Reference Arguments

If we look at a context it consists of a set of pairs, each pair being a name (an identifier) and a value (that is said to be assigned to that identifier).

When a function is applied actual arguments are substituted for formal arguments, and the body of the function is evaluated. The process of evaluating an actual argument yields a value and any associated names are ignored. Magma's evaluation semantics treats identifiers as 'indexes' into the context -- when Magma wants the value of say x it searches through the context looking for a pair whose name component is x. The corresponding value component is then used as the value of x and the name part is simply ignored thereafter.

When we call a procedure with a reference argument, however, the name components of the context become important. When, for example we pass x as an actual reference argument to a formal reference argument y in some procedure, Magma remembers the name x. Then if y is changed (e.g., by assignment) in the called procedure, Magma, knowing the name x, finds the appropriate pair in the calling context and updates it by changing its corresponding value component. To see how this works take the example in the previous section. It was,

> a := 5; b := 6;
> p := procedure( x, ~y ) y := x; end procedure;
> p(a, ~b);