[r6rs-discuss] [Formal] Syntactic datums and datum values

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Name        : Andre van Tonder
Email       : andre at het.brown.edu
Type        : defect
Priority    : minor
Component   : Section 1.9  : Syntactic datums and datum values
               Chapter 3    : Lexical syntax and read syntax
               Section 4.8  : Storage model
Version     : 5.92
Dependencies: None
Summary:
--------
Imprecise description may lead to confusion.
Discussion:
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Section 1.9 says:
  * Each datum value may be represented in textual
    form as a syntactic datum, which can be written out and read back in without
    loss of information.
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?
  * 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)?
  * 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)?
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"?
  * 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.
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?
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).
  * 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))?
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.
The next line says:
  * Literal constants, the strings returned by symbol->string, records with no
    mutable fields, and other values explicitly designated as
    immutable are immutable objects, ...
Again, now datum values are called literal constants instead of the previously 
established terminology.  Also, the line above stated the desirability of 
read-only memory for the first of these cases only, which makes the extra 
items in the laundry list of this line stylistically surprising.