Network Working Group R. Winter
Request for Comments: 515 Computer Corporation of America
NIC 16446 6 June 1973 Specifications for Datalanguage, Version 0/9
Datalanguage is the language processed by the Datacomputer, a data
utility system being developed for the Arpanet. The Datacomputer
performs data storage and data management functions for the benefit
of computers on the network.
Version 0/9 is currently running at CCA. This version is extremely
primitive; however, it does offer an opportunity for experience with
the Datacomputer and with fundamental Datalanguage concepts.
Subsequent versions will provide greater portions of the full
Datalanguage capability, which has been described earlier
(Datalanguage, Working Paper No. 3, Datacomputer Project, October,
1971, NIC 8028). For example, one of the primary restrictions in
0/9--elementary data items must be fixed-length ASCII strings--will
be eliminated in Version 0/10, which is currently being implemented.
Based on the experience gained in the implementation of these early
versions, and based on the feedback from their use, a revised
specification of the full language will be issued.
This document presents a precise and complete specification of
Datalanguage, Version 0/9. It is organized into 11 sections, of
which this introduction is the first. Section 2 discusses the
capabilities of Version 0/9 in general terms. Sections 3 and 4 are
concerned with data description and the directory. Sections 5
through 8 cover the expression of data management operations.
Section 9 discusses the recognition of names. Section 10 covers
miscellaneous topics and Section 11 specifies the syntax in BNF.
This specification is to be followed with a user manual, which will
present the language in tutorial form and treat components of the
Datacomputer-user interface other than the language.
2. Capabilities of Version 0/9
Version 0/9 of Datalanguage has capabilities for the storage of
files; for addition of data to existing files, and for the deletion
of files. Retrievals can output whole files as well as subsets of
files. Data can be selected from files by content, using expressions
formed from boolean and inequality operators.
At the option of the file creator, an inversion is constructed and
maintained by the Datacomputer. The inversion increases the
efficiency of selective retrieval, at the cost of storage space and
file maintenance effort. Users other than the file creator need not
be aware of the existence of the inversion, or of which fields are
inverted file keys. The language is designed so that they state the
desired result of a retrieval, and the Datacomputer uses the
inversion as much as the request permits.
Elementary data items are fixed-length ASCII strings. Files are a
restricted class of hierarchical structures.
Many of the restrictions mentioned in this memo will be short-lived.
In particular, those statements followed with 3 asterisks (***) refer
to restrictions that will be considerably weakened or eliminated
entirely in the next version of the software.
3. Data Description
A container is a variable whose value is a data object of general
character and arbitrary size (In Version 0/9, size is restricted.
See section 3.4). Examples of containers which are implemented in
other systems are files, records, fields, groups, and entries.
The container is distinct from the data in the container. For
example, space allocation is an operation on a container, while
changing the unit price field from 25 to 50 is an operation on data
in a container.
A container may enclose other containers. When a container is not
enclosed by another container, it is said to be outermost. If
container A encloses container B, and no other container in A also
encloses B, then A immediately encloses B.
A Datalanguage description is a statement of the properties of a
All containers have the attributes ident and type. Ident is a
character string by which users refer to the container. Type
determines the form of the container's value; the value can be
elementary, or it can consist of other containers. There are 3
types: LIST, STRUCT, and STRING(***). A LIST contains a group of
containers having the same description. A STRUCT contains a group of
containers, each of which has its own description. A STRING is a
sequence of ASCII characters. While a STRING is not really an
elementary item, it is handled as one in Version 0/9.
Certain containers can have other attributes. An outermost container
has a function. The function attribute specifies whether the
container is to be used for storage or for transmission.
Size is some meaningful dimension of the container, which is type-
dependent. It is used for space allocation and data stream parsing.
An aggregate container (i.e., one that contains other containers) has
as an attribute the description or descriptions of its components.
Thus if S is a STRUCT containing A, B, and C, then the descriptions
of A, B, and C are attributes of S.
A STRING defined in certain contexts can have an inversion attribute.
This is an access property that is not really local to the STRING,
but is associated with it for convenience.
