docs/_docs/reference/error-codes/E059.md
This error is emitted when attempting to instantiate a class that has a self type which the class itself doesn't satisfy.
When a class declares a self type, it promises that any instance will conform to that type. If the class doesn't implement or mix in the required types, it cannot be instantiated.
trait Database:
def query(sql: String): String
class Repository:
self: Database =>
def findAll(): String = query("SELECT *")
val repo = new Repository
-- [E059] Type Mismatch Error: example.scala:8:15 ------------------------------
8 |val repo = new Repository
| ^^^^^^^^^^
|Repository does not conform to its self type Database; cannot be instantiated
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| I tried to show that
| Repository
| conforms to
| Database
| but none of the attempts shown below succeeded:
|
| ==> Repository <: Database = false
|
| The tests were made under the empty constraint
-----------------------------------------------------------------------------
// Implement the required self type when instantiating
trait Database:
def query(sql: String): String
class Repository:
self: Database =>
def findAll(): String = query("SELECT *")
val repo = new Repository with Database:
def query(sql: String): String = s"Executed: $sql"
// Or create a concrete class that mixes in the required trait
trait Database:
def query(sql: String): String
class Repository:
self: Database =>
def findAll(): String = query("SELECT *")
class SqlRepository extends Repository with Database:
def query(sql: String): String = s"Executed: $sql"
val repo = new SqlRepository