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.
For example, given this data type:
type Person = {
Name: string
Age: int
Children: Person list
}You can map it to JSON like this:
open Fleece
open Fleece.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 = [] }
] }
// Test with System.Text.Json
open Fleece.SystemTextJson
printfn "%s" (toJsonText 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 .@ "name"
let age = o .@ "age"
let children = o .@ "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 <| Uncategorized (sprintf "Error parsing person: %A" x)
| x -> Decode.Fail.objExpected x
let john : Person ParseResult = ofJsonText """{"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 .@ "name") <*> (o .@ "age") <*> (o .@ "children")
| x -> Decode.Fail.objExpected x
or with applicatives:
open FSharpPlus
type Person with
static member OfJson json =
match json with
| JObject o ->
monad {
let! name = o .@ "name"
and! age = o .@ "age"
and! children = o .@ "children"
return {
Person.Name = name
Age = age
Children = children
}
}
| x -> Decode.Fail.objExpected xOr you can use the Choice monad/applicative in FSharpx.Extras instead, if you prefer.
You can see more examples in the EdmundsNet project.
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 get_Codec () =
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 can use 'jopt'
<*> jreq "children" (fun x -> Some x.children)
|> ofObjCodec
let john: Person ParseResult = ofJsonText """{"name": "John", "age": 44, "children": [{"name": "Katy", "age": 5, "children": []}, {"name": "Johnny", "age": 7, "children": []}]}"""Discriminated unions can be modeled with alternatives:
type Shape =
| Rectangle of width : float * length : float
| Circle of radius : float
| Prism of width : float * float * height : float
with
static member get_Codec () =
(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))
|> ofObjCodecor using the jchoice combinator:
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)
]
|> ofObjCodec
But codecs for both types can easily be written with the codec computation expressions
type Person = {
name : string * string
age : int option
children: Person list }
with
static member get_Codec () =
codec {
let! f = jreq "firstName" (Some << fun x -> fst x.name)
and! l = jreq "lastName" (Some << fun x -> snd x.name)
and! a = jopt "age" (fun x -> x.age) // Optional fields can use 'jopt'
and! c = jreq "children" (fun x -> Some x.children)
return { name = (f, l); age = a; children = c } }
|> ofObjCodec
type Shape =
| Rectangle of width : float * length : float
| Circle of radius : float
| Prism of width : float * float * height : float
with
static member get_Codec () =
codec {
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)
}
|> ofObjCodec
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:
open System.Text
open Fleece.SystemTextJson.Operators
type Person = {
name : string * string
age : int option
children: Person list }
with
static member JsonObjCodec: Codec<PropertyList<Fleece.SystemTextJson.Encoding>, Person> = codec {
let! f = jreq "firstName" (Some << fun x -> fst x.name)
and! l = jreq "lastName" (Some << fun x -> snd x.name)
and! a = jopt "age" (fun x -> x.age) // Optional fields can use 'jopt'
and! c = jreq "children" (fun x -> Some x.children)
return { name = (f, l); age = a; children = c } }
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: byte [] -> string) Encoding.UTF8.GetBytes // This is a pair of of isomorphic functions
let p = { name = "John", "Smith"; age = Some 42; children = [] }
let bytePerson = Codec.encode personBytesCodec p
// val bytePerson : byte [] = [|123uy; 34uy; 102uy; 105uy; 114uy; 115uy; ... |]
let p' = Codec.decode personBytesCodec bytePersonSo 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 () =
codec {
let! i = jreqWith Codecs.string "id" (fun x -> Some x.Id)
and! c = jreqWith colorCodec "color" (fun x -> Some x.Color)
and! k = jreqWith Codecs.int "kms" (fun x -> Some x.Kms)
return { Id = i; Color = c; Kms = k }
}
|> Codec.compose (Codecs.propList Codecs.id)
let car = { Id = "xyz"; Color = Red; Kms = 0 }
let jsonCar : Fleece.SystemTextJson.Encoding = Codec.encode (carCodec ()) car
// val jsonCar: SystemTextJson.Encoding = {"id":"xyz","color":"red","kms":0}Json lenses allow to focus on a specific part of the json structure to perform operations like view, write and update.
For a quick reference have a look at this test file