You use := in EAL action expressions to assign a value to a property or variable; this page is for developers who need to understand the value and type returned by an assignment.

Assign a value

The assignment operator := writes the value on its right-hand side (RHS) to the property or variable on its left-hand side (LHS).

self.Product.RegulatoryAffair:=Biocide.Create

In this example, the newly created Biocide is assigned to RegulatoryAffair on self.Product.

You can also assign to a variable:

vSomeDouble:=44/33

Result type of an assignment

An assignment expression has the type of its left-hand side. This rule applies from 2025-May.

For example, if SomeDouble has type Double, the assignment below has type Double:

x.SomeDouble:=4

The integer literal 4 can be assigned to the Double property. The assignment result is nevertheless typed as Double, because SomeDouble is the LHS.

Use assignment in a collection expression

The LHS type rule is important when an assignment is evaluated inside a collection operation. For example:

somelist->collect(x|x.SomeDouble:=4)

If SomeDouble is a Double, collect produces a collection of Double. It does not produce a collection of Integer merely because 4 is an integer literal.

When the assigned value has a more specific type

The assignment result can be less specific than the object created on the RHS. This occurs when the LHS property is declared as a superclass.

Assume that RegulatoryAffair is a superclass of Biocide:

self.Product.RegulatoryAffair:=Biocide.Create

The expression above now has type RegulatoryAffair, not Biocide. Before 2025-May, it was typed from the RHS and therefore had type Biocide.

If the following expression needs a Biocide, use one of these patterns.

Cast the assignment result

Use safeCast when you need the assignment expression itself to be treated as Biocide:

(self.Product.RegulatoryAffair:=Biocide.Create).safeCast(Biocide)

Keep the created object in a variable

Use a let expression when you want to retain the created object's specific type without casting the assignment result:

let x:=Biocide.Create in(
  self.Product.RegulatoryAffair:=x;
  x
)

Here, x has type Biocide. The assignment stores x in the RegulatoryAffair property, and the final x expression returns the Biocide value.

Choose the right pattern

Your need	Pattern	Result type
Assign a value and use the property's declared type	property:=value	The type of property
Assign a value in collect and retain the property element type	x.SomeDouble:=4)	The type of SomeDouble
Assign a derived object and continue as the derived type	(property:=Derived.Create).safeCast(Derived)	Derived
Create once, assign it, and return the derived object	let x:=Derived.Create in(property:=x; x)	Derived

Migration guidance for expressions written before 2025-May

Review assignments where the RHS is more specific than the LHS property, especially assignments followed by an operation available only on the derived type.

1. Find an expression of the form SupertypeProperty:=Derived.Create.
2. Check whether later parts of the expression require the Derived type.
3. If they do, add safeCast(Derived) to the assignment result, or create the object in a let variable and return that variable.
4. If later code only needs the declared property type, no change is needed.

See also

• Documentation:OCL General Operators
• Documentation:OCL Boolean Operators
• Documentation:OCLOperators let