The ident of a container is composed of alphanumeric characters,
the first of which is alphabetic. It may not consist of more than
The elements of a STRUCT must have idents unique in the STRUCT.
The function of a container is either FILE, PORT, or TEMPORARY
PORT. When the function is FILE, then the container is used for
storage of data at the Datacomputer. When the function is PORT,
then the container is used for transmission of data into or out of
the Datacomputer. When the function is TEMPORARY PORT (which may
be abbreviated TEMP PORT), the container behaves like a PORT;
however, its description is not retained in the Datacomputer
beyond the session in which it is created.
Type is one of: LIST, STRUCT, or STRING. These are defined on the
In an occurrence of a STRUCT, the elements appear in the order in
which their descriptions appear in the STRUCT description. All
elements are present in each occurrence of the STRUCT.
An element of a STRUCT or LIST can be a container of any datatype.
However, the outermost container must be a LIST(***).
The size of a STRING is the number of characters in it. The size
of a STRUCT is not defined (***). The meaning of the size of a
LIST depends upon other properties of the LIST (***).
Ordinarily, the size of a LIST is the number of LIST-members. An
exception is the case of the outermost-LIST. In an outermost-LIST
with a function of FILE, the size is the number of LIST-members
for which space should be allocated. When no size is present in
this case, the system computes a default. In an outermost-LIST
with a function of PORT, the size is ignored (***).
Only outermost containers may be larger than a TENEX page (2560
An inversion is an auxiliary data structure used to facilitate
retrieval by content.
Its basic application is the fast retrieval of sets of outermost-
LIST-members (this can be extended to other container sets, and
will be after release 1). Consider a list of weather
observations, stored as a file on the Datacomputer. If quick
retrieval of observations by COUNTRY is desired, then this is
indicated in the description of the COUNTRY container. According
to common usage in information retrieval, this makes COUNTRY a key
in the retrieval of observations.
Note that the inversion option only affects the efficiency of
retrieval by COUNTRY, not the ability to retrieve by COUNTRY.
There are restrictions on use of the inversion option. First, it
can be applied only to STRINGs. Second a STRING having the
inversion option must occur only once in each
outermost-LIST-member. Third, it is ignored when applied to
STRINGs in PORT descriptions.
Eventually there will be several types of inversion option; in
Version 0/9 there is only the 'D' option (for distinct).
The description is simply an enumeration of properties; these
properties are specified in the order:
<ident> <function> <type> <size> <other>
Properties which do not apply are omitted. An example:
F FILE LIST (25) A STR (10)
Here 'F' is the <ident>, 'FILE' is the <function>, 'LIST' is the
<type>, '(25)' is the size, and 'A STR (10)' is the <other> of one
description. Of course, 'A STR (10)' is itself another
description: the description for members of the LIST named F.
An example of a complete description for a file of weather
observations keyed on location:
WEATHER FILE LIST
CITY STR (10), I=D
COUNTRY STR (10), I=D
YEAR STR (2)
DAY STR (3)
HOUR STR (2)
TEMPERATURE STR (3)
RAINFALL STR (3)
HUMIDITY STR (2)
The ENDs are needed to delimit the list of elements of a STRUCT.
`, I=D' indicates that the string is to be an inversion key for
the retrieval of outermost-LIST-members.
The directory is a system file in which the names and descriptions of
all user-defined containers are kept.
The directory is structured as a tree. Each node has an ident, which
need not be unique. There is a single path from the root of the tree
to any node. The idents of the nodes along this path are
concatenated, separated by periods, to form a pathname, which
unambiguously identifies the node (e.g., A.B.C could be a pathname
for node with an ident of C).
In a later version of the software, the directory will be generalized
to provide for links between nodes, so that it will not properly be a
tree. For now, however, the tree model is convenient and adequate.
A node may represent a container, or it may simply hold a place in
the space of pathnames. When it represents a container, it cannot
(currently) have subordinate nodes.
Eventually, it is planned to model the directory as a structure of
containers, with its description distributed throughout the
structure. Most operations defined on the directory will be defined
on user data, and vice versa. Access privileges and privacy locks
will be part of the data description and will likewise be applicable
both to directory nodes and data structures below the node level.
