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Update JSON.Encoder moduledoc around :only and :except #14784

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Sep 22, 2025
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Update JSON.Encoder moduledoc around :only and :except #14784

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19 changes: 12 additions & 7 deletions lib/elixir/lib/json.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -8,24 +8,29 @@ defprotocol JSON.Encoder do
If you have a struct, you can derive the implementation of this protocol
by specifying which fields should be encoded to JSON:

@derive {JSON.Encoder, only: [....]}
@derive {JSON.Encoder, only: [...]}
defstruct ...

It is also possible to encode all fields or skip some fields via the
`:except` option:
Additionally, you can exclude specific fields using the `:except` option or
encode all fields by omitting both options entirely, but these should be used
with caution:

@derive {JSON.Encoder, except: [...]}
defstruct ...

@derive JSON.Encoder
defstruct ...

> #### Leaking Private Information {: .error}
>
> The `:except` approach should be used carefully to avoid
> accidentally leaking private information when new fields are added.
> Prefer using `:only` to avoid accidentally leaking private information when
> new fields are added. Other approaches should be used with auction.

Finally, if you don't own the struct you want to encode to JSON,
you may use `Protocol.derive/3` placed outside of any module:
You can also use `Protocol.derive/3` if you don't own the struct that you want
to encode to JSON:

Protocol.derive(JSON.Encoder, NameOfTheStruct, only: [...])
Protocol.derive(JSON.Encoder, NameOfTheStruct, except: [...])
Protocol.derive(JSON.Encoder, NameOfTheStruct)

"""
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