PS: a practical example for #:rest

> For define-typed* it would be possible to set a rest argument checking
> procedure via #:rest, but this check procedure then also receives all
> arguments defined via #:key.

(define (all-integer? x) (not (member #f (map integer? x))))
(define-typed* (foo name #:rest r) (string? #:rest all-integer? -> list?)
  r)

(foo "hello" 1 2 3 4 5)
$1 = (1 2 3 4 5)

(foo "me" "hello" 1 2 3 4 5)
type error ~a ~a #<procedure all-integer? (x)> ("hello" 1 2 3 4 5)

(foo 1 2 3 4 5)
type error ~a ~a #<procedure string? (_)> 1

And a small aside: it’s pretty awesome that we can add this to Scheme
without needing any changes in the implementation.

An option to support rest arguments in define-typed (not only in
define-typed*) would be to go back to the initial version which had the
type in the first position instead of after ->

But that would require some more changes, because supporting return
types with lambda would require to either put single return types into
parentheses, too, or to use a specific marker for lists of return types.
This form:

(define-typed (count . a) (integer? . string?) (length a))

could be possible. But currently that’s a syntax error.

Then multiple value return would need:

(define-typed (vals . a) ((-> all-string?) . string?) (apply values a))

The simpler version

(define-typed (count a) (integer? list?) (length a))
(count '(1 2 3))

already works.

Best wishes,
Arne
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