Introduction

The manual API allows you to have full control over the decoding process.

This is useful when your JSON doesn't match your F# types one-to-one or if you prefer to not rely on reflection.

For example, if your JSON contains the information inside of data property you can directly access it without having to create a record just for this property:

{
    "data": {
        "id": "9d9d9d9d-9d9d-9d9d-9d9d-9d9d9d9d9d9",
        "name": "Triss Merigold",
        "age": 42
    }
}

Then you can write a decoder like this:

type User =
    {
        Id : System.Guid
        Name : string
        Age : int
    }

module User =

    // Decoder specific to the user type
    let decoder : Decoder<User> =
        Decode.object (fun get ->
            {
                Id = get.Required.Field "id" Decode.guid
                Name = get.Required.Field "name" Decode.string
                Age = get.Required.Field "age" Decode.int
            }
        )

Decode.fromString
    // Access the `data` property directly
    // allow us direct access to the 'User' object
    (Decode.field "data" User.decoder)
    json