Advanced Structures

You can simplify the writing of lots of “struct copying” functions by making a separate file that contains the powerful Racket function struct-copy, which is disabled in the student modes.

The example file below creates a struct called “m” that contains a position, velocity, score, and a boolean for if the game is lost. The file also creates “updating” functions that change only one part of the struct.

NOTE: In this file you must change the language to racket (Language -> Choose Language… -> Racket).

#lang racket
(require lang/posn)
(provide (all-defined-out)

(define-struct m (pos vel score lost?) #:transparent)

(define (m-update-pos x w)
    (struct-copy m x [pos w]))
(define (m-update-vel x w)
    (struct-copy m x [vel w]))
(define (m-update-score x w)
    (struct-copy m x [score w]))
(define (m-update-lost x w)
    (struct-copy m x [lost? w]))

Save this file as “m-struct.rkt” and then use (require "m-struct.rkt") in your main program.

Summary

1. Using #lang racket allows some advanced features at a cost of some complexity.
2. Every struct needs #:transparent. This is best explained as a design mistake in the Racket language.
3. Read about struct-copy or follow the examples below.

Example

(require picturing-programs)
(require "m-struct.rkt")

(define example-1
    (make-m (make-posn 5 12) 
            (make-posn 2 -1) 
            120 
            #false))
(define example-2
    (m-update-score example-1 150))
(define example-3
    (m-update-pos example-1 (make-posn 8 15)))