A CREATE-request either; (a) adds a node to the directory,
optionally associating the description of either a PORT or a FILE
with the node, or (b) creates a temporary container which is not
entered in the directory, but has a description and can be
referenced in requests. If the description defines a file, CREATE
causes space to be allocated for the file.
To create a node with a description:
CREATE <pathname> <description> ;
To create a node with no description:
CREATE <pathname> ;
Note that the description determines whether or not the container
is temporary (see section 3.2 for details).
A CREATE-request adds a single node to the directory. Thus to add
CCA.RAW.F to an empty directory, three requests are needed:
CREATE CCA ;
CREATE CCA.RAW ;
CREATE CCA.RAW.F ;
Notice that the last ident of the pathname doubles as the first
ident of the description:
CREATE CCA.RAW.G FILE LIST A STR (5) ;
That is, G is both the ident of a node and the ident of an
outermost container of type LIST.
A DELETE-request deletes a tree of nodes and any associated
descriptions or data. The syntax is:
DELETE <pathname> ;
The named node and any subordinates are deleted.
Note that to delete data while retaining the directory entry and
description, DELETE should not be used (see section 6.3 for the
The LIST-request is used to display system data of interest to a
user. It causes the data specified to be transmitted through the
Datalanguage output port.
Several arguments of LIST apply to the directory. LIST %ALL
transmits all pathnames in the directory. LIST %ALL.%SOURCE
transmits all descriptions in the directory. Instead of %ALL, a
pathname can be used:
Lists pathnames subordinate to <pn>.
lists descriptions subordinate to the node represented by <pn>.
For details about the LIST-request, see section 10.1.
5. Opening and closing containers
Containers must be open before they can be operated on.
A container is open when it is first created. It remains open until
closed explicitly by a CLOSE-request or implicitly by a DELETE-
request or by session end.
A closed container is opened by an OPEN-request. A temporary
container is always open; a CLOSE-request deletes it.
An open container has a mode, which is one of: READ, WRITE, or
APPEND. The mode determines the meaning and/or legitimacy of
certain operations on the container.
The mode is established by the operation which opens the
container. It can be changed at any time by a MODE-request. A
CREATE leaves the container in WRITE mode. An OPEN either
specifies the mode explicitly or implicitly sets the mode to READ.
To open a container:
OPEN <pathname> <mode> ;
OPEN <pathname> ;
where <mode> is defaulted to READ.
To close a container:
CLOSE <ident> ;
where <ident> is the name of an outermost container.
Two containers with the same outermost <ident> can not be
opened at the same time (***).
To change the mode of an open container:
MODE <ident> <newmode> ;
LIST %OPEN transmits name, mode and connection status of each open
outermost container through the Datalanguage output port. (The
Datalanguage output port is the destination to which all
Datacomputer diagnostics and replies are sent. It is established
when the user initially connects to the Datacomputer.) For
details of the LIST-request, see section 10.1.
Assignment transfers data from one container to another.
The equal sign ('=') is the symbol for assignment. The value of the
operand on the right of the equal sign is transferred to the operand
on the left. (Eventually, both operands will be weakly-restricted
Datalanguage expressions, which may evaluate to sets as well as to
single containers. Now, the left must be a container name, the right
may be a container name or a constant.)
Assignment is defined for all types of containers. When the
containers are aggregates, their elements are paired and data is
transferred between paired elements. Elements of the target
container that do not pair with some source element are handled with
a default operation (currently they are filled with blanks).
The operands of an assignment must have descriptions that match. The
idea of matching is that the descriptions must be similar enough so
that it is obvious how to map one into the other.
6.1 Conditions for legitimate assignment
Assignment must reference objects, not sets. An object is:
(a) an outermost container, or
(b) a constant, or
(c) in the body of a FOR-loop, either
(c1) a member of a set defined by a FOR-OPERAND, or
(c2) a container which occurs once in such a member
In the case of a reference of type (c1), the object referenced is
taken to be the current member. In the case of (c2), the object
referenced is that which occurs in the current member. This is
explained further in section 7.
