Optional struct field

Question

I have searched the documentation for Racket (the language; non-typed) and have been unable to decide if it is possible to have optional arguments for a non-mutable struct. I would like to be able to do: ``` (struct q-a-website (name interest-value #syntax? some-optional-field)) ... (q-a-website "stack-overflow" 42 "My name is Jon Skeet") ... ... (q-a-website "quora" -inf.0) ... ``` In this pseudo-example, `#syntax?` is just a placeholder for where I suspect some special syntax might reside to make the following field optional. Is there a way to make your everyday, immutable, run-of-the-mill struct have optional arguments in base Racket? Clarification: If a struct is created without the optional parameter, it is filled in with a default value that must be supplied at creation time. In this instance, that information would have to be contained inside of the (perhaps poorly named) `#syntax?` block. (Note: I have reservations regarding the inclusion of the `struct` tag to this question a…

Accepted Answer

I think the easiest way to do what you want is to create a "constructor" that has an optional argument, like this:

```
#lang racket

(struct q-a-website (name interest-value optional-field))

;; make a q-a-website
(define (make-q-a-website name interest-value [optional-field #f])
  (q-a-website name interest-value optional-field))

;; try making it with and without the optional argument:
(make-q-a-website "stack-overflow" 42 "My name is Jon Skeet")
(make-q-a-website "quora" -inf.0)
```

Racket also has a full-blown class system, with ... pretty much everything you can imagine. For this use, though, I think I'd just do it like this.

Optional struct field

Solution

Problem