difference in struct printing from default #:transparent versus make-constructor-style-printer

[Jeremy Siek]

I'm seeing some bad behavior from make-constructor-style-printer when

there are lists mixed in with the structs. It seems to switch from print mode to

write mode when going under a list. Whereas the default printer for transparent structs gets this right. The following program demonstrates the difference/problem.

Any thoughts on how to get the make-constructor-style-printer to behave

properly?

#lang racket
(require racket/struct)

(struct Int1 (value) #:transparent)
(struct Prim1 (op arg*) #:transparent)

(struct Int2 (value) #:transparent
  #:methods gen:custom-write
  [(define write-proc
     (make-constructor-style-printer
      (lambda (obj) 'Int2)
      (lambda (obj) (list (Int2-value obj)))))])
(struct Prim2 (op arg*) #:transparent
  #:methods gen:custom-write
  [(define write-proc
     (make-constructor-style-printer
      (lambda (obj) 'Prim2)
      (lambda (obj) (list (Prim2-op obj) (Prim2-arg* obj)))))])

(define p1 (Prim1 '+ (list (Int1 1) (Int1 2))))
(print p1)(newline)

(define p2 (Prim2 '+ (list (Int2 1) (Int2 2))))
(print p2)(newline)

The output is:

(Prim1 '+ (list (Int1 1) (Int1 2)))
(Prim2 '+ '(#<Int2: 1> #<Int2: 2>))

-Jeremy

[Ryan Culpepper]

The Racket printer uses a separate property to determine whether a struct is quotable. If you add 

  #:property prop:custom-print-quotable 'never

to the declarations of Int2 and Prim2, your program should produce the output you expect. I'll add a note to the docs for make-constructor-style-printer.

Ryan

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[Jeremy Siek]

Thanks Ryan!