Deserializing specific scalar type

[willflitch]

I'm trying to create a scalar (implemented as an ISO 8601 string) called Instant that will be automatically deserialized to a TypeScript Date object from responses.

I've added an exchange that serializes correctly and deserializes correctly when I use regexp to identify ISO 8601 strings, but sometimes I want date strings to stay strings and sometimes I want them to be converted to Date objects.

How can I get my deserializer to know only to convert Instants and not Strings?

[kitten]

This is probably not the answer you're looking for but it sounds like the operation you're doing is inherently "unsafe" or rather not a sane type derivation in a type system.

Basically, given that you have several inputs that serialise to JSON, some of them strings, technically speaking, only the schema can tell you if a given field's scalar is a Date (let's just call your type a date for now)

However, shipping a schema to the client-side (while possible and we do have a minification tool for that and someone has made an exchange for scalar deserialisation that uses this) is expensive inherently.
Similarly, crawling and cloning a response is similarly expensive and what you're doing is basically inefficient and may not do the right thing (basically, your serialisation can always disagree with reality and be subject to XSS attacks that force your code to deserialise incorrectly if you naively match all strings.

This question is similar to this btw: 0no-co/gql.tada#439 (comment)

This also contains my preferred and recommended solution: tagged types and deserialisers.

Basically, add a "shadow type" to your Date types and let your type generator take care of the problem. Basically, these are called branded types in TS most of the time

Then have a helper that deserialises this branded type in your UI/view/consumer code outside of the GraphQL client.

The branded/shadow type enforces that you must use your helper and it solves this problems with a zero-cost abstraction

[willflitch]

Understood! I appreciate the response.