Network Working Group R. Winter
Request for Comments: 515 Computer Corporation of America
NIC 16446 6 June 1973
 Specifications for Datalanguage, Version 0/9
Preface
 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.
1. Introduction
 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
 container.
 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.
3.1 Ident
 The ident of a container is composed of alphanumeric characters,
 the first of which is alphabetic. It may not consist of more than
 100 characters.
 The elements of a STRUCT must have idents unique in the STRUCT.
3.2 Function
 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.
3.3 Type
 Type is one of: LIST, STRUCT, or STRING. These are defined on the
 preceding page.
 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(***).
3.4 Size
 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
 ASCII characters)(***).
3.5 Inversion
 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).
3.6 Syntax
 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
 OBSERVATION STRUCT
 LOCATION STRUCT
 CITY STR (10), I=D
 COUNTRY STR (10), I=D
 END
 TIME STRUCT
 YEAR STR (2)
 DAY STR (3)
 HOUR STR (2)
 END
 DATE STRUCT
 TEMPERATURE STR (3)
 RAINFALL STR (3)
 HUMIDITY STR (2)
 END
 END
 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.
4. Directory
 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.
4.1 CREATE
 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.
4.2 DELETE
 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
 proper method).
4.3 LIST
 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:
 LIST <pn>.%ALL
 Lists pathnames subordinate to <pn>.
 LIST <pn>.%SOURCE
 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.
5.1 Modes
 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.
5.2 Syntax
 To open a container:
 OPEN <pathname> <mode> ;
 or:
 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> ;
5.3 LIST
 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.
6. Assignment
 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-
 mode.
 If A is not an outermost-LIST, then it is always effectively in
 WRITE-mode.
 After taking the mode of A into account, as described above, the
 procedure is:
 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).
7. FOR
 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
 all blanks.
 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
 output LIST-members.
 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
 B STR
 C STR
 END
 (1) P = F ;
 (2) FOR P.R, F.R
 P.R = F.R ;
 END ;
 (3) FOR P.R, F.R WITH A1 EQ 'XY' OR A2 GE 'AB'
 B = C ;
 C = A2 ;
 END
 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
 match.
 These restrictions are:
 (a) both LISTs must be objects in the context in which the FOR
 appears.
 (b) both LISTs must be open or contained in open outermost
 containers.
 (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
 FOR.
 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
 the description:
 F FILE LIST
 R STRUCT
 A STR (1)
 B STR (2)
 END
 the expression:
 F.R
 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
 form:
 <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
 with NOT.
 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>
 where:
 (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:
 EQ equal
 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).
7.2 FOR-body
 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
 completeness.
 The restriction is most easily understood by example:
 given the description
 F LIST
 R STRUCT
 A STR (3)
 BL LIST (3)
 B STR (3)
 C STR (3)
 END
 and the request fragment:
 FOR ...,R
 FOR ...,B
 ... = A ;
 ... = C ;
 END
 END
 observe:
 (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:
 F LIST
 R STRUCT
 A STR (3)
 C STR (3)
 BL LIST (3)
 B STR (3)
 END
 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
 transmission.
 Such an address is supplied through a CONNECT-request, which
 associates a container (having a function of PORT) with an external
 address:
 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
 transmitted.
 The address associated with a port can be changed by issuing another
 CONNECT-request.
 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 is:
 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
 Datacomputer, or
 (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
 session.
 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
 qualified names.
 For example:
 WEATHER FILE LIST
 STATION STRUCT
 CITY STR (15)
 STATE STR (15)
 DATA LIST (24)
 OBSERVATION STRUCT
 HOUR STR (2)
 TEMPERATURE STR (3)
 HUMIDITY STR (2)
 PRESSURE STR (4)
 END
 END
 RESULTS PORT LIST
 RESULT STRUCT
 CITY STR (15)
 HOUR STR (2)
 TEMPERATURE STR (3)
 END
 FOR STATION WITH STATE EQ 'CALIFORNIA'
 FOR RESULT, OBSERVATION WITH HOUR GT '12'
 AND HUMIDITY LT '75'
 CITY = CITY ;
 HOUR = HOUR ;
 TEMPERATURE = TEMPERATURE ;
 END ;
 END ;
 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
 description:
 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,
 are made:
 (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
 R STRUCT
 A STR
 B STR
 S STRUCT
 R STR
 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)
 and:
 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
 to %OPEN.
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
 or port.
 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
 listed.
 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
 container.
 Examples:
 Let P be the ident of an open PORT. Let A.B.C be a
 directory pathname.
 LIST %ALL
 LIST A.B.C.%ALL
 LIST %OPEN
 LIST %ALL.%SOURCE
 LIST %OPEN.%SOURCE
 LIST P.%SOURCE
 LIST %OPEN.%DESC
 LIST P.%DESC
 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:
 'ABC'
 A single quote is included in a constant by preceding it with a
 double quote:
 'FATHER"'S'
 Likewise, a double quote is included by preceding it with a double
 quote:
 '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
 STRING constants(***).
