next up previous
Next: Evaluation Up: The ClassAd Language Reference Previous: Basic Concepts

Subsections

Syntax

The ClassAd language has both an abstract syntax and several concrete syntaxes. The abstract syntax describes each expression as an abstract data structure called its internal form. Each concrete syntax maps an expression to a string of ASCII characters called its external form or representation. There are currently three concrete syntaxes defined: the native syntax, the old classad syntax, and the xml syntax. Each implementation should provide an application programming-language interface (API) for manipulating internal-form expressions as data structures in the host programming language and converting between representations: parsing to convert from external to internal form, and unparsing to convert from internal to external form.

Figure 1 shows a sample expression in abstract, native, and XML syntaxes.

Figure 1: Example ClassAd Expression
\resizebox{!}{1.2in}{\includegraphics{example.eps}}


(a) Abstract Syntax


[ a = 1; b = a + 1.5; c = { a, "xxx" }; d = c[3] ]


(b) Native Syntax 


<c><a name="a"><i>1</i></a>
    <a name="b"><e>a+1.5</e></a>
    <a name="c"><l><e>a<e><s>xxx</s></l></a>
    <a name="d"><e>c[3]</e></a></c>


(c) XML Syntax


Abstract Syntax

An internal-form expression is an ordered tree: Each node is either a leaf or an internal node with a sequence of child nodes. There is a unique root node; every other node has a unique parent. The leaves of an expression tree are literal constants and references. Each internal node contains an operator. Its children are called operands.

A literal constant is an integer, real, string, boolean, absolute time, relative time, error, or undefined. All literal constants have values as indicated in Table 1. The value of an error literal is the unique value error of type Error. An error literal also has an annotation, which is a sequence of zero or more non-null ASCII characters meant for human consumption. Similarly, an undefined literal has the value undefined and an annotation. Two literals are equivalent if and only if they have the same value.


Table 1: Literal Constants
Type Value or annotation
Integer 32-bit signed, two's complement integer
Real 64-bit IEEE 754 double-precision floating point number
String Zero or more non-null characters3
Boolean The value true or false
AbsTime An indication of an instant in time and the time zone where this time was measured4
RelTime Interval between two absolute times5
Error Zero or more non-null ASCII characters providing a human-readable explanation
Undefined Zero or more non-null ASCII characters providing a human-readable explanation


A reference contains an attribute name, which is a sequence of zero or more non-null characters, or the reserved word parent. Two attribute names match if the character sequences are the same length and corresponding characters are the same except for differences in case. The following words are reserved, meaning that they may not be used as attribute names. 

    error false is isnt parent true undefined
Recognition of reserved words is independent of case. For example, false, FALSE, and False are all reserved words.

An internal node consists of an operator and a sequence of child nodes called its operands. Each operator has an arity constraining the number of operands it may have. Operators are classified as unary (one operand), binary (two operands), ternary (three operands), or varargs (any number of operands). The operators and their arities are listed in the Table 2. Table 2 also indicates the symbol used to represent each operator in the native syntax described in the following section.


Table 2: Operators
Arity Symbol Name
1 + UPLUS
1 - UMINUS
1 ~ BIT_COMPLEMENT
1 ! NOT
2 || OR
2 && AND
2 | BITOR
2 ^ BITXOR
2 & BITAND
2 == EQUAL
2 != NOT_EQUAL
2 is SAME
2 isnt DIFFERENT
2 < LESS
2 > GREATER
2 <= LESS_EQ
Arity Symbol Name
2 >= GREATER_EQ
2 << LEFT_SHIFT
2 >> RIGHT_SHIFT
2 >>> URIGHT_SHIFT
2 + PLUS
2 - MINUS
2 * TIMES
2 / DIV
2 % MOD
2 [] SUBSCRIPT
2 . SELECT
3 ?: COND
var { ... } LIST
var [ ... ] RECORD
var name(...) FUNCTION_CALL
     


