Client Server communication over HTTP

Communicating over raw HTTP using Saturn has three main steps.

1. Load your data

Start by creating a function on your server that returns some data:

let loadCustomersFromDb() =
    [ { Id = 1; Name = "Joe Bloggs" } ]

Next, create a method which returns the data as JSON within Giraffe's HTTP context.

/// Returns the results of loadCustomersFromDb as JSON.
let getCustomers next ctx =
    json (loadCustomersFromDb()) next ctx

You can opt to combine both of the above functions into one, depending on your preferences, but it's often good practice to separate your data access from serving data in HTTP endpoints.

Also note the next and ctx arguments. These are used by Giraffe as part of its HTTP pipeline and are required by the json function (Note you can also use Successful.Ok instead of json, which will offer XML serialization as well).

2. Expose data through Saturn

Now expose the api method using Saturn's router construct and add it to your overall application scope:

let myApis = router {
    get "/api/customers/" getCustomers
}

For simple endpoints you may elect to embed the API call directly in the scope (and use partial application to omit the next and ctx arguments):

let myApis = router {
    get "/api/customers/" (json (loadCustomersFromDb()))
}

3. Consume the endpoint from the client

Finally, call the endpoint from your client application.

promise {    
    let! customers = Fetch.fetchAs<Customer list> "api/customers" (Decode.Auto.generateDecoder()) []
    // do more with customers here...
}

Note the use of the promise { } computation expression. This behaves similarly to async { } blocks that you might already know, whilst the fetchAs function retrieves data from the HTTP endpoint specified. The JSON is deserialized as a Customer array using an automatically-generated "decoder" (see the section on serialization for more information).