Class contract on abstract method

Questions & Answers
1

In the following example code:

#lang racket/base

(require
  racket/class
  racket/contract)

(define/contract parent%
  (class/c (override [meth (->m string? void?)]))
  (class object%
    (super-new)
    (abstract meth)))

(define/contract child-1%
  (class/c [meth (->m string? void?)])
  (class parent%
    (super-new)
    (define/override (meth str)
      (displayln str))))

(define child-2%
  (class parent%
    (super-new)
    (define/override (meth str)
      (displayln str))))

(module+ test
  (require rackunit)
  (let ([child-1 (new child-1%)]
        [child-2 (new child-2%)])
    (check-exn exn:fail? (lambda() (send child-1 meth 1)))
    (check-exn exn:fail? (lambda() (send child-2 meth 1)))
    ))

the first contract exception check succeeds (a contract exception is raised) but the second fails:

$ raco test contract-abstract-override.rkt 
raco test: (submod "contract-abstract-override.rkt" test)
1
--------------------
FAILURE
name:       check-exn
location:   contract-abstract-override.rkt:31:4
params:     '(#<procedure:exn:fail?> #<procedure:...stract-override.rkt:31:25>)
message:    "child-2"
--------------------
1/2 test failures

I was kind of expecting the contract on meth to be inherited in child-2%. It is, after all, attached to the original, overridden method. Am I doing this wrong or is it always necessary to redefine contracts for overridden methods?

I also tried this without the abstract part, using define/public in parent%. I got the same result.

2

From the docs:

“”
A method contract tagged with override describes the behavior expected by the contracted class for method-id when called directly (i.e. by the application (method-id...)).
“”

The code in question defines meth, it doesn’t call (any existing) meth directly.