RFC 195 (rfc195) - Page 2 of 4
Data computers-data descriptions and access language
Alternative Format: Original Text Document
RFC 195 Data Computers July 1971
4. Data items in files may be stored in arbitrary representations
(e.g., those of the originating user's HOST rather than that of
the data computer or other "standard" representation).
5. Access to a file will normally be to some subset of it. (I.e.,
the unit for transmission will usually be part of a file rather
than the whole file, and access will not necessarily be
sequential).
Consequences
------------
1. A method of data description significantly more powerful than
now commonly available (as with COBOL or PL/I) is required.
The descriptions must be stored with the files. Data item
representations and storage organizations must be describable.
2. The data computer must offer a "data reconfiguration service",
based on use of the data descriptions.
3. A representation and organization-independent level of
discourse must be made available for controlling access.
Data Description
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As it happens, the descriptive facilities in ELl (References 1 and 2)
are almost adequate as they stand. ELl is an extensible language --
the compiler and interpreter for ELl are principal components of a
system implemented on the PDP-lO at Harvard -- which allows the
definition of arbitrary data structures in terms of a few primitive
data types (BOOL, CHAR, INT, REAL, SYMBOL, MODE, FORM, and ROUTINE).
These data types are of the sort I called "generic" in Reference 3.
To the EL1 implementation on the PDP-10, say, we would have to add
methods to describe a specific representation of INT, etc. and
primitive routines to convert between specific representations.
In the ECL system (in which EL1 is embedded), there is no rigid
distinction between compile time and run time. In particular, if the
arguments and free variables of a routine are evaluable at compile
time, then the routine is evaluated and the value replaces the call.
More generally, arbitrarily large amounts of a routine being compiled
may collapse into values. As far as the data computer is concerned,
this offers the possibility of producing tailor-made data
reconfiguration programs, taking maximum advantage of the data
descriptions at compile time rather than using a strictly
interpretative mode of operation.
Healy