Is there a shorthand way to update a specific struct field in racket?

Question

Suppose I have a struct with many fields:

(struct my-struct (f1 f2 f3 f4))

If I am to return a new struct with f2 updated, I have

Answer 1

I like Alexis' macro! It has more of the "lens" flavor you wanted.

I also want to point out struct-copy. Given:

#lang racket
(struct my-struct (f1 f2 f3 f4) #:transparent)
(define s (my-struct 1 2 3 4))

You can use struct-copy to set a value:

(struct-copy my-struct s [f2 200])
;;=> (my-struct 1 200 3 4)

Or to update a value:

(struct-copy my-struct s [f2 (* 100 (my-struct-f2 s))])
;;=> (my-struct 1 200 3 4)

---

Update: Thinking about this more, here are a few more ideas.

You could also update using match's struct* pattern:

(match s
  [(struct* my-struct ([f2 f2]))
   (struct-copy my-struct s [f2 (* 100 f2)])])

Of course, that's very verbose. On the other hand the struct* pattern makes it easy to define a macro using the simpler define-syntax-rule:

;; Given a structure type and an instance of it, a field-id, and a
;; function, return a new structure instance where the field is the
;; value of applying the function to the original value.
(define-syntax-rule (struct-update struct-type st field-id fn)
  (match st
    [(struct* struct-type ([field-id v]))
     (struct-copy struct-type st [field-id (fn v)])]))

(struct-update my-struct s f2 (curry * 100))
;;=> (my-struct 1 200 3 4)

Of course, setting is the special case where you give update a const function:

(struct-update my-struct s f2 (const 42))
;;=> (my-struct 1 42 3 4)

Finally, this is like struct-update, but returns an updater function, in the spirit of Alexis' macro:

(define-syntax-rule (struct-updater struct-type field-id)
  (λ (st fn)
    (struct-update struct-type st field-id fn)))

(define update-f2 (struct-updater my-struct f2))

(update-f2 s (curry * 100))
;;=> (my-struct 1 200 3 4)

I'm not saying that any of this is idiomatic or efficient. But it's possible. :)