The left operand of an assignment is subject to further
restriction. If it is an outermost container, it must be open in
either WRITE- or APPEND-mode. If it is not an outermost
container, then the reference is of type (c), which means that
some FOR-operand has established a context in which the assign-
operand is an object. The FOR-operand which establishes this
context must be the output-operand of the FOR.
When the assign-operand is an outermost container, it must be
open. Such an operand must be referenced by its simple container
ident(***), not its directory pathname.
In the body of a loop nested in one or more other loops,
assignments are further restricted, due to a 0/9 implementation
problem. See section 7.2 for details.
Finally, the descriptions of the operands must match. If one is a
constant, then the other must be a STRING(***). If both are
containers, then in the expression:
A = B;
the descriptions of containers A and B match if:
1. A and B have the same type
2. If A and B are LISTs, then they have equal numbers of
LIST-members, or else A is an outermost-LIST.
3. If A and B are aggregates, then at least one container
immediately enclosed in A matches, and has the same ident as, one
container immediately enclosed in B.
6.2 Result of assignment
If the operands are STRINGs, then the value of B, left-justified,
replaces the value of A. If B is longer than A, the value is
truncated. If B is shorter than A, then A is filled on the right
with blanks as necessary.
If the operands are STRUCTs, then assignment is defined in terms
of the STRUCT members. If a member of A, mA, matches and has the
same name as a member of B, mB, then mB is assigned to mA. If no
such mB exists, then mA is filled with blanks.
If the operands are LISTs, the result depends on several factors.
First, notice that the descriptions of the LIST-members must
match; otherwise the assignment would not be legitimate by the
matching rules of 6.1.
If A is an outermost-LIST, then it can be in either of two modes:
WRITE or APPEND. If A is in WRITE-mode, its previous contents are
first discarded; it is then handled as though it were in APPEND-
If A is not an outermost-LIST, then it is always effectively in
After taking the mode of A into account, as described above, the
for each member of LIST B
(a) add a new member to the end of A
(b) assign the current number of B to the new member of A
6.3 Deletion of Data Through Assignment
If A is an outermost container in WRITE-mode, and B is a container
with description that matches A, and if B contains no data, then
A=B has the effect of deleting all data from A. Note that if A is
in APPEND-mode in these circumstances, then A=B is a no-operation
(i.e., has no effect).
FOR <output set spec>, <input set spec> <body> END ;
The output set is optional: that is, FOR need not produce output.
When the output set is omitted, the syntax is:
FOR <input set spec> <body> END ;
The operations specified in the body are performed once for each
member of the input set. References in the body to the input set
member are treated as references to the current input set member.
When an output set is present, a new member is created and added to
the output set for each iteration (i.e., for each input set member).
References to the output set member, similarly, are treated as
references to the current output set member.
The output set spec must be the name of a LIST member. Each
iteration of the FOR will create one such member, and add it to its
LIST (hereafter called the output LIST). The body terminates the
value that the new member receives. Any STRING in the new member
which is not given a value by the body receives he default value of
The input set spec must be an expression evaluating to a set of
LIST-members (see section 7.1 for details of input set
specification). Each iteration for the FOR will input one such
member; the FOR will terminate when each member of the set has been
processed. The LIST from which the input set members are drawn is
called the input LIST.
FOR is effectively a means of accomplishing variants of assignment
between a pair of LISTS. FOR is less concise than assignment, but
offers more flexibility. Its advantages are:
(a) not all the input LIST-members need be transferred to the
output LIST. A subset can be selected by content.
(b) the user has explicit control over the assignment of values to
This is most easily understood by an example:
P PORT LIST F FILE LIST
R STRUCT R STRUCT
B STR A STRUCT
C STR A1 STR
END A2 STR
(1) P = F ;
(2) FOR P.R, F.R
P.R = F.R ;
(3) FOR P.R, F.R WITH A1 EQ 'XY' OR A2 GE 'AB'
B = C ;
C = A2 ;
Here, (1) and (2) are entirely equivalent requests. However, (3) is
quite different and is not expressible as assignment. It selects a
subset of F.Rs. The values it gives to the P.Rs could not result
from application of the matching rules to F and P.
