When instantiating a SchemaMapper you should provide a Schema. When calling 'map' the Schema provided will be used to format the data to the correct format. This class is meant to do the opposite of the EntityAssembler
 public function testShouldParseFieldIntoCastableType()
 {
     // Arrange
     $schema = m::mock(Schema::class);
     $schemaMapper = new SchemaMapper($schema);
     // Assert
     $this->assertSame(23, $schemaMapper->parseField('23', 'int'));
     $this->assertSame('1234', $schemaMapper->parseField(1234, 'string'));
 }