Struct subtype with default/fixed value of super-type

[Marc Kaufmann]

Hi all,

suppose I have a struct `(struct fish (size name) #:transparent)` and a sub-type `(struct shark fish (scares-people?) #:transparent)`. What if I want all sharks to have size 'large by default whenever I create one, so that I create them via:

(define white-shark (shark "The big white" #t))

rather than

(define white-shark (shark 'big "The big white" #t))

Is that possible to do, other than defining my own wrapper? If it is not possible with the current structures, I guess it would be bad practice to overwrite the `shark` constructor, and rather create a `my-shark` constructor to avoid (hypothetical) reader of my code wouldn't get confused.

Best,
Marc

[Luis Sanjuán]

Maybe #:auto/#:auto-value is what you want:

```
(struct fish (name [size #:auto])
#:auto-value 'big
#:transparent)

(struct shark fish (scares-people?) #:transparent)

(define white-shark (shark "The big white" #t))
```

[Marc Kaufmann]

Yes, I saw that, the problem is that I want to be able to set a (potentially different) default value for the sub-type shark than for the sub-type guppy. So if I set the value to 'big for all `fish`, then I can't do that. Essentially, I'd need an #:auto for every sub-type, but the #:auto refers to a parent/super field.

I'll just run with defining a custom constructor.

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[Jérôme Martin]

Once you discover that the default constructor for a struct is just a simple procedure that was generated for you, it's not that big of deal to just make your own and export it like this:

(provide

  (except-out (struct-out fish) fish)

  (rename-out (make-fish fish)))

All the code relying on it will still work.

Except I guess if you pass through different phase levels, it becomes a bit more complicated because you have a syntax attached at compile-time to the identifier. But that's another issue.

Correct me if I'm wrong.

[Marc Kaufmann]

While I did know that I could define my own, I was going to do it while providing-out another function make-fish and make-shark, but it's nice to know that this allows me to locally define make-fish, while it looks to the outside like (fish ...).

Thanks to including sufficient numbers of mundane bugs in my code (all well-maintained in version control, for quick deployment on docker), I rarely get to worry about phases - it just isn't where the highest gains are. :-)

--

[Jérôme Martin]

To go further, if you wonder how to make this work for `match` and inheriting, see this stackoverflow question: https://stackoverflow.com/questions/55262818/how-to-override-a-struct-constructor-while-still-providing-the-struct-metadata-f