# Typed Struct Property: Problem with prop:custom-write **URL:** \ **Category:** Questions & Answers\ **Tags:** typed-racket\ **Created:** [December 15, 2024, 10:48am UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419 "2024-12-15T10:48:23Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![wurmli](https://avatars.discourse-cdn.com/v4/letter/w/dbc845/32.png) [@wurmli](https://racket.discourse.group/u/wurmli)\ **Post date:** [December 15, 2024, 10:48am UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419/1 "2024-12-15T10:48:23Z")
This is probably a question for Suzanne Soy. I tried her struct type properties, but the result is not exactly what I expected. ```racket #lang typed/racket (: display-point (-> Point Output-Port (U #t #f 0 1) Any)) (define (display-point p o-p opt) (display (format "(~a, ~a)" (point-x p) (point-y p)))) (struct point ([x : Number] [y : Number]) #:type-name Point #:transparent #:property prop:custom-write display-point ) (define p (point 1 2)) ``` My output at the REPL looks like this: ```scheme > p - : Point (1, 2)(1, 2) > (display-point p (current-output-port) #t) (1, 2)- : Any ``` What, by the way, is the meaning of the third argument for the `custom-write`? ---
**Author:** ![bakgatviooldoos](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/bakgatviooldoos/32/1381_2.png) [@bakgatviooldoos](https://racket.discourse.group/u/bakgatviooldoos)\ **Post date:** [December 15, 2024, 11:07am UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419/2 "2024-12-15T11:07:16Z")
Hi, @wurmli. No idea about the type properties, but on the question about `custom-write`, from [the docs](https://docs.racket-lang.org/reference/Printer_Extension.html#(part._.Printer_.Extension)): > A write-proc method takes three arguments: the structure to be printed, the target port, and an argument that is #t for write mode, #f for display mode, or 0 or 1 indicating the current quoting depth for print mode. The procedure should print the value to the given port using write, display, print, fprintf, write-special, etc. Also, do note: > Using the prop:custom-write property is discouraged; use the gen:custom-write generic interface instead. ---
**Author:** ![wurmli](https://avatars.discourse-cdn.com/v4/letter/w/dbc845/32.png) [@wurmli](https://racket.discourse.group/u/wurmli)\ **Post date:** [December 16, 2024, 5:34am UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419/3 "2024-12-16T05:34:37Z")
Thank you! I tried to follow Suzanne Soy's " Struct type properties for Typed/Racket", but made mistakes. The definitions could look like this ```racket #lang typed/racket (require typed-struct-props) (: display-point (-> point Output-Port (U #t #f 0 1) Any)) (define (display-point p o-p opt) (case opt [(#t) (display (format "(~a, ~a)" (point-x p) (point-y p)))] [(#f) (write `(point ,(point-x p) ,(point-y p)))] )) (struct/props point ([x : Number] [y : Number]) #:transparent #:property prop:custom-write display-point ) (define p (point 1 2)) ``` but still produce errors: ```scheme > p - : point > (write p) (1, 2)(1, 2) > (display p) (point 1 2)(point 1 2) ``` As using "struct/props" instead of "struct" would disallow, for example, #:type-name, it wouldn't serve my purpose anyway. Still, I wonder why the doubling of output happens. ---
**Author:** ![shhyou](https://avatars.discourse-cdn.com/v4/letter/s/ccd318/32.png) [@shhyou](https://racket.discourse.group/u/shhyou)\ **Post date:** [December 16, 2024, 7:00am UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419/4 "2024-12-16T07:00:26Z")
The code needs to write to `o-p`, not the current output port. ---
**Author:** ![wurmli](https://avatars.discourse-cdn.com/v4/letter/w/dbc845/32.png) [@wurmli](https://racket.discourse.group/u/wurmli)\ **Post date:** [December 16, 2024, 8:10am UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419/5 "2024-12-16T08:10:25Z")
Thank you, that almost works. What is still missing is the printing of a value by the REPL: ```racket #lang typed/racket (require typed-struct-props) (: display-point (-> point Output-Port (U #t #f 0 1) Any)) (define (display-point p o-p opt) (case opt [(#f) (display (format "(~a, ~a)" (point-x p) (point-y p)) o-p)] [(#t) (write `(pt ,(point-x p) ,(point-y p)) o-p)] )) (struct/props point ([x : Number] [y : Number]) #:transparent #:property prop:custom-write display-point ) (define p (point 1 2)) ``` ```scheme > p - : point > (write p) (pt 1 2) > (display p) (1, 2) ``` ---
**Author:** ![shhyou](https://avatars.discourse-cdn.com/v4/letter/s/ccd318/32.png) [@shhyou](https://racket.discourse.group/u/shhyou)\ **Post date:** [December 16, 2024, 8:32am UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419/6 "2024-12-16T08:32:15Z")
See @bakgatviooldoos's post and the example code in it: > A write-proc method takes three arguments: the structure to be printed, the target port, and an argument that is #t for write mode, #f for display mode, or 0 or 1 indicating the current quoting depth for print mode. So `case opt` has to handle all four of `#f`, `#t`, `0`, and `1`. ---
**Author:** ![wurmli](https://avatars.discourse-cdn.com/v4/letter/w/dbc845/32.png) [@wurmli](https://racket.discourse.group/u/wurmli)\ **Post date:** [December 16, 2024, 12:01pm UTC](https://racket.discourse.group/t/typed-struct-property-problem-with-prop-custom-write/3419/7 "2024-12-16T12:01:38Z")
Thank you! I think I got it now. What is described in " Struct type properties for Typed/Racket" seems to be implemented for structs in typed/racket, but not documented, at least it produces identical results.