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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:
-----------
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.
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