Get default value from case class default paramters

[liewhite]

It's very convenient to define default value in case class, and zio-json do so.

[jdegoes]

/bounty $150

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Attempt	Started (GMT+0)	Solution
🟢 @987Nabil	May 19, 2023, 3:19:03 AM	#562

[andrzejressel]

From what I see schema does not have notion of default values for case classes.

To keep it as generic as possible I guess something like that should be added (+ support for custom deserializers like json)?

[liewhite]

From what I see schema does not have notion of default values for case classes.

But Schema[T] is the only thing json codec can get from schema.

[jdegoes]

We already have an annotation that can be used by a user to add a default value to a case class field. I think we should reuse that, and, in the Scala 2/3 macros, we see if a parameter to a case class constructor has a default value, and if so, we bundle that up and add the annotation to the Schema, unless the annotation does / will exist because the user added it themselves.

This way, users of a Schema only need to check the annotation, rather than look at 2 possibly conflicting sources for default values.

[liewhite]

Create instance of class can't make use of annotations.
This is a language feature, many people use it, I think we should use default parameter as far as possible.

[987Nabil]

I'll /attempt #552 since this this is also useful for OpenAPI gen in zio-http.

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