Unexpected Key Messages | Effect Schema

By default, Schema strips extra keys from the input without complaint. But when you enable strict mode with { onExcessProperty: "error" }, extra keys cause UnexpectedKey issues. You can customize the error message for this scenario with the messageUnexpectedKey annotation on the struct:

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import { Schema } from "effect";
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const Strict = Schema.Struct({
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  name: Schema.String,
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}).annotate({ messageUnexpectedKey: "Unknown field. Only 'name' is allowed" });
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const result = Schema.decodeUnknownExit(Strict)(
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  { name: "Alice", extra: "oops" },
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  { onExcessProperty: "error" },
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);
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console.log(String(result));

Output:

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Failure(Cause([Fail(SchemaError(Unknown field. Only 'name' is allowed
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  at ["extra"]))]))

Without the annotation, the error would say: Unexpected key with value "oops" at ["extra"].

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