With the exception of the binary operator SELECT and the varags operators RECORD and FUNCTION_CALL, the operands may be arbitrary expressions.6 The second operand of the SELECT operator, called the selector, is an attribute name. The first operand of the FUNCTION_CALL operator, the function name, is a non-empty sequence of letters, digits, and underscores not starting with a digit; the remaining operands are called actual parameters. Section 4.3.9 lists a set of built-in functions that must be provided by all implementations.7The RECORD operator has zero or more operands, called attribute definitions. Each attribute definition is an ordered pair consisting of an attribute name and an arbitrary expression. The attribute names must be distinct (ignoring case)8We say that the attribute definition N = E defines the attribute name N' if N and N' match.


Escaped Strings

Each of the concrete syntaxes represents an attribute name or string literal as a sequence of printable ASCII characters (characters with values in the range 32 - 126 inclusive).9 The translation from concrete to abstract form (parsing) and from abstract to concrete form (unparsing) in each case shares the following two algorithms.

In the concrete representation, a backslash may be followed by one of the characters 

            b  t  n  f  r  "  '  \  0  1  2  3  4  5  6  7
In the value of the resulting String or attribute name, the backslash and one or more following characters are replaced by a single ASCII character as indicated by Table 3.


Table 3: Backslash escapes
Escape Sequence Value (decimal)
\b 8 (Backspace)
\t 9 (Horizontal Tab)
\n 10 (Linefeed)
\f 12 (Formfeed)
\r 13 (Carriage Return)
\\ 92 (Backslash)
\" 34 (Quote)
\' 39 (Apostrophe)
\ $octal\_digit^+$ value of octal constant


If the character following the backslash is an octal digit, the octal constant consists of three digits if the first digit is 0, 1, 2, or 3 and two digits otherwise. However, the constant is terminated by the first character that is not an octal digit. The string is ill-formed if a backslash is followed by any character other than those listed in Table 3, or if the value of an octal constant following a backslash is zero.

Multiple canonical representations may parse to the sequence of characters, any of which may be considered an ``unparsing'' of that sequence. The canonical unparsing of a sequence of characters is defined relative to an optional delimiter chararacter, which may be quote (34), apostrophe (39), or neither. Printable ASCII characters (in the range 32 - 126 inclusive) are represented as themselves, with the exception that each backslash (92) or delimiter character (if any) is preceded by an extra backslash. Each character with the value 8, 9, 10, 12, or 13 is replaced by a backslash followed by the character b, t, n, f, or r, respectively. Any other character is replaced by a backslash followed by a three-digit octal representation of its value.

For example, the strings 

    a'\n
    a\'\n
    a\47\012
    \141\047\012
all parse to the three-character sequence consisting of a lower-case a, an apostrophe, and a newline (97, 39, 10 decimal). The canonical unparsing is the second of these if apostrophe is the delimiter, and the first otherwise.


Native Syntax.

The native syntax of the ClassAd language is quite similar to C, C++, or Java. The external representation of an expression consists of a sequence of tokens separated by optional white space and/or comments. Comments are as in C++ or Java: The string // introduces a comment that continues until the end of the current line, and /* introduces a comment that continues until the next occurrence of */ (such comments do not nest).

Tokens.

Tokens include reserved words, integer, floating-point, and string literals, attribute names, operators, and the additional lexical tokens equal sign (=), parentheses (( )), braces ({ }), brackets ([ ]), comma (,), and semicolon (;). The operators are shown in Table 2 in the previous section. The syntax of literals and attribute names is shown in Table 4.


