# Question on Racket Guide 5.2 statement **URL:** \ **Category:** Questions & Answers\ **Created:** [August 16, 2025, 1:03am UTC](https://racket.discourse.group/t/question-on-racket-guide-5-2-statement/3903 "2025-08-16T01:03:46Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Dashman](https://avatars.discourse-cdn.com/v4/letter/d/e19b73/32.png) [@Dashman](https://racket.discourse.group/u/Dashman)\ **Post date:** [August 16, 2025, 1:03am UTC](https://racket.discourse.group/t/question-on-racket-guide-5-2-statement/3903/1 "2025-08-16T01:03:46Z")
"The struct-copy form clones a structure and optionally updates specified fields in the clone. This process is sometimes called a functional update, because the result is a structure with updated field values. but the original structure is not modified. struct-copy struct-id struct-expr [field-id expr] ...) The struct-id that appears after struct-copy must be a structure type name bound by struct (i.e., **the name that cannot be used directly as an expression** ). The struct-expr must produce an instance of the structure type. The result is a new instance of the structure type that is like the old one, except that the field indicated by each field-id gets the value of the corresponding expr. Examples: > > define p1 ( **posn** 1 2)) > > define p2 (struct-copy **posn** p1 [x 3])) Is the statement about "cannot be directly used in an expression" out of date? It appears it is being used in an expression: define p1 (posn 1 2)) ---
**Author:** ![spdegabrielle](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/spdegabrielle/32/95_2.png) [@spdegabrielle](https://racket.discourse.group/u/spdegabrielle)\ **Post date:** [August 16, 2025, 4:57pm UTC](https://racket.discourse.group/t/question-on-racket-guide-5-2-statement/3903/2 "2025-08-16T16:57:48Z")
> [@Dashman](#): > > Is the statement about "cannot be directly used in an expression" out of date? It appears it is being used in an expression: (define p1 (posn 1 2)) Hi @Dashman , Welcome! I believe the statement ["the name that cannot be used directly as an expression"](https://docs.racket-lang.org/guide/define-struct.html#(part._struct-copy):~:text=(i.e.%2C-,the%20name%20that%20cannot%20be%20used%20directly%20as%20an%20expression,-).%20The%20struct%2Dexpr) means you can only use `posn` as a constructor like your example `(define p1 (posn 1 2))`, unlike `posn-x` and `posn-y`, which are used in expressions to get those components of the struct e.g. `(list (posn-x p1) (posn-y p1))` Best regards, Stephen PS We are always looking to improve the documentation so if you feel you can make an improvement please don't hesitate to make a pull request with your proposed changes. ---
**Author:** ![LiberalArtist](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/liberalartist/32/151_2.png) [@LiberalArtist](https://racket.discourse.group/u/LiberalArtist)\ **Post date:** [August 16, 2025, 8:24pm UTC](https://racket.discourse.group/t/question-on-racket-guide-5-2-statement/3903/3 "2025-08-16T20:24:08Z")
> [@Dashman](#): > > Is the statement about "cannot be directly used in an expression" out of date? Yes, I think so—good catch! IIRC, historically, `(define-struct posn (x y))` bound `posn` in such a way that it could not be used as an expression, which is why it bound `make-posn` to the constructor. After the invention of `(struct posn (x y))`, where the same identifier is used for the name and the constructor by default, `define-struct` was changed to do the same, easing compatibility and migration to the new style. Whatever the historical details, `posn` and, more explicitly, `(#%expression posn)` are definitely valid expressions. It is still possible to supply keyword options to `struct` to specially arrange for `posn` not to be directly usable as an expression. Whether or not the identifier can be directly used as an expression is irrelevant as long as it has the static info. I think this is just an outdated, and now wrong, attempt to clarify which identifier to use there. ---
**Author:** ![soegaard](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/soegaard/32/19_2.png) [@soegaard](https://racket.discourse.group/u/soegaard)\ **Post date:** [August 16, 2025, 10:34pm UTC](https://racket.discourse.group/t/question-on-racket-guide-5-2-statement/3903/4 "2025-08-16T22:34:06Z")
I think the confusion is do the the double duty of the name `posn` here. In phase 0 it stands for the constructor - which can be used as an expression. In phase 1 it stands for the structure descriptor. But regardless, I too think the Guide should be reworded.