capability-expressions.md

Capability Expressions

The reference capability types require some special expressions to work with them. However, these also find use with movable types.

capability_expression
    : move_expression
    | freeze_expression
    | id_expression
    ;

Move Expression

A move expression allows for moving the value out of a variable or field. This is useful both for move types and to recover isolation on reference types. Without a move expression, ownership cannot be passed from a variable or field to a called function.

move_expression
    : "move" identifier
    | "move" embedded_expression "." identifier
    ;

For variables, if it is a reference type then the flow typing of that variable is changed to id. If it is a value type, then the variable becomes inaccessible on all code paths after the move. The compiler issues fatal errors for attempts to access a potentially moved value type.

For fields, the field must be an optional type and have a mutable binding. The value will be moved from the field leaving the value none in the field.

Freeze Expression

A freeze expression allows one to convert a reference to const. After freezing a variable, that variable is changed to const T. The freeze expression also returns the constant reference.

freeze_expression
    : "freeze" identifier
    ;

Implicit Move and Freeze

When calling a method, the self parameter can cause an implicit move or freeze. If the self parameter is iso self then an implicit move can occur when calling the method. If the self parameter is const self then an implicit freeze can occur when calling the method.