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Serialization in SAFE

Serialization basics with Thoth

When using basic HTTP communication between the client and server, you'll need to consider how to deserialize data from JSON to F# types.

In order to guarantee that the serialization / deserialization routines between client and server are compatible, you should replace the JSON converter in Giraffe / Saturn with the Thoth library's serializer. This is the same library as that used in Fable for deserialization, and so will work seamlessly together.

let configureSerialization (services:IServiceCollection) =

If you are using the SAFE Template, this will automatically be done for you in Server.fs.

Approaches to deserialization

Fable 2 uses the Thoth library for JSON deserialization, which makes use of decoders to convert JSON into F# values. There are generally two main approaches to take when doing this: automatic and manual decoders.

Assume the following Customer record for the remaining examples.

type Customer =
    { Id : int
      Name : string }

Automatic Decoders

Automatic decoders are the quickest and easier way to deserialize data. It works by Thoth trying to decode JSON automatically from a raw string to an F# type using automatic mapping rules. In the sample below, we fetch data from the /api/customers endpoint and have Thoth create a strongly-typed Decoder for a Customer array.

fetchAs<Customer []> "/api/customers" (Decode.Auto.generateDecoder()) []

If the serialization fails, Thoth will create an Error (rather than Ok) value for this.

Be aware that automatic decoders are designed to work with primitives, collections, F# records, tuples and discriminated unions but cannot deserialize classes.

Improving efficiency with cached decoders

You can reuse decoders when you know you'll be calling them often:

// let-bound value that exists outside of the update function
let customerDecoder = Decode.Auto.generateDecoder<Customer>()

// inside the update function
Fetch.fetchAs (sprintf "api/customers") (Decode.array customerDecoder [])

Notice how the decoder is bound to a single Customer, and not an array. This way, we can also reuse the decoder on other routes, for example api/customers/1 which would return a single Customer object rather than a collection.

Manual Decoders

Manual decoders give you total control over how you rehydrate an object from JSON. Use them when:

  • The JSON does not directly map 1:1 with your F# types
  • You want flexibility to evolve JSON and F# types independently
  • You are calling an external service and need fine-grained control over the deserialization process
  • You are using F# on the client and another language on the server

You create a manual decoder as follows:

let customerDecoder : Decoder<Customer> =
        (fun get ->
            { Id = get.Required.Field "id"
              Name = get.Optional.Field "customerName" Decode.string |> Option.defaultValue "" })

You can now replace the automatically generated decoder from earlier. You can also "manually" decode JSON to Customers as follows:

Decode.fromString customerDecoder """{ "id": 67, "customerName": "Joe Bloggs" }"""

If decoding fails on any field, an error case will be returned.