Duplicate definition of identifiers allowed in eval in Racket?

Question

While playing around with Racket in DrRacket, I accidently entered in the interactions window.

(define x 7)
(define x 8)

and DrRacket allowed it, i.e. I didn't get a "duplicate definition of identifiers error message.

So to try and figure out what was going onI then entered this in the definitions window of DrRacket:

Code snippet A

(define-namespace-anchor a)  
(define ns (namespace-anchor->namespace a))  
(eval '(begin (define x 7) (define x 8)) ns)  
(eval 'x ns)  

and still no "duplicate definitions error".

I then tried this also in the definitions window:

Code snippet B

(define x 9)  
(define-namespace-anchor a)  
(define ns (namespace-anchor->namespace a))  
(eval '(begin (define x 7) (define x 8)) ns)  
(eval 'x ns)  

and I got the error message: cannot redefine a constant x.

Can someone explain to me why in code snippet A in the definitions window (and also simply entering (define x 7) followed by (define x 8) in the interactions window) doesn't give me a duplicate definitions error.

Answer 1

Historically, Scheme (not Racket) has always allowed re-definitions. A re-definition at the top level is equivalent to the mutation of an existing binding. Racket has tightened this up a bit, in that code at the top level of a module is not allowed to re-define an identifier. However, the old behavior persists in the "top level" that's used for "eval" and the interactions window.

I may be missing details here, but I believe that everything I'm telling you is true.