Because FOR is effectively assignment between a pair of LISTs, the
LISTs referenced by a legitimate FOR-operation are largely subject to
the same restrictions as LISTs referenced in an assignment. One
exception is that the descriptions of the LIST-members need not
These restrictions are:
(a) both LISTs must be objects in the context in which the FOR
(b) both LISTs must be open or contained in open outermost
(c) if the output LIST is an outermost container, it must be in
WRITE- or APPEND-mode.
(d) If the output LIST is not outermost, the LIST which most
immediately encloses it must be the output LIST of an enclosing
The mode of the output LIST of the FOR affects the result much as it
would in an assignment: that is, a FOR outputting to a LIST in
WRITE-mode overwrites previous contents, while a FOR outputting to a
LIST in APPEND-mode adds to previous contents.
CAUTION TO THE READER: For convenience, these specifications use
phrases such as 'LISTs referenced by a FOR'. Recall that such a
phrase is not literally correct, in the sense that the operands of a
FOR are always LIST members, not LISTs.
7.1 Details of input set specification
The input set is specified by a Datalanguage expression that
evaluates to a set of LIST-members. Such an expression can be
simply the set of all members of a LIST, or it can be a subset of
the members of a LIST, specified by content. For example, with
F FILE LIST
A STR (1)
B STR (2)
references all R's on the LIST F. However:
F.R WITH A EQ '5'
references only those R's containing an A having the value '5'.
The expressions permitted as input set specifications are of the
<list-member-name> WITH <boolexp>
The <boolexp> is constructed of comparison expressions joined by
the Boolean operators AND and OR. Any expression can be negated
Comparison operators have the highest precedence. Next highest is
AND, then OR, then NOT.
The comparison expressions are restricted to the form:
<container name> <comop> <constant>
(a) <constant> is a string constant enclosed in single quotes
(see section 10.2 for a discussion of constants)
(b) <comop> is one of six operators:
NE not equal
LT less than
GT greater than
LE less than or equal to
GE greater than or equal to
(c) <container name> is the name of a STRING that appears once
in each LIST-member.
The constant is truncated or padded with blanks on the right to
make it equal in size to the container to which it is being
compared. Notice that padding on the right is not always
desirable (users will have control over the padding in a future
release). In particular, care must be exercised when using
numbers in Version 0/9. (A number represented as a STRING should
actually be described as a number; eventually it will be possible
to do this).
Two operations are legitimate in a FOR-body: FOR and assignment.
These are subject to the restrictions discussed in Section 6.1 and
in the introduction to Section 7. The restrictions are related to
three requirements: (1) that the names be recognizable (see
Section 9 for details), (2) that a request be consistent regarding
direction of data transfer between containers, both within itself
and with the MODE of outermost containers, and (3) that transfers
occur between objects, not sets of objects. The first two
requirements are permanent, but will become weaker in later
versions of the language. The last requirement is temporary and
will be present only in early versions.
Due to an implementation problem associated with Version 0/9,
there is a somewhat bizarre restriction applied to references made
in the body of a loop nested in another loop. This restriction is
not expected to pose any practical problems for users, and is not
part of the language design, but is discussed here for
The restriction is most easily understood by example:
given the description
A STR (3)
BL LIST (3)
B STR (3)
C STR (3)
and the request fragment:
... = A ;
... = C ;
(a) The outer loop processes the set of R's in F.
(b) For each R in F, the inner loop processes the set of B's in
the BL contained in that R.
(c) In the body of the inner loop, there are references to A and
C, which do not occur in B, but do occur in R. That is, the
objects referenced in the inner loop body are defined by the
outer loop, not the inner loop. In general, this is fine;
in the case of C, however, we have a problem.
(d) C occurs beyond the end of BL.
The 0/9 compiler is capable of neither (1) looking ahead enough to
locate C before it compiles code for the loop, nor (2) while
generating code to loop on the B's in BL, generating a separate
body of code that skips to the end of BL to locate C. Thus it can
handle A, which has been located before it begins loop generation,
but it cannot handle C, which requires a little foresight.
