Demo

Closes #519

/claim #519

Summary

Implements a complete, algebraic, serializable Schema Migration System for ZIO Blocks, resolving issue #519.

Architecture

Two-layer design mirroring the existing DynamicPatch / Patch[A] pattern:

DynamicMigration - untyped, pure data, fully serializable

• actions: Vector[MigrationAction] - a sequence of named operations applied in order
• apply(DynamicValue): Either[MigrationError, DynamicValue]
• reverse: DynamicMigration - structural inverse of all actions
• ++ composition, isEmpty

Migration[A, B] - typed wrapper

• Converts A to DynamicValue via sourceSchema, applies DynamicMigration, converts back to B via targetSchema
• reverse: Migration[B, A]
• ++[C](that: Migration[B, C]): Migration[A, C]

MigrationAction ADT

All actions carry at: DynamicOptic for precise path addressing:

• AddField(at, default) - Add a field with a default value
• DropField(at, defaultForReverse) - Remove a field
• Rename(at, to) - Rename a field
• TransformValue(at, expr) - Transform a value using a DynamicMigrationExpr
• Mandate(at, default) - Make optional field mandatory
• Optionalize(at) - Wrap mandatory field in Some
• RenameCase(at, from, to) - Rename an enum variant
• TransformCase(at, caseName, actions) - Apply nested migrations to a variant payload
• TransformElements(at, expr) - Map over sequence elements
• TransformKeys(at, expr) - Map over map keys
• TransformValues(at, expr) - Map over map values

DynamicMigrationExpr

Serializable expression ADT: Identity, Constant, IntToLong/LongToInt, IntToString/StringToInt, LongToString/StringToLong, DoubleToString/StringToDouble, FloatToDouble/DoubleToFloat, BooleanToString/StringToBoolean, ConcatFields, Compose

MigrationBuilder DSL

val migration = Migration.newBuilder[PersonV1, PersonV2]
  .renameField("name", "fullName")
  .addField("country", Schema[String], "Unknown")
  .build

.build validates paths, .buildPartial skips validation

Laws

• Identity: Migration.identity[A].apply(a) == Right(a)
• Associativity: (m1 ++ m2) ++ m3 == m1 ++ (m2 ++ m3)
• Reverse: m.reverse.reverse has structurally equal actions to m

Files Added

• migration/MigrationError.scala
• migration/DynamicMigrationExpr.scala
• migration/MigrationAction.scala
• migration/DynamicMigration.scala
• migration/Migration.scala
• migration/MigrationBuilder.scala
• migration/DynamicMigrationSpec.scala (tests)
• migration/MigrationSpec.scala (tests)