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

8.8Assignment Expressions

An assignment operator has two operand expressions. The first is known as the left-hand side and has a form restricted to representing a local name, an output parameter or a (possibly indexed) attribute. The left-hand side denotes the target to be assigned by the assignment expression. The second operand expression is known as the right-hand side. The right-hand side expression evaluates to the value or values that are assigned to the target designated by the left-hand side.

All assignment operators are syntactically right-associative (they group right-to-left). Thus, an expression of the form a=b=c is equivalent to a=(b=c), which assigns the value of c to b and then assigns the value of b to a.

Examples

customer = new Customer() // Local name assignment

customer[i] = new Customer() // Indexed local name assignment

reply = this.createReply(request,result) // Output parameter assignment

customer.email = checkout.customerEmail // Attribute assignment

customer.address[i] = newAddress // Indexed attribute assignment

x += 4 // Compound assignment

filename += ".doc" // Compound assignment

Figure 8 41 Abstract Syntax for Assignment Expressions

1. 
   Expression see Subclause 8.1
   
2. 
   QualifiedName see Subclause 8.2
   
3. 
   FeatureReference see Subclause 8.3.6
   
4. 
   Index see Subclause 8.3.13
   

Left-Hand Side

The left-hand side of an assignment expression may be one of the following.

• 
  Local name. The assigned source of the local name before the assignment expression must not be a loop variable definition (see Subclause 9.12), a @parallel annotation (see Subclause 9.12) or a sequence expansion expression (see Subclause 8.3.19).
  
• 
  Output parameter name. The named parameter must have mode out or inout. A parameter name may be qualified with the name of the behavior or operation that owns it, though this is not required. The identified parameter must not be a template. The assignment expression must appear within the definition of the behavior that owns the parameter or the method of the operation that owns the parameter.
  
• 
  Property reference. A property reference is a feature reference that names a property of the type of its primary expression. As for a property access expression (see Subclause 8.3.6), the identified property may be either a structural feature or an association end, but must not be a template. The primary expression of the property reference must have a multiplicity upper bound of 1.
  

The type of the left-hand side is determined as given below.

• 
  Local Name. Determined by its first assignment. If this is the first assignment of the local name, then the type of the left-hand side is the type of the right-hand side expression.
  
• 
  Parameter Name. As declared for the named parameter.
  
• 
  Property Reference. As given for the named property.
  

Assignability

The right-hand side of an assignment must be assignable to the left-hand side. In general, this means that the right-hand side is statically compatible in type and multiplicity with the left-hand side, either directly or after the application of a conversion.

A right-hand side is assignable to a left-hand side if any of the following conditions hold.

1. 
   Conformance. The left-hand is untyped or the right-hand side has a type that conforms to the type of the left-hand side (see Subclause 8.2 for the definition of type conformance). If the multiplicity upper bound of the left-hand side is less than or equal to 1, then the multiplicity upper bound of the right-hand side cannot be greater than that of the left-hand side.
   
2. 
   Null Conversion. The right-hand side is untyped with a multiplicity of [0..0] (i.e., guaranteed to be null) and the left-hand side has a multiplicity lower bound of 0 (regardless of type).
   
3. 
   Collection Conversion. The type of the right-hand side is a collection class (see Subclause 11.6), the right-hand side has a multiplicity upper bound of 1 and the type and multiplicity of the result of applying the toSequence operation to the right-hand side would be assignable to the left-hand side. The assigned value is the result of implicitly calling the toSequence operation on the result of evaluating the right-hand side expression.
   
4. 
   Bit String Converstion. The type of the right-hand side conforms to Integer, and the type of the left-hand side is BitString. If the multiplicity upper bound of the left-hand side is less than or equal to 1, then the multiplicity upper bound of the right-hand side cannot be greater than that of the left-hand side. The assigned value is the result of implicitly invocating the standard library BitStringFunctions::toInteger function (see Subclause 11.3.5) on each of the values in the result of evaluating the right-hand side expression. Note that both collection conversion and bit string conversion may apply. In this case, bit string conversion is applied after collection conversion.
   

Simple Assignment

A simple assignment has the form lhs = expr. The right-hand side expression of a simple assignment must be assignable to the left-hand side, as defined above.

When a simple assignment expression is evaluated, the right-hand side expression is evaluated first. If the left-hand side contains an index expression, this is evaluated after the right-hand side expression. The result of the right-hand side expression is then assigned to the left-hand side, as described below (possibly after conversion as discussed above).

If the left-hand side does not have an index expression, then the assignment proceeds as follows.

• 
  Local name. If the local name already exists, then it is assigned a new value. Otherwise, the assignment expression acts as the definition of a new local name with the type of the right-hand side expression.
  

• 
  Output parameter name. A new value is assigned to the named parameter. Note that any previously assigned value is effectively overwritten. That is, at the completion of the execution of a behavior, output parameters are given their last assigned value.
  
• 
  Property reference. A new value is assigned to the named property of the referenced object or structured data value.
  

If the left-hand side includes an index, then only the value at the index position is overwritten by the right-hand side value. However, if the right-hand side value is an empty collection, then the former value at the index position is removed without being replaced by a new value.

A simple assignment expression has the type and multiplicity of its right-hand side expression.

As noted earlier for a property reference left-hand side, an assignment expression may also be used to update a binary association in which an instance participates via reference to the opposite association end. In this case, the effect of the assignment is equivalent to an appropriate link operation (see Subclause 8.3.13).

As an example of an association end update, consider the following association (represented in Alf notation—see Subclause 10.4.5).

assoc Owns {
owner: Person;
house: House[*];
}

If the association Owns is in the current scope (that is, visible without qualification), and newHouse is a House, then the expression

newHouse.owner = jack;

is equivalent to the link operation

Owns.createLink(owner => jack, house => newHouse)

The result value of an assignment expression is the value of the right-hand side. Thus, the expression

WriteLine(a = 3)

assigns 3 to a and then writes the value “3”. Note that this should not be confused with the named tuple notation, such as

WriteLine(value => 3)

which uses the “=>” symbol, rather than “=”.

Compound Assignment

A compound assignment has the form lhs op= expr, where op is any arithmetic (see Subclauses 8.6.1) or logical (see Subclause 8.6.7) operator. It is equivalent to lhs = lhs op expr (except that, if lhs contains an index expression, it is only evaluated once).

In a compound assignment expression, if the left-hand side is a name (qualified or unqualified), it must also satisfy the semantics of a name expression (see Subclause 8.3.3). If the left-hand side is a feature reference, then it must also satisfy the static semantics of a property access expression (see Subclause 8.3.6). If the left-hand side has an index, then, in addition to the requirements for its name or feature reference, the left-hand side overall must also satisfy the semantics of a sequence access expression (see Subclause 8.3.16).

Both the left-hand side and the right-hand side must have multiplicity upper bounds of 1 and must have the same type, consistent with the arithmetic or logical operator used in the compound assignment operator (see Subclauses 8.6.1 or 8.6.7, respectively).