The request fragment shown would not cause problems if the
description were changed to:
A STR (3)
C STR (3)
BL LIST (3)
B STR (3)
Then both A and C would have been found before code for the inner
loop was generated.
8. Data Transmission
Data is transferred from container to container by execution of
assignment and FOR operations. The outermost containers involved in
transfers can be files or ports. If both are files, then the
transfer is internal to the Datacomputer. If either is a port, then
an address in the external world is needed to accomplish the data
Such an address is supplied through a CONNECT-request, which
associates a container (having a function of PORT) with an external
CONNECT <ident> TO <address> ;
Here <address> is either a specifications of host and socket number,
or a TENEX file designator (for CCA's TENEX) enclosed in single
quotes. The host and socket form is:
<socket> AT <host>
where <socket> is a decimal number, and <host> is either a decimal
number or a standard host name (since standard host names don't exist
right now, it has to be the TENEX 'standard' name for the host.
Contact the author for the latest list.) If <host> is omitted, it is
taken to be the host from which the Datalanguage is being
The address associated with a port can be changed by issuing another
A DISCONNECT-request simply breaks an existing port/address
association without establishing a new one. (A CLOSE-request that
references an open port executes a DISCONNECT.) The syntax of
DISCONNECT <ident> ;
A port is disconnected when: (a) no successful CONNECT-request has
ever been issued for it, or (b) a DISCONNECT for the port has been
executed since the last successful CONNECT.
When a disconnected port is referenced in an assignment, it is
connected by default either to:
(a) the connection used for the transmission of Datalanguage to the
(b) the connection used for the transmission of Datacomputer
diagnostics to the user
The choice between (a) and (b), of course, depends on whether the
reference is for input or output. These connections are established
by the network user's ICP to the Datacomputer at the beginning of the
Note that CONNECT and DISCONNECT do not open files or network
connections. They simply make address associations within the
Datacomputer. The files and connections are opened before each
request and closed after each request.
9. Names in Datalanguage
A name is recognized when it has been associated with a particular
data container or set of containers.
Datalanguage has mechanisms for the recognition of names in contexts.
That is, the meaning of the name is related to where it appears.
This makes it possible to attach natural meanings to partially
WEATHER FILE LIST
CITY STR (15)
STATE STR (15)
DATA LIST (24)
HOUR STR (2)
TEMPERATURE STR (3)
HUMIDITY STR (2)
PRESSURE STR (4)
RESULTS PORT LIST
CITY STR (15)
HOUR STR (2)
TEMPERATURE STR (3)
FOR STATION WITH STATE EQ 'CALIFORNIA'
FOR RESULT, OBSERVATION WITH HOUR GT '12'
AND HUMIDITY LT '75'
CITY = CITY ;
HOUR = HOUR ;
TEMPERATURE = TEMPERATURE ;
in the assignment 'CITY = CITY', the first CITY is understood to be
RESULT.CITY and the second is understood to be STATION.CITY.
9.1 Informal Presentation of Recognition Rules
'Ident' is used in the sense of section 3. For example, in the
F FILE LIST R STRUCT A STR (1) B STR (1) END
F, R, A and B are idents.
A context is a tree whose nodes are idents. In such a tree, the
terminal nodes are idents of STRINGs. The ident of a LIST is
superior to the ident of the LIST-member. The ident of a STRUCT
is superior to the idents of the STRUCT elements. The context
whose top node is F is said to be the context of F.
! F !
! R !
! A ! ! B !
Figure 9.1-1 The context of F
A pathname is a sequence of idents, naming nodes along a path from
one node to another. A full pathname in the context starts at the
topmost node. Thus F.R.B is a full pathname in the context of F.
A partial pathname starts at a node other than the topmost node
(e.g. R.B, B).
In Datalanguage, pathnames omitting intermediate nodes, such as
F.B (which omits 'R'), are not permitted. Thus partial pathnames
are partial only in that additional names are implied on the left.
Three attempts at recognition of a pathname, PN, in a context, CX,
(a) recognition of PN as a full pathname in CX
(b) recognition of PN as a partial pathname in which only the
topmost node of CX is omitted
(c) recognition of PN as an arbitrary partial pathname occurring
only once in CX.