 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
 (ignored).
 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.
10.4 Comments
 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
 a separator.
10.5 Reserved Identifiers
 Certain identifiers are reserved in Datalanguage, and should not
 be used to name containers or directory nodes. These are:
 AND
 APPEND
 AT
 CLOSE
 CONNECT
 CREATE
 DELETE
 DISCONNECT
 END
 EQ
 FILE
 FOR
 GE
 GT
 LE
 LIST
 LT
 NODE
 NE
 NOT
 OPEN
 OR
 PORT
 READ
 STR
 STRUCT
 TO
 WITH
 WRITE
 More reserved identifiers will be added in Version 0/10.
11. Datalanguage Syntax Expressed in BNF
11.1 Requests
 11.1.01 <request> ::= ;
 11.1.02 <request> ::= <create>
 11.1.03 <request> ::= OPEN <pn> ;
 11.1.04 <request> ::= OPEN <pn> <mode> ;
 11.1.05 <request> ::= CLOSE <ident> ;
 11.1.06 <request> ::= CONNECT <ident> TO <address> ;
 11.1.07 <request> ::= DISCONNECT <ident> ;
 11.1.08 <request> ::= MODE <ident> <mode> ;
 11.1.09 <request> ::= DELETE <pn> ;
 11.1.10 <request> ::= LIST <listarg> ;
 11.1.11 <request> ::= <sr-request> ;
11.2 Data Description and Creation
 11.2.01 <create> ::= CREATE <pn>
 11.2.02 <create> ::= CREATE <pn> <ftn> LIST <desc>
 11.2.03 <create> ::= CREATE <pn> <ftn> LIST <size> <desc>
 11.2.04 <desc> ::= <ident> <attributes>
 11.2.05 <attributes> ::= LIST <size> <desc>
 11.2.06 <attributes> ::= STRUCT <descs> END
 11.2.07 <attributes> ::= STR <size>
 11.2.08 <attributes> ::= STR <size> ,I=D
 11.2.09 <descs> ::= <desc>
 11.2.10 <descs> ::= <descs> <desc>
 11.2.11 <ftn> ::= PORT
 11.2.12 <ftn> ::= FILE
 11.2.13 <ftn> ::= TEMP PORT
 11.2.14 <ftn> ::= TEMPORARY PORT
 11.2.15 <size> ::= ( <integer constant> )
11.3 Data Storage and Retrieval
 11.3.01 <sr-request> ::= <assign>
 11.3.01 <sr-request> ::= <loop>
 11.3.03 <assign> ::= <pn> = <object>
 11.3.04 <loop> ::= FOR <looparg> <loopbody> END
 11.3.05 <looparg> ::= <exp>
 11.3.06 <looparg> ::= <pn> , <exp>
 11.3.07 <loopbody> ::= <sr-request>
 11.3.08 <loopbody> ::= <loopbody1> <sr-request>
 11.3.09 <loopbody> ::= <loopbody1>
 11.3.10 <loopbody1> ::= <sr-request> ;
 11.3.11 <loopbody1> ::= <loopbody1> <sr-request> ;
11.4 Expressions
 11.4.01 <exp> ::= <pn>
 11.4.02 <exp> ::= <pn> WITH <boolexp>
 11.4.03 <boolexp> ::= <pn> <comop> <string constant>
 11.4.04 <boolexp> ::= ( <boolexp> )
 11.4.05 <boolexp> ::= NOT <boolexp>
 11.4.06 <boolexp> ::= <boolexp> AND <boolexp>
 11.4.07 <boolexp> ::= <boolexp> OR <boolexp>
 11.4.08 <comop> ::= EQ
 11.4.09 <comop> ::= NE
 11.4.10 <comop> ::= GT
 11.4.11 <comop> ::= LT
 11.4.12 <comop> ::= GE
 11.4.13 <comop> ::= LE
11.5 Miscellaneous
 11.5.01 <address> ::= <quote> <TENEX file designator>
 <quote>
 11.5.02 <address> ::= <socket> AT <host>
 11.5.03 <address> ::= <socket>
 11.5.04 <socket> ::= <integer constant> //INTERPRETED AS
 DECIMAL
 11.5.05 <host> ::= <integer constant> //INTERPRETED AS
 DECIMAL
 11.5.06 <host> ::= ***** TENEX host names *****
 11.5.07 <object> ::= <pn>
 11.5.08 <object> ::= <string constant>
 11.5.09 <mode> ::= READ
 11.5.10 <mode> ::= APPEND
 11.5.11 <mode> ::= WRITE
 11.5.12 <listarg> ::= %ALL
 11.5.13 <listarg> ::= <pn>.%ALL
 11.5.14 <listarg> ::= %OPEN
 11.5.15 <listarg> ::= %ALL.%SOURCE
 11.5.16 [<LISTARG>] ::= <IDENT>.%SOURCE
 11.5.17 <listarg> ::= %OPEN.%SOURCE
 11.5.18 <listarg> ::= %OPEN.%DESC
 11.5.19 <listarg> ::= <ident>.%DESC
 11.5.20 <pn> ::= <ident>
 11.5.21 <pn> ::= <pn>.<ident>
 11.5.22 <ident> ::= <letter>
 11.5.23 <ident> ::= <ident> <letter>
 11.5.24 <ident> ::= <ident> <digit>
 11.5.25 <integer constant> ::= <digit>
 11.5.26 <integer constant> ::= <integer constant> <digit>
 11.5.27 <string constant> ::= <quote> <string conbody>
 <quote>
 11.5.28 <string conbody> ::= <nonquote>
 11.5.28 <string conbody> ::= <string conbody> <nonquote>
11.6 Character Set
 11.6.01 <separator> ::= //SPACE(32)
 11.6.02 <separator> ::= //TAB(9)
 11.6.03 <separator> ::= <eol>
 11.6.04 <special> ::= <quote>
 11.6.05 <special> ::= <superquote>
 11.6.06 <special> ::= <special1>
 11.6.07 <letter> ::= A
 11.6.08 <letter> ::= B
 ...............
