[plt-dev] Symptom of REPL's hopelessness?

Can someone explain why this macro expands without error in a module
but not in the REPL?

(define-syntax (m stx)
 (syntax-case stx ()
   [(_ x)
    (with-syntax ([(y) (generate-temporaries (syntax (x)))])
      (syntax (define (y) y)))]))

> (m q)
compile: unbound identifier (and no #%top syntax transformer is bound) in: q1