feat(Schema): add UndefinedOrFromNullOr - #8168
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The `T | undefined` counterpart of `OptionFromNullOr`: decoding maps `null` to `undefined`, encoding maps `undefined` back to `null`. For programs that model absence as `undefined` (the `Array.find` / `Map.get` idiom) but must emit `null` on a JSON wire, where `JSON.stringify` would otherwise drop the key. Adds `SchemaTransformation.undefinedOrFromNullOr`, unit and type tests, and doc examples. Claude-Session: https://claude.ai/code/session_01XpmvW3MzCahnWN6PD1UCsc
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@davetorbeck Very nice! I have been re-implementing this by hand in different projects. It would be a great first-class citizen, imo. |
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For serialization to and deserialization from JSON, the idiomatic approach in v4 is not to worry about it at all and simply use import { Schema } from "effect"
const domainModel = Schema.UndefinedOr(Schema.FiniteFromString)
const codec = Schema.toCodecJson(domainModel)
const decode = Schema.decodeExit(codec)
const encode = Schema.encodeExit(codec)
console.log(String(encode(1))) // Success("1")
console.log(String(encode(undefined))) // Success(null)
console.log(String(decode("1"))) // Success(1)
console.log(String(decode(null))) // Success(undefined)video clip: https://x.com/GiulioCanti/status/2090409195807903924 |
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@gcanti Thank you again for another explainer. That video was helpful. What program did you used to create it? It looked slick. |
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What
Adds
Schema.UndefinedOrFromNullOr(schema)andSchemaTransformation.undefinedOrFromNullOr().The schema is
NullOr(schema)decoded toUndefinedOr(toType(schema)). Decoding mapsnulltoundefined. Encoding mapsundefinedtonull. Other values pass through in both directions. It follows theOptionFromNullOr/OptionFromUndefinedOr/OptionFromNullishOrfamily in shape, naming, JSDoc and placement.Includes a unit test with
TestSchema.Asserts(decoding, encoding, arbitrary generation), atstychetype test for the wrapper type and itsType/Encodedsides, doc examples that run underpnpm doctest, and apatchchangeset foreffect.pnpm checkforpackages/effect, the Schema unit test file,pnpm test-typesfor the Schema type tests,pnpm doctestfor both modules,oxlintanddprint checkall pass.Why
JSON has no
undefined.JSON.stringifydrops a key whose value isundefined, so a payload with a stable shape (a health probe, a status response) must carrynullfor absence on the wire. Many programs model absence asT | undefinedrather thanOption<T>, following theArray.find/Map.getidiom. Today that combination has no first-class codec:NullOrisnullon both sides,UndefinedOrisundefinedon both sides,optional/optionalKeydrop the key, andOptionFromNullOrrequiresOptionin the domain. The user-land answer is a per-field helper such asjsonNull(value)at every call site, which is exactly the kind of manual boundary code Schema exists to replace, or a hand-writtenencodeTowith two getters in every codebase. This PR provides the codec once, in the library, with the same name pattern as theOptionvariants so it is discoverable next to them.https://claude.ai/code/session_01XpmvW3MzCahnWN6PD1UCsc