Function flatMap

flatMap<T, O>(decoder: Decoder<T>, fn: (value: T) => Decoder<O>): Decoder<O>
Chains a decoder into another decoder that depends on the decoded value.
Type Parameters
- T
- O
Parameters
- decoder: Decoder<T>
- fn: (value: T) => Decoder<O>
Returns Decoder<O>
Example
```
const adult = flatMap(number(), age =>
  age >= 18 ? succeed() : fail(`Age ${age} is less than 18`)
);
```