3.17 Assignment: set! and set!-values
Assignment: set! in The Racket Guide introduces set!.
syntax
(set! id expr)
If id has a transformer binding to an assignment
transformer, as produced by make-set!-transformer or as an
instance of a structure type with the prop:set!-transformer
property, then this form is expanded by calling the assignment
transformer with the full expressions. If id has a
transformer binding to a rename transformer as produced
by make-rename-transformer or as an instance of a structure
type with the prop:rename-transformer property, then this
form is expanded by replacing id with the target identifier
(e.g., the one provided to make-rename-transformer). If a
transformer binding has both prop:set!-transformer and
prop:rename-transformer properties, the latter takes
precedence.
Otherwise, evaluates expr and installs the result into the location for id, which must be bound as a local variable or defined as a top-level variable or module-level variable. If id refers to an imported binding, a syntax error is reported. If id refers to a top-level variable that has not been defined, the exn:fail:contract exception is raised.
See also compile-allow-set!-undefined.
Examples: | |||||||||||||||||||
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syntax
(set!-values (id ...) expr)
Assuming that all ids refer to variables, this form evaluates
expr, which must produce as many values as supplied
ids. The location of each id is filled with the
corresponding value from expr in the same way as for
set!.
Example: | ||||||
|
More generally, the set!-values form is expanded to
(let-values ([(tmp-id ...) expr]) (set! id tmp-id) ...)
which triggers further expansion if any id has a transformer binding to an assignment transformer.