[r6rs-discuss] [Formal] Vector size in the read syntax

[Jens Axel Søgaard]

Submitter: Jens Axel Søgaard
Email address: jensaxel at soegaard.net
Issue type: Enchancement
Priority: Minor
Component: Lexical Syntax
Report version: 5.91
Summary:  Vector size in read syntax


PROPOSAL
--------

In section "3.3.2 Vectors" the read syntax of vector
datums are

     #(<datum> ...) .

I propose to allow the following syntax:

     #<natural number>(<datum> ...)

The natural number between the # and the opening parenthesis
denotes the vector size.

If there are fewer <datum>s than the given size, the vector
is filled with the value denoted by the last <datum>.
E.g #4(0 1) denotes #(0 1 1 1).

If no datums are present the vector is filled with 0.
E.g. #4() denotes #(0 0 0 0).

More more <datum>s than the given size is not allowed.


RATIONALE
---------

The #<number>(<datum> ...) requires less memory to parse.

During the parsing of, say, #100000(1 2 ...), the reader
can allocate the vector after seeing the #100000( and then
fill in the vector slots.

With the #(1 2 ...) syntax the reader needs to allocate a
temporary data structure to hold the data, until all datums
have been read. Then the vector can be allocated and the
slots be filled.


CURRENT PRACTICE
----------------

Chez Scheme
- - - - - -

Vectors may be printed with an explicit length prefix, and when the 
explicit length prefix is specified, duplicate trailing elements may be 
omitted. For example, #(a b c) may be printed as #3(a b c), and a vector 
of length 100 containing all zeros may be printed as #100(0). The 
printer generally prints the prefix and suppresses duplicate trailing 
elements unless the parameter print-vector-length is set to false.

<http://www.scheme.com/csug7/intro.html#./intro:h1>


PLT Scheme
- - - - -

Vector constants can be unquoted, and a vector size can be specified 
with a decimal integer between the # and opening parenthesis. If the 
specified size is larger than the number of vector elements that are 
provided, the last specified element is used to fill the remaining 
vector slots. For example, #4(1 2) is equivalent to #(1 2 2 2). If no 
vector elements are specified, the vector is filled with 0. If a vector 
size is provided and it is smaller than the number of elements provided, 
the exn:fail:read exception is raised.

<http://download.plt-scheme.org/doc/352/html/mzscheme/mzscheme-Z-H-11.html#node_sec_11.2.3>


SISC
- - -

SISC supports the length prefix method of creating Vector constants. For 
example, '#5(x)  creates a vector constant containing five identical 
symbols. In addition, the length-prefix form is used when printing 
vectors, and if elements repeat at the end of a Vector, only the last 
unique element is printed. This form is referred to as the compact 
vector representation. The unprefixed form with all elements displayed 
is called the verbose representation.

<http://sisc.sourceforge.net/manual/html/ch03.html#Vectors>