Assignment

An assignment is an association of an identifier to a value. The statement,

> a := 6;

establishes an association between the identifier \(a\) and the value 6 (6 is said to be the value of \(a\), or to be assigned to \(a\)). A collection of such assignments is called a context. When a value \(V\) is assigned to an identifier \(I\) one of two things happens:

(1)

if \(I\) has not been previously assigned to, it is added to the current context and associated with \(V\). \(I\) is said to be declared when it is assigned to for the first time.

(2)

if \(I\) has been previously assigned to, the value associated with \(I\) is changed to \(V\). \(I\) is said to be re-assigned.

The ability to assign and re-assign to identifiers is why Magma is called an imperative language. One very important point about assignment is illustrated by the following example. Say we type,

> a := 6;
> b := a+7;

After executing these two lines the context is [ (a,6), (b,13) ]. Now say we type,

> a := 0;

The context is now [ (a,0), (b,13) ]. Note that changing the value of \(a\) does not change the value of \(b\) because \(b\)’s value is statically determined at the point where it is assigned. Changing \(a\) does not produce the context [ (a,0), (b,7) ].