Table 4: Names and Literals
token ::= attribute_name  | literal  | operator  | punctuation  | reserved_word
attribute_name ::= unquoted_name | quoted_name
literal ::= integer_literal
     | floating_point_literal
     | string_literal
punctuation ::= ' = ' |  ' ( ' |  ' ) ' |  ' { '  |  ' } ' |  ' [ ' |  ' ] ' |  ' , '  |  ' ; '
unquoted_name ::= (  letter | '_' )   (  letter  | decimal_digit  | '_'  )*
quoted_name ::= ''' (  non_quote |  ' " ' )+ '''
integer_literal ::= nonzero_digit  decimal_digit*
     |  ' 0 '  octal_digit*
     |  ' 0 '  (  ' x ' |  ' X ' )  hex_digit+
floating_point_literal ::= digit+  ' . '  digit*  exponent?
     |  ' . '  digit+  exponent?
     | digit+  exponent
exponent ::= (  ' e ' |  ' E ' )   (  '+' | '-' ) ?   decimal_digit+
string_literal ::= ' " '  (  non_quote | ''' )*   ' " '
letter ::= ' a '... ' z ' |  ' A '... ' Z '
non_quote ::= '\'  escaped_char
     | '\'  octal_digit  octal_digit?
     | '\'  (  ' 0 '... ' 3 ' )  octal_digit   octal_digit
     | other_character
escaped_char ::= ' n '  |  ' t '  |  ' b '  |  ' r '  |  ' f '  | '\'  |  ' " ' | '''
decimal_digit ::= ' 0 '... ' 9 '
nonzero_digit ::= ' 1 '... ' 9 '
octal_digit ::= ' 0 '... ' 7 '
hex_digit ::= decimal_digit  |  ' a '... ' f ' |  ' A '... ' F '


In Table 4, parentheses are used for grouping, literal characters are enclosed in single quotes, and the postfix meta-operators *, +, and ? are used to denote zero or more repetitions, one or more repetitions, and zero or one repetitions, respectively. The name other_character denotes any ASCII character other than Quote (34)10apostrophe (39), backslash (92), linefeed (10), carriage-return (13), or null (0). The name reserved_word denotes any of the strings 

    error false is isnt parent true undefined
in any combination of upper or lower case letters.

A ClassAd expression consists of a sequence of tokens, separated by whitespace. Whitespace consists of one or more occurrences of the ASCII characters Space (32), Horizontal Tab (9), Linefeed (10), Vertical Tab (11), Formfeed (12), or Carriage Return (13).11Whitespace is only required between two tokens if the first character of the second token could be construed as an extension of the first token according to the rules in Table 4. Two consecutive string_literals separated by whitespace are equivalent to a single literal whose value is the concatenation of the values of the two literals.

A string_literal or quoted_name is parsed according to the common rules for Escaped Strings in Section 3.2. The quoted_name syntax allows an attribute name to be any sequence of non-null ASCII characters. If the sequence conforms to the syntax of unquoted_name, the attribute name may be represented using either syntax. For example, the following are three representations in the native syntax of the same attribute name: 

    _abc        '_abc'       '_ab\143'

Grammar.

The native syntax is defined by the context-free grammar in Table 5.


Table 5: Native Expression Grammar
expression ::= binary_expression
     | binary_expression ' ? ' expression ' : ' expression
binary_expression ::= binary_expression binary_operator prefix_expression
     | prefix_expression
binary_operator ::= '||'
     | '&&'
     |  ' | '
     | '^'
     | '&'
     | '=='  | '!='  | 'is'  | 'isnt'
     |  ' < '  |  ' > '  | '<='  | '>='
     | '<<'  | '>>'  | '>>>'
     |  ' + ' |  ' - '
     |  ' * ' |  ' / ' | '%'
prefix_expression ::= suffix_expression
     | unary_operator suffix_expression
unary_operator ::= ' + ' | '-' | '~' |  ' ! '
suffix_expression ::= atom
     | suffix_expression ' . ' attribute_name
     | suffix_expression ' [ ' expression ' ] '
atom ::= attribute_name
     | 'error'  | 'false'  | 'true'  | 'undefined'  | 'parent'
     | integer_literal
     | floating_point_literal
     | string_literal+
     | list_expression
     | record_expression
     | function_call
     |  ' ( ' expression ' ) '
list_expression ::= ' { ' (  expression (  ' , ' expression )* ' , '? )? ' } '
record_expression ::= ' [ ' (  attribute_definition (  ' ; ' attribute_definition )* ' ; '? )? ' ] '
attribute_definition ::= attribute_name ' = ' expression
function_call ::= unquoted_name ' ( ' (  expression (  ' , ' expression )* )? ' ) '


