Ad/2010-08-01 Concrete Syntax for a uml action Language for Foundational uml (Alf) Second Revised Submission

8.1Overview

Figure 8 2 Base Abstract Syntax for Expressions

1. 
   Syntax Element see Subclause 6.5
   
2. 
   ConditionalExpression see Subclause 8.7
   
3. 
   AssignmentExpression see Subclause 8.8
   

Integration with UML

An Alf expression can be inserted into a UML model using an opaque expression (UML Specification, Subclause 7.3.35) in which the unprocessed text of the Alf expression is used as the body of the opaque expression and the corresponding language string is "Alf". Opaque expressions are kinds of value specifications (UML Specification, Subclause 7.3.54). Thus, an Alf expression may be used in a UML model any place that a value specification is allowed.

In addition, a special form of initialization expression may be for instance creation and sequence construction expressions acting as the specification of default values of properties or parameters. In such initialization expressions, an explicit description of the type of the expression does not need to be included, as is the case for normal instance creation and sequence construction expressions, since this can be inferred from the declared type of the property or parameter. (Instance creation expressions are described in Subclause 8.3.12 and sequence construction expressions are described in Subclause 8.3.15. The use of their special forms in initialization descriptions within Alf is discussed for local name declaration statements in Subclause 9.6 and for property initialization in Subclause 10.5.2.)

The execution semantics of an Alf expression are given formally by the mapping to UML activity graphs given in the following subclauses. An Alf expression can therefore always be compiled to a part or all of a UML activity model (which does not necessarily need to be the same as the formal mapping, but must have an equivalent effect to it—see Subclause 2.2). If the expression appears as part of a statement, then the compilation of the expression will be part of the compilation of the statement (see Clause 9). Otherwise, the compilation of the expression may be inserted as an activity that is the associated behavior of a corresponding opaque expression (see UML Superstructure 13.3.21) that is constructed as follows:

• 
  A single return parameter of the type of the Alf expression.
  
• 
  The activity nodes and edges resulting from the compilation of the Alf expression (semantically equivalent to the formal mapping as specified in the following subclauses).
  
• 
  A single activity parameter node associated with the return parameter, which becomes the target of an outgoing object flow from the result source element from the mapping of the Alf expression (see the definition of “result source element” given under Mapping below).
  

An expression either has a statically determined type or can be statically determined to be untyped. All values resulting from evaluation of the expression will be of the type of the expression, if it has one. If the expression is untyped, then the result values may have any type. For simplicity of terminology in this clause, an untyped expression will be referred to as having “type any”.

An expression also has a statically determined multiplicity. The number of values resulting from the evaluation of the expression will be no smaller than the multiplicity lower bound and no higher than the multiplicity upper bound (or unbounded if the upper bound is the unbounded value “*”).

The evaluation of an expression may depend on the values assigned to local names used in the expression. Local names are used in Alf to denote intermediate values in computations within a statement sequence (see Subclause 9.1). Alf is designed so, on the surface, local names can be assigned and reassigned in a similar way to variables in more traditional programming languages. However, the underlying fUML metamodel for actions and activities is fundamentally based on data flow, not on an implicit underlying store of variables. Therefore, the assignments of and references to local names in an Alf input text need to be mapped to appropriate object flow edges from the mapping of the appropriate assignment to the mapping of the reference that requires that assigned value.

Carrying out this mapping requires an analysis of the set of local names that are statically known to have assigned values during the execution of any Alf statement. The assigned source for a local name is the syntax element that, when executed, will provide the actual assigned value for that local name. If the assigned source for a local name is known, then a reference to the assigned value of that local name can be mapped to an object flow from the mapping of the assigned source. (See also the discussion of local names relative to statements in Clause 9.)

Since a local name may also be defined or reassigned within an expression, one can refer to the assignments (i.e., the statically determined assigned sources) for local names both before and after the evaluation of an expression. Most kinds of expressions do not actually themselves change the assigned source of a name. An assignment expression (see Subclause 8.8) is, of course, the main kind of expression that changes the assigned source for a name. However, invocation expressions (see Subclause 8.3.7) may assign names via out and inout parameters. Increment and decrement expressions also act as assignments (see Subclause 8.4).

Other than the kinds of expressions listed above, the only way that an assigned source may change in an expression is if it contains one of the above kinds of expression, directly or indirectly, as a subexpression. Unless otherwise stated, it can be assumed that the assigned source of a name before any subexpression of an expression is the same as the assigned source before the expression.

Further, in general, it is only legal to assign a name in at most one subexpression of any expression. Therefore, unless otherwise stated, the assigned source for a name after an expression is the same as after any subexpression. Specific assignment rules for various kinds of expressions are described in their respective subclauses.