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Description

Json mapper for F#

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Fleece

Fleece is a JSON mapper for F#. It simplifies mapping from a Json library's JsonValue onto your types, and mapping from your types onto JsonValue.

The Json library could be System.Json, System.Text.Json, FSharp.Data's or NewtonSoft's Json.NET.

Its design is strongly influenced by Haskell's Aeson. Like Aeson, Fleece is designed around two typeclasses (in FSharpPlus style) ToJson and OfJson.

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Example

For example, given this data type:

type Person = {
    Name: string
    Age: int
    Children: Person list
}

You can map it to JSON like this:

open System.Json
open Fleece.SystemJson
open Fleece.SystemJson.Operators

type Person with static member ToJson (x: Person) = jobj [ "name" .= x.Name "age" .= x.Age "children" .= x.Children ]

let p = { Person.Name = "John" Age = 44 Children = [ { Person.Name = "Katy" Age = 5 Children = [] } { Person.Name = "Johnny" Age = 7 Children = [] } ] }

printfn "%s" (string (toJson p))

And you can map it from JSON like this:

type Person with
    static member OfJson json =
        match json with
        | JObject o ->
            let name = o [email protected] "name"
            let age = o [email protected] "age"
            let children = o [email protected] "children"
            match name, age, children with
            | Decode.Success name, Decode.Success age, Decode.Success children ->
                Decode.Success {
                    Person.Name = name
                    Age = age
                    Children = children
                }
            | x -> Error  Decode.Fail.objExpected x

let john : Person ParseResult = parseJson """{"name": "John", "age": 44, "children": [{"name": "Katy", "age": 5, "children": []}, {"name": "Johnny", "age": 7, "children": []}]}"""

Though it's much easier to do this in a monadic or applicative way. For example, using FSharpPlus (which is already a dependency of Fleece):

open FSharpPlus

type Person with static member Create name age children = { Person.Name = name; Age = age; Children = children }

static member OfJson json =
    match json with
    | JObject o -> Person.Create  (o [email protected] "name")  (o [email protected] "age")  (o [email protected] "children")
    | x -> Decode.Fail.objExpected x

Or monadically:

type Person with
    static member OfJson json =
        match json with
        | JObject o -> 
            monad {
                let! name = o [email protected] "name"
                let! age = o [email protected] "age"
                let! children = o [email protected] "children"
                return {
                    Person.Name = name
                    Age = age
                    Children = children
                }
            }
        | x -> Decode.Fail.objExpected x

Or you can use the Choice monad/applicative in FSharpx.Extras instead, if you prefer.

You can see more examples in the EdmundsNet project.

CODEC

For types that deserialize to Json Objets, typically (but not limited to) records, you can alternatively use codecs and have a single method which maps between fields and values.

type Person = { 
    name : string * string
    age : int option
    children: Person list } 
    with
    static member JsonObjCodec =
        fun f l a c -> { name = (f, l); age = a; children = c }
         jreq  "firstName" (Some << fun x -> fst x.name)
         jreq  "lastName"  (Some << fun x -> snd x.name)
         jopt  "age"       (fun x -> x.age) // Optional fields: use 'jopt'
         jreq  "children"  (fun x -> Some x.children)


let p = {name = ("John", "Doe"); age = None; children = [{name = ("Johnny", "Doe"); age = Some 21; children = []}]} printfn "%s" (string (toJson p))

let john = parseJson """{"children": [{"children": [],"age": 21,"lastName": "Doe","firstName": "Johnny"}],"lastName": "Doe","firstName": "John"}"""

If you prefer you can write the same with functions:

type Person = { 
    name : string * string
    age : int option
    children: Person list }
    with
    static member JsonObjCodec =
        fun f l a c -> { name = (f, l); age = a; children = c }
        |> withFields
        |> jfield    "firstName" (fun x -> fst x.name)
        |> jfield    "lastName"  (fun x -> snd x.name)
        |> jfieldOpt "age"       (fun x -> x.age)
        |> jfield    "children"  (fun x -> x.children)

Discriminated unions can be modeled with alternatives:

fsharp
type Shape =
    | Rectangle of width : float * length : float
    | Circle of radius : float
    | Prism of width : float * float * height : float
    with 
        static member JsonObjCodec =
            Rectangle  jreq "rectangle" (function Rectangle (x, y) -> Some (x, y) | _ -> None)
             ( Circle  jreq "radius" (function Circle x -> Some x | _ -> None) )
             ( Prism  jreq "prism"   (function Prism (x, y, z) -> Some (x, y, z) | _ -> None) )
or using the jchoice combinator: ```fsharp type Shape with static member JsonObjCodec = jchoice [ Rectangle <!> jreq "rectangle" (function Rectangle (x, y) -> Some (x, y) | _ -> None) Circle <!> jreq "radius" (function Circle x -> Some x | _ -> None) Prism <!> jreq "prism" (function Prism (x, y, z) -> Some (x, y, z) | _ -> None) ]
What's happening here is that we're getting a Codec to/from a Json Object (not neccesarily a JsonValue) which Fleece is able to take it and fill the gap by composing it with a codec from JsonObject to/from JsonValue.

We can also do that by hand, we can manipulate codecs by using functions in the Codec module. Here's an example:

```fsharp open System.Text

let personBytesCodec = Person.JsonObjCodec |> Codec.compose jsonObjToValueCodec // this is the codec that fills the gap to/from JsonValue |> Codec.compose jsonValueToTextCodec // this is a codec between JsonValue and JsonText |> Codec.invmap Encoding.UTF8.GetString Encoding.UTF8.GetBytes // This is a pair of of isomorphic functions

let bytePerson = Codec.encode personBytesCodec p // val bytePerson : byte [] = [|123uy; 13uy; 10uy; 32uy; 32uy; ... |] let p' = Codec.decode personBytesCodec bytePerson

Combinators

So far we've seen how Fleece is capable of encoding/decoding by deriving automatically a codec from static members in the type.

But for those cases where we don't have control over the types (extension members won't be taken into account) we can explicitly specify combinators.

To do so, a set of the available functions exists, ending with the

With
suffix, which accepts a combinator as first parameter:
type Color = Red | Blue | White

type Car = { Id : string Color : Color Kms : int }

let colorDecoder = function | JString "red" -> Decode.Success Red
| JString "blue" -> Decode.Success Blue | JString "white" -> Decode.Success White | JString x as v -> Decode.Fail.invalidValue v ("Wrong color: " + x) | x -> Decode.Fail.strExpected x

let colorEncoder = function | Red -> JString "red" | Blue -> JString "blue" | White -> JString "white"

let colorCodec = colorDecoder, colorEncoder

let []carCodec = fun i c k -> { Id = i; Color = c; Kms = k } |> withFields |> jfieldWith JsonCodec.string "id" (fun x -> x.Id) |> jfieldWith colorCodec "color" (fun x -> x.Color) |> jfieldWith JsonCodec.int "kms" (fun x -> x.Kms) |> Codec.compose jsonObjToValueCodec

let car = { Id = "xyz"; Color = Red; Kms = 0 }

let jsonCar = Codec.encode carCodec car // val jsonCar : JsonValue = {"id": "xyz", "color": "red", "kms": 0}

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