12.2 Arity

Each procedure has an arity, which is the minimum and (optionally) maximum number of arguments that it will accept. MIT/GNU Scheme provides an abstraction that represents arity, and tests for the apparent arity of a procedure.

Arity objects come in two forms: the simple form, an exact non-negative integer, represents a fixed number of arguments. The general form is a pair whose car represents the minimum number of arguments and whose cdr is the maximum number of arguments.

procedure: make-procedure-arity min [max [simple-ok?]]

Returns an arity object made from min and max. Min must be an exact non-negative integer. Max must be an exact non-negative integer at least as large as min. Alternatively, max may be omitted or given as ‘#f’, which represents an arity with no upper bound.

If simple-ok? is true, the returned arity is in the simple form (an exact non-negative integer) when possible, and otherwise is always in the general form. Simple-ok? defaults to ‘#f’.

procedure: procedure-arity? object

Returns ‘#t’ if object is an arity object, and ‘#f’ otherwise.

procedure: procedure-arity-min arity

procedure: procedure-arity-max arity

Return the lower and upper bounds of arity, respectively.

The following procedures test for the apparent arity of a procedure. The results of the test may be less restrictive than the effect of calling the procedure. In other words, these procedures may indicate that the procedure will accept a given number of arguments, but if you call the procedure it may signal a condition-type:wrong-number-of-arguments error. For example, here is a procedure that appears to accept any number of arguments, but when called will signal an error if the number of arguments is not one:

(lambda arguments (apply car arguments))

procedure: procedure-arity procedure

Returns the arity that procedure accepts. The result may be in either simple or general form.

(procedure-arity (lambda () 3))         ⇒  (0 . 0)
(procedure-arity (lambda (x) x))        ⇒  (1 . 1)
(procedure-arity car)                   ⇒  (1 . 1)
(procedure-arity (lambda x x))          ⇒  (0 . #f)
(procedure-arity (lambda (x . y) x))    ⇒  (1 . #f)
(procedure-arity (lambda (x #!optional y) x))
                                        ⇒  (1 . 2)

procedure: procedure-arity-valid? procedure arity

Returns ‘#t’ if procedure accepts arity, and ‘#f’ otherwise.

procedure: procedure-of-arity? object arity

Returns ‘#t’ if object is a procedure that accepts arity, and ‘#f’ otherwise. Equivalent to:

(and (procedure? object)
     (procedure-arity-valid? object arity))

procedure: guarantee-procedure-of-arity object arity caller

Signals an error if object is not a procedure accepting arity. Caller is a symbol that is printed as part of the error message and is intended to be the name of the procedure where the error occurs.

procedure: thunk? object

Returns ‘#t’ if object is a procedure that accepts zero arguments, and ‘#f’ otherwise. Equivalent to:

(procedure-of-arity? object 0)