The attempts are made in the above order, and the recognition
process halts with the first successful attempt.
As an example, consider the description:
F FILE LIST
which defines the context in Figure 9.1-2.
! F !
! R !
! ! !
! ! !
+-----+ +-----+ +-----+
! A ! ! B ! ! S !
+-----+ +-----+ +-----+
! R !
Figure 9.1-2 Example Context
In this context, F.R.A is a full pathname. Thus, F.R.A is
recognized in attempt (a). R is a partial pathname in which only
the topmost node is omitted. Thus R is recognized in attempt (b).
Note carefully that R is recognized as a reference to F.R, not to
F.R.S.R. Finally, B is an arbitrary partial pathname occurring
only once in the context. Thus B is recognized in attempt (c).
Two stacks of contexts are maintained: one for names used in an
input sense, and one for names used in an output sense. When a
name is to be recognized, it is first decided whether the
reference is an input reference or an output reference. An input
reference is (a) the right hand operand of an assign, or (b) a
name in the input set spec of a FOR. An output reference is (a)
the left operand of an assign, or (b) the output operand of a FOR.
The first context on the appropriate context stack is then
searched, according to the procedure outlined on the previous
page. If the name is neither recognized nor ambiguous in that
context, search continues in the next context on the stack. If
the name can be recognized in none of the contexts on the
appropriate stack, it is unrecognizable.
When a stack is empty, the recognition procedure is different.
The search is carried on in a special context: The context of
%OPEN. Its top node, %OPEN, is a built in system ident.
Subordinate to %OPEN is a context for each open directory node.
Each such context represents all the idents defined in the
directory nodes having data descriptions:
F FILE LIST R STRUCT A STR (1) B STR (1)
P PORT LIST R STRUCT A STR (1) B STR (1)
then the context of %OPEN would be as in Figure 9.1-3.
! %OPEN !
! A ! ! S !
! R ! ! R !
! ! ! !
! ! ! !
+-----+ +-----+ +-----+ +-----+
! A ! ! B ! ! A ! ! B !
+-----+ +-----+ +-----+ +-----+
Figure 9.1-3 The Context of %OPEN
When a directory node is closed, the corresponding context is
removed from the context of %OPEN. When a node is opened, the
associated context is added as the rightmost context subordinate
9.2 Context Stack Maintenance
The context stacks are always empty between requests. The FOR-
operator adds entries to the stacks. FOR A adds the context of A
to the input context stack. FOR A, B ... adds the context of A to
the output stack and the context of B to the input stack.
When adding to an empty stack, FOR adds two contexts instead of
one. The second addition to the stack is the context of the
looparg; the first addition is the context of the outermost
container which encloses the looparg.
For example, given a context of %OPEN as in Figure 9.1-3, and
empty context stacks, the fragment 'FOR F.R ...' adds two
contexts: (1) the context of F, and (2) the context of F.R.
Contexts are removed from the stacks by the END matching the FOR
which added them.
10. Miscellaneous Topics
10.1 The LIST-request
The LIST-request provides a means for the user to inspect system
data of interest to him. The user can determine the contents of
the directory, the source or parsed form of any data description
in the system, and the mode and connection status of any open file
The LIST operator has a single operand, which can have any of
several forms. The action of the operator is to output a list of
values on the Datalanguage output port.
To output a list of pathnames, the operand used is '%ALL'. When
'%ALL' appears alone, all pathnames in the directory are listed.
When '%ALL' appears after the last ident in a directory pathname,
the full pathnames of all nodes subordinate to the named node are
To output one or more source descriptions, the operand '.%SOURCE'
is used. '.%SOURCE' is preceded with one of (a) '%ALL', (b)
'%OPEN', or (c) the ident of an open outermost container. The
output is either (a) all descriptions, (b) all open descriptions,
or (c) a particular open description.
To output a parsed description, the operand '.%DESC' is used
('%DESCRIPTION' is also accepted). This operand is preceded
either with (a) '%OPEN', or (b) the ident of an open outermost
Let P be the ident of an open PORT. Let A.B.C be a
Note that 'LIST A.B.C.%SOURCE' is not legal - '.%SOURCE'
must be preceded with the ident of an open container, not a
pathname. A similar restriction applies to '.%DESC'.
