4.9.1 Define and Set

Special Form: defined? symbol

Equivalent to #t if symbol is a syntactic keyword (such as if) or a symbol with a value in the top level environment (see Variables and regions in Revised(5) Scheme). Otherwise equivalent to #f.

Special Form: defvar identifier initial-value

If identifier is unbound in the top level environment, then identifier is defined to the result of evaluating the form initial-value as if the defvar form were instead the form (define identifier initial-value) . If identifier already has a value, then initial-value is not evaluated and identifier’s value is not changed. defvar is valid only when used at top-level.

Special Form: defconst identifier value

If identifier is unbound in the top level environment, then identifier is defined to the result of evaluating the form value as if the defconst form were instead the form (define identifier value) . If identifier already has a value, then value is not evaluated, identifier’s value is not changed, and an error is signaled. defconst is valid only when used at top-level.

Special Form: set! (variable1 variable2 …) <expression>

The identifiers variable1, variable2, … must be bound either in some region enclosing the ‘set!’ expression or at top level.

<Expression> is evaluated, and the elements of the resulting list are stored in the locations to which each corresponding variable is bound. The result of the ‘set!’ expression is unspecified.

(define x 2)
(define y 3)
(+ x y)                              ⇒ 5
(set! (x y) (list 4 5))              ⇒ unspecified
(+ x y)                              ⇒ 9

Special Form: qase key clause1 clause2 …

qase is an extension of standard Scheme case: Each clause of a qase statement must have as first element a list containing elements which are:

• literal datums, or
• a comma followed by the name of a symbolic constant, or
• a comma followed by an at-sign (@) followed by the name of a symbolic constant whose value is a list.

A qase statement is equivalent to a case statement in which these symbolic constants preceded by commas have been replaced by the values of the constants, and all symbolic constants preceded by comma-at-signs have been replaced by the elements of the list values of the constants. This use of comma, (or, equivalently, unquote) is similar to that of quasiquote except that the unquoted expressions must be symbolic constants.

Symbolic constants are defined using defconst, their values are substituted in the head of each qase clause during macro expansion. defconst constants should be defined before use. qase can be substituted for any correct use of case.

(defconst unit '1)
(defconst semivowels '(w y))
(qase (* 2 3)
  ((2 3 5 7) 'prime)
  ((,unit 4 6 8 9) 'composite))        ==>  composite
(qase (car '(c d))
  ((a) 'a)
  ((b) 'b))                            ==>  unspecified
(qase (car '(c d))
  ((a e i o u) 'vowel)
  ((,@semivowels) 'semivowel)
  (else 'consonant))                   ==>  consonant