To simplify the grammar, productions enforcing the precedence of operators have been omitted. The actual precedences are indicated by the order in which operators are listed. Operators are listed in order of increasing precedence (most tightly binding last), with operators on the same line having equal precedence. All binary operators are left-associative. Unary operators have higher precedence than binary operators other than subscripting and attribute selection. The meta-syntactic symbols ()+? are used as in the definition in the previous section. A complete YACC grammar for the ClassAd language is contained in Appendix A.


Unparsing.

Parsing associates with each expression, binary_expression, prefix_expression, or suffix_expression a node in the internal representation of an expression. For example, a binary_expression of the form expr1 + expr2 parses to an internal expression with operator PLUS and operands that are the results of parsing expr1 and expr2. Multiple distinct strings may parse to the same internal form, due to presence of optional comments, whitespace, and parentheses. To support certain built-in functions such as string, we define for each expression a canonical unparsing, which parses to that expression.

The canonical unparsing of an expression is completely parenthesized and has no comments and no whitespace outside of string literals or attribute names. For example, the strings ``-x + 3 * (y + 1)'' and ``((-x)+(3*(y+1)))'' both parse to the same internal form. The second string is the canonical unparsing.

The canonical unparsing of an attribute name is simply the name itself if it conforms to the syntax of unquoted_name in Table 4. Otherwise, an attribute name is unparsed using the quoted_name syntax. A string literal is unparsed as a string. Inside a quoted_name or string, the unparsing uses the rules defined in Section 3.2, with apostrophe as the delimiter for a quoted_name and quote as the delimter for a string.

The literals true, false, undefined, and error unparse to true, false, undefined, and error, respectively. The annotation of an Error or Undefined literal is not included in the canonical unparsing.

The canonical unparsing of an integer literal is the decimal representation of its value, with no leading zeros unless the value is exactly zero and no sign unless the value is negative.

The canonical unparsing of a real literal is one of the strings 0.0, -0.0, real("INF"), real("-INF"), or real("NaN"), or a normalized ``scientific'' representation such as 6.02E24 or 3.14159265E0, with one non-zero digit before the decimal point and as many digits following the decimal point as necessary to represent the value exactly (but at least one digit must follow the decimal point). For details, see Section 3.5 on the XML representation for details.

The canonical unparsing of an absolute time literal has the form absTime("yyyy-mm-ddThh:mm:ss$\pm$zz:zz") where the argument is a string representation of the time and date in ISO 8601 syntax, including the local time zone as hours and minutes from the prime meridian (negative for west). The canonical unparsing of a relative time has the form relTime("d+hh:mm:ss.mmm") where the argument is a string representation of the duration in days, hours, minutes, seconds, and milliseconds.12The suffix .mmm is omitted if the number of milliseconds is zero, and leading fields are omitted, together with the following punctuation, if their values are zero. Leading digits of the first non-zero field are also omitted. The string begins with a minus sign if the value is negative. Examples are absTime("1949-03-11T08:17:00-06:00"), relTime("-5:00"), and relTime("0").

The canonical unparsing of a list or record expression omits the optional comma or semicolon following the last item. In particular, the empty record unparses as [] and the empty list unparses as {}.