10.2 Constants in Datalanguage
A constant of type STRING can be included in a Datalanguage
request by enclosing it in single quotes:
A single quote is included in a constant by preceding it with a
Likewise, a double quote is included by preceding it with a double
'JOHN SAID ""HELLO""'
Such constants can be used on the right of comparison operators
and of assignment operators which reference strings.
Eventually, Datalanguage will contain facilities for the inclusion
of constants of all datatypes; such constants are simply a special
case of the Datalanguage expression and will be permitted wherever
such expressions are permitted.
10.3 Character Set
Internally, Version 0/9 of the Datacomputer software operates in
7-bit ASCII characters. Its output to the ARPANET is converted to
8-bit ASCII. On input from the ARPANET, it expects 8-bit
characters, which it converts to 7-bit characters.
To convert from 7- to 8-bit characters, a '0' bit is prefixed. To
convert from 8- to 7-bit characters, the high-order bit is
checked. If the high-order bit is a '0', the bit is discarded and
the character is accepted as a 7-bit character. If the high-order
bit is a '1', then the character is discarded.
(In the following discussion, as elsewhere in this memo, all
references to numerical character codes are in decimal).
The remainder of this section discusses treatment of codes 0
through 127, when they appear in Datalanguage requests.
In general, printing characters are acceptable in requests, while
control characters are not. There are some exceptions, which are
detailed below. The printing characters are codes 32-126. The
control characters are codes 0-31 and 127.
Certain control characters are accepted:
Tab(9) is accepted freely in requests. It functions as a
separator (explained below).
EOL(31), meaning end-of-line, is accepted in requests,
functioning both as a separator and an activator (a). EOL has
a special meaning in data, and should not be introduced into
Control-L(12) is an activator and a high-level request
delimiter. It terminates the test of any request being
processed when it is encounter in the input stream. It is
useful in Datacomputer-user program synchronization.
Control-Z(26) means end-of-session when encountered in
Datalanguage. It has the properties of control-L, and in
addition, causes the Datacomputer to execute an end-of-session
procedure, which results in closing the Datalanguage
connections, closing any open files or ports, etc. The effect
is equivalent to a [LOGOUT(which] does not exist yet).
The two-character sequence <carriage return(13), line feed(10)>
is equivalent to EOL (and is translated to EOL on input from
the network). The reverse sequence, as well as either
character alone, is treated simply as other control characters
All other control characters are ignored.
The printing characters are further divided into four groups:
special characters, uppercase letters, lower case letters, and
digits (the membership of these groups is defined in section 11).
Corresponding upper and lower case letters are equivalent in
requests, except with quoted strings.
Certain special characters have a lexical function, which is
either break or separator. A break character terminates the
current lexical item and returned itself as the next item. A
separator character terminates the current item but does not begin
a new item (i.e., its only function is to separate items).
Multiple separators are equivalent to a single separator. A
separator can always be inserted before or after a break
character, without altering the meaning of the request.
The separators are tab(9), space(32), and end-of-line(31).
The break characters are left parenthesis(40), right
parenthesis(41), equals(61), semicolon(59), period(46), comma(44),
quote(39), and slash(47).
(a) An activator character causes the Datacomputer to process
whatever has been received since the previous activator or
the beginning of the request. The meaning of a request is
independent of the presence/absence of activators. However,
a request will not be executed until an activator beyond the
end of the request is received.
While Version 0/9 defines (carriage return, linefeed) in terms of
EOL, network users should not think in terms of sending EOL's to
the Datacomputer over the network. EOL is not part of the network
ASCII character set, and has no definite permanent place in
Datacomputer implementation plans.
Comments can be included in Datalanguage requests. A comment is
begun with the two-character sequence '/*', and ended with the
two-character sequence '*/'. Since slash is a break character, a
comment does cause a lexical break; its overall effect is that of
10.5 Reserved Identifiers
Certain identifiers are reserved in Datalanguage, and should not
be used to name containers or directory nodes. These are:
More reserved identifiers will be added in Version 0/10.