Print values, not types (Racket)

Question

When I print in Racket, only the type is printed of structures, not the value. (I'm working in DrRacket, in the interactions area.)

For example, I have a tree structure:

#lang racket
(define-struct node [name left right])

An example could be:

(define SALLY (make-node 'sally BOBBY SUSIE))
(define BOBBY (make-node 'bobby NONE NONE))
(define SUSIE (make-node 'susie NONE NONE))

What I see:

> (print SALLY)
#<node>

What I want to see:

> (print SALLY)
(make-node 'sally (make-node 'bobby NONE NONE)
                  (make-node 'susie NONE NONE))

How can I see the value and not the type?

Answer 1

Use #:transparent keyword:

(define-struct node [name left right] #:transparent)

---

> (define-struct node [name left right] #:transparent)
> (define NONE '())
> (define BOBBY (make-node 'bobby NONE NONE))
> (define SUSIE (make-node 'susie NONE NONE))
> (define SALLY (make-node 'sally BOBBY SUSIE))
> (print SALLY)
(node 'sally (node 'bobby '() '()) (node 'susie '() '()))

Answer 2

NB: Today struct is preferred over define-struct so I've written the code with struct.

It doesn't really print the type but it's the default representation of the object. There are two options you can do:

1. Use #:transparent keyword.

(struct node [name left right] #:transparent)
(define root (node 'd 
                   (node 'b (node 'a '() '()) 
                            (node 'c '() '())) 
                   (node 'f (node 'e '() '())
                            (node 'g '() '()))))
root ; ==>
     ; (node 'd 
     ;       (node 'b (node 'a '() '()) 
     ;                (node 'c '() '())) 
     ;       (node 'f (node 'e '() '())
     ;                (node 'g '() '()))))

Notice how they look like the construction?

1. Add a writer for the object

(struct node [name left right]
  #:methods gen:custom-write
  [;; needs to be named write-proc
   (define (write-proc x port mode)
     ((if (eq? mode #t) write display) (fancy-writer x 0) port))

   ;; helper used by write-proc
   (define (fancy-writer x ident)
     (if (null? x)
         ""
         (let ([new-ident (+ 5 ident)])
           (string-append (fancy-writer (node-left x) new-ident)
                          (string-append (make-string ident #\space)
                                         (symbol->string (node-name x))
                                         "\n")
                          (fancy-writer (node-right x) new-ident)))))])

root ; ==>
     ;           a
     ;      b
     ;           c
     ; d
     ;           e
     ;      f
     ;           g