220.127.116.11 Setf Expansions
Sometimes it is possible to avoid evaluating subforms of a place multiple times or in the wrong order. A setf expansion for a given access form can be expressed as an ordered collection of five objects:
- List of temporary variables
a list of symbols naming temporary variables to be bound sequentially, as if by let*, to values resulting from value forms.
- List of value forms
a list of forms (typically, subforms of the place) which when evaluated yield the values to which the corresponding temporary variables should be bound.
- List of store variables
a list of symbols naming temporary store variables which are to hold the new values that will be assigned to the place.
- Storing form
a form which can reference both the temporary and the store variables, and which changes the value of the place and guarantees to return as its values the values of the store variables, which are the correct values for setf to return.
- Accessing form
a form which can reference the temporary variables, and which returns the value of the place.
The value returned by the accessing form is affected by execution of the storing form, but either of these forms might be evaluated any number of times.
It is possible to do more than one setf in parallel via psetf, shiftf, and rotatef. Because of this, the setf expander must produce new temporary and store variable names every time. For examples of how to do this, see gensym.
For each standardized accessor function F, unless it is explicitly documented otherwise, it is implementation-dependent whether the ability to use an F form as a setf place is implemented by a setf expander or a setf function. Also, it follows from this that it is implementation-dependent whether the name (setf F) is fbound.
18.104.22.168.1 Examples of Setf Expansions
The following X3J13 cleanup issues, not part of the specification, apply to this section:
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