Old ClassAd Syntax

The so-called ``old'' classad syntax is used in some versions of Condor to transmit classads (Record Expressions) over a network connection. It is deprecated. Not all expressions can be represented in the old syntax. To be so represented, an expression must satisfy the following conditions.13

If the root RECORD node has N children, the representation consists of 4 bytes representing the binary value N, most significant byte first, followed by N strings of ASCII characters, each terminated by a null character. The last two strings are representations, in the native syntax, of the expressions in the attribute definitions corresponding to attribute names MyType and TargetType, in that order. The remaining N-2 strings correspond to the other N-2 operands of the RECORD operator. Each of these strings has the form "name = expr", where name is the attribute name of the definition, and expr is a representation, using the native syntax, of the corresponding expression.


XML Syntax

A Document Type Definition (DTD) for the XML representation may be found in Appendix B, and a schema definition compliant with the XML Schema standarad [5] is in Appendix C.

In the following paragraphs, xml_escape(string) denotes a string of printing ASCII characters that results from the following two steps, excuted in order. First, the string is replaced by its canonical unparsing, as defined in Section 3.2. Then, each occurrence of <, &, or > is replaced by &lt;, &amp;, or &gt;, respectively. The function xml_escape_attr is defined similarly, except that in the second step, " is also replaced by &quot;.

The canonical XML representation of an expression depends on the operator at the root of the expression. All ``simple'' (atomic) values conform to types defined in XML Schema Part 2: Datatypes [3]. In this section, these types are referenced as if they were in the namespace xsd, e.g., xsd:string, where xmlns:xsd=http://www.w3.org/2001/XMLSchema. A complete XML Schema is provided in Appendix C.

A string literal is represented as <s>xml_escape(s)</s>, where s is the value of the literal. The content of an <s> element has type xsd:string.

An integer literal is represented as <i>dddd</i>, where dddd is the decimal representation of its value, with no excess leading zeros (that is, the first digit is zero if and only if the value is zero), preceded by a minus sign if the value is negative. The content of an <i> element has type xsd:int.

A real literal is represented as <r>s</r>, where s is the representation of the value of the literal in ``scientific'' notation, for example, <r>3.141592653589793E+00</r>. More specifically

The content of an <i> element has type xsd:double. Except for the representations for positive and negative infinity and not-a-number, which conform to the specification of xsd:double, this representation is meant to conform to the printf format %1.15E.

A true or false boolean literal is represented by <b v="t"/> or <b v="f"/>, respectively.

An Error literal with non-empty annotation a is represented by the string <er a="annot"/>, where annot is xml_escape_attr(a). If the annotation is empty, the representation is <er/>.

An Undefined literal with non-empty annotation a is represented by the string <un a="annot"/>, where annot is xml_escape_attr(a). If the annotation is empty, the representation is <un/>.

An absolute time literal is represented as <at>yyyy-mm-ddThh:mm:ss$\pm$zz:zz</at>, where the element content is a string representation of the time and date in a restricted ISO 8601 syntax, including the local time zone as hours and minutes from the prime meridian (negative for west). The content of an <at> element has type xsd:dateTime. An example is <at>2003-01-25T09:00:00-06:00</at>.

A relative time literal is represented as <rt>sPnDTnHnMn.mmmS</rt>, where s is - for negative values and omitted for positive values, each n is a non-negative integer, representing days, hours, minutes, and seconds, mmm represents a number of milliseconds, and the letters PDTHMS are included verbatim. The suffixes represent scale factors: DT for days (24*60*60), H for hours (60*60), M for minutes (60) and S for seconds (1). A field (including its following scale factor) may be omitted if its value is zero. If the value is an integral number of seconds, the number of milliseconds, along with the preceding decimal point, may be omitted, but if present, it must be represented as exactly three digits. Finally, the letter T is omitted if the H, M, and S fields are all omitted. Note that these rules permit multiple representation of the same value. For example, <rt>PT1H2S</rt>, <rt>PT60M2S</rt>, and <rt>PT3602.000S</rt> represent the same value. The canonical representation is defined by the limitation that the number of hours is less than 24, and the numbers of minutes and seconds are each less than 60. Moreover, each field (including milliseonds) is omitted if its value is zero. However as a special case, the value zero is represented as <rt>PT0S</rt>. Thus the first of the three examples above is the canonical representation of a duration of 3602 seconds. The content of an <rt> element has type xsd:duration.

