(value: any, schema: MixedSchema) =>
     value === 4 ? schema.required() : schema

    path: 'path',
    errors: ['error'],
    inner: [yup.ValidationError('error', true, 'path')],
    type: 'date',
    value: {start: '2017-11-10'}
};
error.value = 'value';
error.value = true;
error.value = 5;
error.value = {name: 'value'};
error.type = {};
error.type = [];
error.errors = ['error'];

// mixed
let mixed: MixedSchema = yup.mixed();
mixed.clone();
mixed.label('label');
mixed.meta({ meta: 'value' });
mixed.describe().label;
mixed.describe().meta;
mixed.describe().tests;
mixed.describe().type;
mixed.concat(yup.string());
mixed.validate({});
mixed.validate({ hello: 'world' }, { strict: true }).then(value => value);
mixed.isValid(undefined, (valid: true) => true);
mixed.isValid({ hello: 'world' }).then(valid => valid);
mixed.cast({});
mixed.isType('hello');
mixed.strict(true);

 .when("$other", (value: any, schema: MixedSchema) =>
     value === 4 ? schema.required() : schema