also be passed to the
> compiler and thus  made available to M-. and DISASSEMBLE.
> 
> Maintaining source information about a form or subform makes some sense if there's
> a 1:1 relationship between that form and the information; a particular form
> (DEFUN FACT ...) may have been read from a particular file at a particular range
> of positions, and the same is also likely true of a subform of that form (like
> (ZEROP N)).  The loop that you quote below calls REMHASH on entries where there's
> a 1:N relationship, as in:
> ? (trace remhash)
> NIL
> ? (+ 1 2 1 2 3)
> 0> Calling (REMHASH 1 #<HASH-TABLE :TEST EQ size 5/60 #x3020006A488D>) <0 REMHASH returned T
> 0> Calling (REMHASH 2 #<HASH-TABLE :TEST EQ size 4/60 #x3020006A488D>) <0 REMHASH returned T
> 9
> 
> The subforms 1 and 2 each had multiple "source notes" associated with them
> (there was a 1:many relationship between those forms and their location in
> the input stream) so the entries were removed in the loop that you quote
> below.
> 
> Because of the way that it works (the recursive call to READ at essentially
> the same stream position), your reader macro also causes multiple SOURCE-NOTE
> objects to be associated with its result.  (These objects are equivalent but
> not EQ, so I suppose that there's some argument in favor of slowing things down
> further and checking for that ... no, forget that I said that.)
> 
>> I know the toplevel does some things besides simply calling (print (eval (read))) but AFAIK none of those things should care (of even know) that a reader macro had been invoked.  Hence my puzzlement.
> 
> Hopefully, knowing that that isn't what's actually happening is less puzzling.
> 
>> 
>>> If your next question is going to be "aren't there significant costs involved
>>> here, and do the benefits justify those costs ?", my answer would be a resounding ...
>>> oh wait, you haven't really asked that question.  Never mind.
>> 
>> Indeed, I have not asked that question.  I'm really just trying to understand why the toplevel is doing something different when a reader macro is invoked.
> 
> It isn't really.  Reader macros (those associated with #\(, #\', and many
> others) get invoked all the time.  The form that your reader macro returned
> happened to get removed from a hash table because it had multiple source notes
> associated with it (honest), and AFAICT this has to do with the fact that it
> effectively calls READ on the same stream at the same position twice.
> 
> Someone once said that human beings were invented by water as a means of
> transporting itself from one place to another.  I'm not sure about that,
> but I sometimes get the impression that the CCL reader and compiler were
> invented by SOURCE-NOTEs for similar reasons.
> 




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