40.8 Descriptor Expression Language

An expression language is provided for use in FreezeScript descriptors.

40.8.1 Operators

The language supports the usual complement of operators: and, or, not, +, ‑, /, *, %, <, >, ==, !=, <=, >=, (, ). Note that the < character must be escaped as &lt; in order to comply with XML syntax restrictions.

40.8.2 Literals

Literal values can be specified for integer, floating point, boolean, and string. The expression language supports the same syntax for literal values as that of Slice (see Section 4.9.5), with one exception: string literals must be enclosed in single quotes.

40.8.3 Symbols

Certain descriptors introduce symbols that can be used in expressions. These symbols must comply with the naming rules for Slice identifiers (i.e., a leading letter followed by zero or more alphanumeric characters). Data members are accessed using dotted notation, such as value.memberA.memberB.

Expressions can refer to Slice constants and enumerators using scoped names. In a transformdb descriptor, there are two sets of Slice definitions, therefore the expression must indicate which set of definitions it is accessing by prefixing the scoped name with ::Old or ::New. For example, the expression old.fruitMember == ::Old::Pear evaluates to true if the data member fruitMember has the enumerated value Pear. In dumpdb, only one set of Slice definitions is present and therefore the constant or enumerator can be identified without any special prefix.

40.8.4 Nil

The keyword nil represents a nil value of type Object. This keyword can be used in expressions to test for a nil object value, and can also be used to set an object value to nil.

40.8.5 Elements

Dictionary and sequence elements are accessed using array notation, such as userMap['joe'] or stringSeq[5]. An error occurs if an expression attempts to access a dictionary or sequence element that does not exist. For dictio­naries, the recommended practice is to check for the presence of a key before accessing it:

<if test="userMap.containsKey('joe') and userMap['joe'].active">

See Section 40.8.8 for more information on the containsKey function.

Similarly, expressions involving sequences should check the length of the sequence:

<if test="stringSeq.length > 5 and stringSeq[5] == 'fruit'">

See Section 40.8.7 for details on the length member.

40.8.6 Reserved Keywords

The following keywords are reserved: and, or, not, true, false, nil.

40.8.7 Implicit Members

Certain Slice types support implicit data members:

 

• Dictionary and sequence instances have a member length representing the number of elements.

• Object instances have a member ice_id denoting the actual type of the object.

40.8.8 Functions

The expression language supports two forms of function invocation: member functions and global functions. A member function is invoked on a particular data value, whereas global functions are not bound to a data value. For instance, here is an expression that invokes the find member function of a string value:

old.stringValue.find('theSubstring') != ‑1

And here is an example that invokes the global function stringToIdentity:

stringToIdentity(old.stringValue)

If a function takes multiple arguments, the arguments must be separated with commas.

String Member Functions

The string data type supports the following member functions:

• int find(string match[, int start])

Returns the index of the substring, or ‑1 if not found. A starting position can optionally be supplied.

• string replace(int start, int len, string str)

Replaces a given portion of the string with a new substring, and returns the modified string.

• string substr(int start[, int len])

Returns a substring beginning at the given start position. If the optional length argument is supplied, the substring contains at most len characters, otherwise the substring contains the remainder of the string.

Dictionary Member Functions

The dictionary data type supports the following member function:

• bool containsKey(key)

Returns true if the dictionary contains an element with the given key, or false otherwise. The key argument must have a value that is compatible with the dictionary’s key type.

Object Member Functions

Object instances support the following member function:

• bool ice_isA(string id)

Returns true if the object implements the given interface type, or false otherwise. This function cannot be invoked on a nil object.

Global Functions

The following global functions are provided:

• string generateUUID()

Returns a new UUID.

• string identityToString(Ice::Identity id)

Converts an identity into its string representation.

• string lowercase(string str)

Returns a new string converted to lowercase.

• string proxyToString(Ice::ObjectPrx prx)

Returns the string representation of the given proxy.

• Ice::Identity stringToIdentity(string str)

Converts a string into an Ice::Identity.

• Ice::ObjectPrx stringToProxy(string str)

Converts a string into a proxy.

• string typeOf(val)

Returns the formal Slice type of the argument.