Contract for (λ (x . rest) x)

Questions & Answers
1

The documentation for ->* mentions a particular contract not matching (λ (x . rest) x) due to the rest args not being optional, but I was unable to find a contract that does match that function.

What contract matches that function?

2

Rubber duck works again :slight_smile:

[ zipn (-> list? ... list?) ]

Of course, my scenario is simplified since all the rest args are of the same type.

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3

The contract (->* (integer?) #:rest (listof integer?) any) should do the trick.

The REPL seems to agree.

% rlwrap racket
Welcome to Racket v8.2 [cs].
> (define/contract (f x . rest)
    (->* (integer?) #:rest (listof integer?) any)
    x)
> (f 2)
2
> (f 2 3 4)
2
> (f "racket" 2 3 4)
f: contract violation
  expected: integer?
  given: "racket"
  in: the 1st argument of
      (->*
       (integer?)
       #:rest
       (listof integer?)
       any)
  contract from: (function f)
  blaming: top-level
   (assuming the contract is correct)
  at: stdin::50-51
  context...:
   /usr/local/Cellar/minimal-racket/8.2/share/racket/collects/racket/contract/private/blame.rkt:346:0: raise-blame-error
   /usr/local/Cellar/minimal-racket/8.2/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:375:33
   /usr/local/Cellar/minimal-racket/8.2/share/racket/collects/racket/repl.rkt:11:26
> (f 2 "racket" "rhombus" "scribble")
f: contract violation
  expected: integer?
  given: "racket"
  in: an element of
      the rest argument of
      (->*
       (integer?)
       #:rest
       (listof integer?)
       any)
  contract from: (function f)
  blaming: top-level
   (assuming the contract is correct)
  at: stdin::50-51
  context...:
   /usr/local/Cellar/minimal-racket/8.2/share/racket/collects/racket/contract/private/blame.rkt:346:0: raise-blame-error
   /usr/local/Cellar/minimal-racket/8.2/share/racket/collects/racket/contract/private/list.rkt:215:14
   /usr/local/Cellar/minimal-racket/8.2/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:375:33
   /usr/local/Cellar/minimal-racket/8.2/share/racket/collects/racket/repl.rkt:11:26
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