[racket] struct-copy and custom-write bug?

On Mar 1, 2014, at 5:50 AM, Jon Stenerson wrote:

> When I put the following in the DrRacket definitions window and then evaluate t, it works for a few seconds and runs out of memory. Anyone understand the problem?
> 
> #lang racket
> 
> (define (Print stx port mode)
>  (if (Atom? stx)
>      (write-string "ATOM " port)
>      (Print (Pair-cdr stx) port mode)))
> 
> (struct Base ()
>  #:methods gen:custom-write
>  [(define write-proc Print)])
> 
> (struct Atom Base (datum))
> (struct Pair Base (car cdr))
> 
> (define t (struct-copy Base (Atom  3)))


t is _not_ an instance of Atom but Base, which is what the struct-copy expression specifies. When you enter 

 > t

Racket looks for a printer specification. It finds write-proc, which checks Atom? of stx and finds that t is not an Atom. Hence, it extracts Pair-cdr, but t is also not a Pair. Ergo, Pair-cdr raises an exception. The printer needs to print what it found in lieu of a Pair, so Racket looks for a printer specification. It finds write-proc, which ... 

Easiest fix: Change Base in the last line to Atom. 

-- Matthias