[racket-dev] Catching the undefined value

On Apr 15, 2014, at 9:29 PM, Asumu Takikawa <asumu at ccs.neu.edu> wrote:

> On 2014-04-15 18:13:31 -0400, claire alvis wrote:
>> The push below includes changes to letrec expressions, internal
>> definitions, units, classes, and certain ill-formed shared expressions so
>> that they no longer leak the `undefined' value.
> 
> This is great! (especially happy that TR, even with classes, doesn't
> have to worry about #<undefined> anymore)
> 
> BTW, I found this weird behavior:
> 
>  Welcome to Racket v6.0.1.3.
>  -> (require racket/unsafe/ops)
>  -> (let () (+ x 3) (define x 3) 5)
>  ; x: variable used before its definition [,bt for context]
>  -> (let () (unsafe-fx+ x 3) (define x 3) 5)
>  5


I consider this correct in a strange sense. 

Interestingly enough, 

> (let () (displayln  (unsafe-fx+ x 3)) (define x 3) 5)
x: variable used before its definition
  context...:
   /Users/matthias/plt/racket/collects/racket/private/misc.rkt:87:7

which is good too. I don't know how Claire and Matthew did this, 
but it's good :-)