How to use the decoders.compose function in decoders

// $ExpectType Decoder
compose(
    string,
    predicate(s => s.startsWith('x'), 'Must start with x')
);

const a = predicate((foo): foo is string => typeof foo === 'string', 'Is string');
// $ExpectType Decoder
a;

const b = predicate((foo: string): foo is 'a'|'b' => foo === 'a' || foo === 'b', 'Is a or b');
// $ExpectType Decoder<"a" | "b", string>
b;

// $ExpectType Decoder<"a" | "b", unknown>
compose(a, b);

import { compose, map, predicate, string } from 'decoders';

// $ExpectType Decoder
map(string, parseFloat);

// $ExpectType Decoder
compose(
    string,
    predicate(s => s.startsWith('x'), 'Must start with x')
);

const a = predicate((foo): foo is string => typeof foo === 'string', 'Is string');
// $ExpectType Decoder
a;

const b = predicate((foo: string): foo is 'a'|'b' => foo === 'a' || foo === 'b', 'Is a or b');
// $ExpectType Decoder<"a" | "b", string>
b;

// $ExpectType Decoder<"a" | "b", unknown>
compose(a, b);

});

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),
        b4: mapping(string),
        b5: oneOf(['foo', 'bar']),
        b6: instanceOf(TypeError),
    }),
    { style: 'simple' }
)('blah');