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// HELPER FUNCTIONS

// Adds the Access-Control-Allow-Origin header if the request has an Origin header.
let AddAllowOrigin (content: Content<'T>) : Content<'T> =
    Content.CustomContentAsync <| fun ctx -> async {
        let! response = Content.ToResponseAsync content ctx
        let headers =
            ctx.Request.Headers
            |> Seq.tryFind (fun h -> h.Name = "Origin")
            |> Option.map (fun origin ->
                Http.Header.Custom "Access-Control-Allow-Origin" origin.Value)
            |> Option.toList
        return { response with Headers = Seq.append headers response.Headers }
    }

// Creates a preflight response that allows the given methods.
let Preflight (methods: seq<string>) : Content<'T> =
    Content.CustomContent <| fun ctx ->
        let methods = String.concat ", " methods
        let headers = [ Http.Header.Custom "Access-Control-Allow-Methods" methods ]
        { Status = Http.Status.Ok; Headers = headers; WriteBody = ignore }
    |> AddAllowOrigin


// APPLICATION CODE

type MyAction =
    // preflight request
    | [<Method "OPTIONS"; CompiledName "my-api">] MyApiOptions
    // main request
    | [<Method "PUT"    ; CompiledName "my-api">] MyApiPut of SomeArgs

let mySite = Sitelet.Infer <| function
    // Respond to the preflight request
    | MyApiOptions -> Preflight ["PUT"]
    // Respond to the actual request
    | MyApiPut args ->
        Content.JsonContent <| fun ctx ->
            () // Generate your body here...
        |> AddAllowOrigin