Access to structure subtype strings in Schema and Racket

Question

In Racket, this gives me an error:

(struct point-in-plane (pos_x pos_y)) (struct pixel point-in-plane (color)) (define a-pixel (pixel 1 2 "blue")) (pixel-color a-pixel) (pixel-pos_x a-pixel) (pixel-pos_y a-pixel) 

To do this, I need to replace the last two lines:

 (point-in-plane-pos_x a-pixel) (point-in-plane-pos_y a-pixel) 

Similarly in R6RS

 #!r6rs (import (rnrs)) (define-record-type point (fields xy)) (define-record-type cpoint (parent point) (fields color)) (define blue-point (make-cpoint 1 2 "blue")) (write (cpoint-x blue-point)) 

Gives a similar error.

What is the reason why Scheme (and Racket) does not allow you to access the subtype fields that were defined in the parent element: subtypeID-fieldID instead of parenttypeID-fieldID

those. in my case, letting me use pixel-pos_x and pixel-pos_y.

Answer 1

One of the reasons is that struct allows you to define substructures with the same named fields. For instance:

 (struct base (xy)) (struct sub base (xy)) (define rec (sub 1 2 3 4)) (base-x rec) ; => 1 (sub-x rec) ; => 3 

This is because structures do not know the names of the fields. From Racquet Documentation : "Fields of type structure are essentially unnamed, although names are supported for error reporting purposes." You will need to disable this behavior in order to get additional accessors for substructures.

Answer 2

the documentation in the form of a structure says that it provides accessors and setters for the given fields, but does not say that it will automatically re-imagine the existing accessors and setters of the parent type with the additional names that you expect.

When I am structuring and pulling components by name, I often use the racket / match library, especially with struct * pattern matching. I usually have to deal with several components of the structure, and this allows me to do this.