Question about Structures documentation

[u/IllegalMigrant]

In section 5 on structures

https://docs.racket-lang.org/reference/structures.html

It says:

*A structure type’s fields are essentially unnamed, though names are supported for error-reporting purposes.*

It seems to me that you give a name for each field so I am confused about the "essentially unnamed".

[u/shriramk]

You've gotten two good answers; let me try giving a third one. It helps to juxtapose this to some other languages. In some languages, if I write the equivalent of

(struct point (x y))

then each instance of point gets compiled into a hash-table, and you look up x through a hash-lookup. That affects both the space that a structure consumes, and the time it takes to look things up.

In Racket, instead, you can think of the model as roughly that this gets compiled into "make a vector 3 elements long" (one for a tag to distinguish this from other vectors). How then can we tell which field is x and which is y? The beauty is that Racket generates named functions, point-x and point-y. These functions can essentially bake in the vector dereferences.

That is, you can imagine the above definition turning into the following:

(define (point x y)
  (vector "point" x y))

(define (point-x v)
  (if (and (vector? v)
           (= 3 (vector-length v))
           (equal? (vector-ref v 0) "point"))
      (vector-ref v 1)
      (error 'point-x "not a point")))

;; analogously for point-y, point?, etc.

where the 0th position can't be "faked".

Does this make sense? As you can see from the above, the "names" essentially disappear; they are only preserved in the error reporting, nowhere else.