If the root operator is LIST, the XML representation consists of the string <l> followed by the concatenation of the representations of its operands (in order), followed by the string </l>. The content of an <l> element is a sequence of elements of any the types listed in this section except <a>.

If the root operator is RECORD, the XML representation consists of the string <c> followed by the concatenation of the representations of its constituent definitions, followed by the string </c>. The representation of an attribute definition name = expr consists of the string <a n = "xml_escape_attr(name)"> followed by the XML representation of expr followed by the string </a>. The content of a <c> element is a sequence of <a> elements. The content of an <a> element is an element of any of the types listed in this section except <a>.

In all other cases, the XML representation of an expression consists of <e>xml_escape(s)</e>, where s is the canonical unparsing of the expression in the native syntax. The content of an <e> element has type xsd:string.

Non-canonical XML representations may differ from the canonical representation in the following ways:

For example, 
    <c><a n="the value"><e>b</e></a><a n="b"><r>3.14E0<\r></a><c>
    <c>
        <a n="the value"> <e>   b</e> </a>
        <a         n="b"> <e>3.14</e> </a>
    <c>
    <e>[ 'the value' = b; b = 3.14 ]</e>
are three XML representations of the same expression; the first is the canonical representation.


Footnotes

... characters3
Character values are currently limited to the range 1 - 255 (decimal). Future versions of this specification may use Unicode for representing attribute names and string literals.
... measured4
This document does not specify the precision or range of values supported by absolute time literals. The Java implementation currently stores the time as a signed, 64-bit offset in milliseconds from the epoch, while the C++ implementation uses an signed 32-bit offset in seconds (though this may be platform-dependent). Both implementations use an epoch of 00:00, January 1, 1970, Universal Coordinated Time. The time zone is recorded as an offset from UTC.
... times5
This document does not specify the precision or range of values supported by relative time literals. The Java implementation currently stores relative times as signed, 64-bit integer indicating milliseconds, while the C++ implementation uses a signed 32-bit offset indicating seconds.
... expressions.6
The ClassAd language is a dynamically typed language. Mis-typed expressions such as 3/"foo" are allowed by the syntax, but will evaluate to error.
... implementations.7
Other functions may be provided by an extension mechanism to be defined in a subsequent version of this specification.
... case)8
The set of attribute names in a RECORD expression is logically a set of distinct case-independent strings, but the original ordering and case may be retained by an implementation so that the ``unparsing'' of an parsed expression more closely resembles the original.
... inclusive).9
In this section, all characters codes are given in decimal.
... (34)10
In this section, all characters codes are given in decimal.
... (13).11
This definition of whitespace corresponds to the C and C++ programming languages. The Java Language Specification (Second Edition) defines whitespace similarly, but omits Vertical Tab. The latter definition also corresponds to the (deprecated) method Character.isSpace in the standard Java API. The API documentation recommends replacing Character.isSpace with Character.isWhitespace, which adds Vertical Tab as well as characters with codes 28-31 decimal (FS, GS, RS, and US) and several Unicode characters with codes greater than 256.
... milliseconds.12
For this conversion, leap seconds and daylight savings time transitions are not taken into account. Every minute is 60 seconds, and every day is 24 hours.
... conditions.13
The ``old''" classad language had many more restrictions than those listed here. An expression that meets these conditions can be represented in the Old syntax for transmission over a connection, but it may not be intelligible to a legacy application receiving the transmission.
next up previous
Next: Evaluation Up: The ClassAd Language Reference Previous: Basic Concepts
Alain Roy 2004-09-30