Update the whole structure

问题

Suppose I have some function which returns a struct:

(struct layer (points lines areas))

(define (build-new-layer height)
  ...
  (layer list-a list-b list-c))

I want to keep track of the last returned result something like:

(define top-side (build-new-layer 0)) ; store the first result
...
(set! top-side (build-new-layer 0.5)) ; throw away the first result and store the new one

However, for that particular code I get the error:

set!: assignment disallowed;
 cannot modify a constant
  constant: top-side

Please, tell me what would be the right way to do what I want

回答1:

What language are you using? it seems it's a matter of configuration, because in principle what you're doing should work. Go to the "choose language" window (Ctrl+L in Windows), click on "show details" and see if one of the options of the language currently in use disallows redefinition of variables. Alternatively, try using a different language.

Depending on where exactly you're going to use the stored result (I can't tell from the code in the question), you could pass it around as function parameters, in such a way that using a global variable is no longer necessary. This might be a better idea, relying on global state and mutation (the set! operation) is discouraged in Scheme.

回答2:

If you always want to keep around the last layer, then you might prefer setting the last-layer every time one is built. Like this.

(define last-layer #f)
(define build-new-layer
  (let ((save-layer #f))
    (lambda (height)
      (let ((new-layer (layer list-a ...)))
        (set! last-layer save-layer)
        (set! save-layer new-layer)
        new-layer))))

Note: if the real problem is the 'constant-ness' of last-layer then build yourself a little abstraction as:

(define-values (last-layer-get last-layer-set!)
  (begin
    (define last-layer-access
      (let ((last-layer #f))
        (lambda (type . layer)
          (case type
           ((get) last-layer)
           ((set) (set! last-layer (car layer)))))))
    (values (lambda () (last-layer-access 'get))
            (lambda (layer) (last-layer-access 'set layer))))

来源：https://stackoverflow.com/questions/15254713/update-the-whole-structure

标签

struct

scheme

set

racket

assign