 11.6.09 <letter> ::= Z
 11.6.10 <letter> ::= a
 11.6.11 <letter> ::= b
 ...............
 11.6.12 <letter> ::= z
 11.6.13 <digit> ::= 0
 11.6.14 <digit> ::= 1
 ...............
 11.6.15 <digit> ::= 9
 11.6.16 <nonquote> ::= <letter>
 11.6.17 <nonquote> ::= <digit>
 11.6.18 <nonquote> ::= <superquote> <quote>
 11.6.19 <nonquote> ::= <superquote> <superquote>
 11.6.20 <nonquote> ::= <special1>
 11.6.21 <nonquote> ::= <separator>
 11.6.22 <eol> ::= //EOL (31)
 11.6.23 <eol> ::= <carriage return> <line feed>
 11.6.24 <carriage return> ::= //CARRIAGE RETURN (13)
 11.6.25 <line feed> ::= //LINE FEED (10)
 11.6.26 <quote> ::= ' //SINGLE QUOTE(44)
 11.6.27 <superquote> ::= " //DOUBLE QUOTE(34)
 11.6.28 <special1> ::= ! //EXCLAMATION POINT(33)
 11.6.29 <special1> ::= # //NUMBER SIGN(35)
 11.6.30 <special1> ::= $ //DOLLAR SIGN(36)
 11.6.31 <special1> ::= % //PERCENT SIGN(37)
 11.6.32 <special1> ::= & //AMPERSAND(38)
 11.6.33 <special1> ::= ( //LEFT PARENTHESIS(40)
 11.6.34 <special1> ::= ) //RIGHT PARENTHESIS(41)
 11.6.35 <special1> ::= * //ASTERISK(42)
 11.6.36 <special1> ::= + //PLUS SIGN(43)
 11.6.37 <special1> ::= , //COMMA(44)
 11.6.38 <special1> ::= - //MINUS SIGN(45)
 11.6.39 <special1> ::= . //PERIOD(46)
 11.6.40 <special1> ::= / //SLASH(47)
 11.6.41 <special1> ::= : //COLON(58)
 11.6.42 <special1> ::= ; //SEMICOLON(59)
 11.6.43 <special1> ::= < //LEFT ANGLE BRACKET(60)
 11.6.44 <special1> ::= = //EQUAL SIGN(61)
 11.6.45 <special1> ::= > //RIGHT ANGLE BRACKET(62)
 11.6.46 <special1> ::= ? //QUESTION MARK(63)
 11.6.47 <special1> ::= @ //AT-SIGN(64)
 11.6.48 <special1> ::= [ //LEFT SQUARE BRACKET(91)
 11.6.49 <special1> ::= //BACK SLASH(92)
 11.6.50 <special1> ::= ] //RIGHT SQUARE BRACKET(93)
 11.6.51 <special1> ::= ^ //CIRCUMFLEX(94)
 11.6.52 <special1> ::= _ //UNDERBAR(95)
 11.6.53 <special1> ::= ` //ACCENT GRAVE(96)
 11.6.54 <special1> ::= { //LEFT BRACE(123)
 11.6.55 <special1> ::= | //VERTICAL BAR(124)
 11.6.56 <special1> ::= } //RIGHT BRACE(125)
 11.6.57 <special1> ::= ~ //TILDE(126)
 <EOF>
 [ This RFC was put into machine readable form for entry ]
 [ into the online RFC archives by Walter Benton 12/98 ]