How to use the decoders.fail function in decoders

const shape = dispatch(field('type', string), type => {
    switch (type) {
        case 'rect':
            return rect;
        case 'circle':
            return circle;
    }
    return fail('Must be a valid shape');
});

const shape = dispatch(field('type', string), type => {
    switch (type) {
        case 'rect':
            return rect;
        case 'circle':
            return circle;
    }
    return fail('Must be a valid shape');
});

guard(
    object({
        a0: string,
        a1: number,
        a2: optional(nullable(either4(string, email, regex(/x/, 'Must be x'), url()))),
        a3: fail('foo'),
        a4: maybe(mixed),
        a5: either3(constant('foo'), constant('bar'), constant('qux')),
        a6: exact({ c: array(poja), d: pojo }),
        a7: tuple6(hardcoded('foo'), mixed, null_, undefined_, unknown, truthy),
        a8: tuple4(integer, number, positiveInteger, positiveNumber),
        a9: either(boolean, numericBoolean),
        b0: map(
            compose(
                string,
                predicate(s => s.startsWith('x'), 'Must start with x')
            ),
            s => s.toUpperCase()
        ),
        b1: shape,
        b2: date,
        b3: dict(string),