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03·03Schema Transformations

Transforms make the encoded representation and trusted domain type explicit. V4 provides common codecs such as `Schema.NumberFromString` and `Schema.fromJsonString`; custom transformations use `Schema.decodeTo` with `SchemaTransformation`.

20 — Schema transformations

Transforms make the encoded representation and trusted domain type explicit. V4 provides common codecs such as Schema.NumberFromString and Schema.fromJsonString; custom transformations use Schema.decodeTo with SchemaTransformation.

The exercises decode a string port into an integer and encode it back, then decode a JSON string before validating its object shape. Conversion failures remain Schema failures rather than thrown exceptions.

The full test suite

The index.test.ts for this lesson, verbatim from steps/20-schema-transformations/.

index.test.ts
import { describe, expect, it } from "vite-plus/test"
import { Schema } from "effect"


describe("Schema transformations", () => {
  it("keeps encoded and domain types distinct", () => {
    const Port = Schema.NumberFromString.pipe(Schema.check(Schema.isInt()))
    expect(Schema.decodeUnknownSync(Port)("8080")).toBe(8080)
    expect(Schema.encodeSync(Port)(8080)).toBe("8080")
  })


  it("decodes JSON before validating its domain shape", () => {
    const Payload = Schema.fromJsonString(Schema.Struct({ active: Schema.Boolean }))
    expect(Schema.decodeUnknownSync(Payload)('{"active":true}')).toEqual({ active: true })
  })
})
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