If you look at an earlier thread you can see that I also was (and still
am?) confused by this. I think some clarification is needed.
It is not stated what this means. I guess maybe it means that writing
out
(1 2 3) and reading it back in during the same run will give a value
that is equal? to the original. Is that correct?
Yes.
* Several syntactic datums can
represent the same datum value,
I'm unsure what is meant by this. For example, do the two 1s in (cons
1 1) count as distinct syntactic datums that may represent the same
value, or does this only refer to the fact that 1.0 and 1.00000 may
represent the same value (as stated in chapter 3)?
Syntactic datums are just strings of characters. So yes the strings "1"
are the same.
* but the datum value corresponding to a
syntactic datum is uniquely determined.
I also find this confusing. Clearly this cannot mean uniqueness with
respect to eq? Does this mean uniqueness with respect to equal? ?
Or could the two syntactic datums in (cons "abc" "abc") represent
different datum values (given that they are considered as separate
syntactic datums, which is unclear)?
I think you are on the wrong track here. Syntactic datums and datum
values are not Scheme objects/values, and it makes no sense to apply
Scheme function to them.
Chapter 3 (introduction) says:
* A syntactic datum can be used in a
program to obtain the corresponding value using quote.
What is meant by "the"?
_The_ (unique) datum value corresponding to a syntactic datum is
described by grammar and text spread out over chapter 3.
* Moreover, valid Scheme expressions form a subset of the syntactic
datums.
Is this actually true? I believe that there are valid Scheme
expressions represented by syntax objects that are not syntactic
datums as described by
the grammar of this section. For example, macro-generated Scheme
expressions may be valid only if identifiers are interpreted as
entities containing substitutions/renamings. If this extra data is
removed, the expression would cease to be valid.
I think "Scheme expression" here is to be interpreted as the program
text before any macro expansion.
Also, is the intent to talk only about expressions or forms?
* Note that several syntactic datums may represent the same object, a
so-called
datum value. For example, both "#e28.000"
and "#x1c" are syntactic datums representing the exact integer 28;
The
syntactic datums "(8 13)", "( 08 13 )", "(8 . (13 . ()))" (and
more) all
represent a list containing the integers 8 and 13.
This paragraph promises to illustrate syntactic datums that represent
the same object. However, I believe the language "the same object" at
least strongly suggests eq? to many Schemers. However, the examples
are of syntactic datums that give rise to objects that may not be eq?
I agree, "same" implies "eq?" for me too. And here the report actually
talk about objects.
The next line says:
* Syntactic datums that denote equal objects are
always equivalent as forms of a program.
Perhaps changing equal to equal? may clarify what is meant by this (if
it is in fact true - see comment about syntax objects above).
Michael Sperber already agreed to add a reference to "equal?" here.
* A syntactic datum occurring in program text is often called a form.
Does this mean that the datum (x 1) in (let ((x 1)) x) is a form? How
about
the 1 in (display (quote 1))?
Pass. (I think the answer is "no" but I prefer if someone else fill in.)
Section 4.8 says:
* It is desirable for constants (i.e. the values of literal
expressions) to
reside in read-only-memory.
I assume that the term "constant" here refers to what is called "datum
value"
in section 1.9. If so, it may be better to use the same terminology.
No not the same as datum value. Of the following three expression the
first to are "literals" but the third is not. All are datum values.
"abc"
(quote (1 2 3))
(list 1 2 3)
To sum up my understanding of the issue: "Syntactic datums" are just
strings of characters as defined by the grammar. "Datum values" are
something more abstract, described only indirect via a function from
syntactic datums. Datum values can be "instantiated" as Scheme values,
for example with "quote" or by using the "read" procedure. No all Scheme
values have a corresponding datum value, for example a circular list.
/Mikael
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