Write JSONs in F#!

JsonBuilder.fs

open System.Collections.Generic
open Quotations.Patterns
let inline ( ==> ) a b = KeyValuePair(a, b)

let isCall = function Call _ -> true | _ -> false

let findQuotes q =
    let rec findQuotes' q res = seq {
        match q with
        | Call (_, _, [Lambda (_, Call (_, _, [Call (_, _, [Call (_, _, l)]); next]))] )  ->
            yield! findQuotes' next (seq { yield! res; l })
        | Call (_, _, [Lambda (_, Call (_, _, l))] ) ->
            match l with
            | [Call (None, op, list); q] when op.Name = nameof(==>) ->
                yield! findQuotes' q (seq { yield! res; list })
            | [Call (None, op, list)] when op.Name = nameof(==>) -> yield! (seq { yield! res; list })
            | q -> yield! (seq { yield! res; q })
        | q -> failwithf "Unexpected quote {%A}" q 
    }
    findQuotes' q [] |> Seq.toList
    
let rec toJson quotes =
    let rec getString quote =
        match quote with
        | NewArray(_, list) ->
            list |> List.map getString
            |> fun s -> System.String.Join(",", s)
            |> fun s -> "[" + s + "]"
        | Value(v, _) -> sprintf "%A" v
        | Application (Lambda (_, Call (_, _, [Quote call])), _) ->
            findQuotes call
            |> toJson
        | q -> failwithf "Unexpected {%A}" q
    let rec toJson' quotes res =
        match quotes with
        | q::[] ->
            res + (getString q)
        | h :: t :: [] ->
            (getString h) + ":" + (getString t)
    quotes
    |> List.map (fun x -> toJson' x "")
    |> fun s ->
        let result = System.String.Join(",", s)
        if s.Length = 1 then result else "{" + result + "}" 
        

type JsonBuilder() =
    member _.Yield (a: KeyValuePair<string, string>) = a
    member _.Yield (a: KeyValuePair<string, int>) = a
    member _.Yield (a: KeyValuePair<string, float>) = a
    member _.Yield (a: KeyValuePair<string, 'a []>) = a
    member _.Yield (a: 'a []) = a
    member _.Combine(KeyValue(a1, a2), KeyValue(b1, b2)) =
        seq {yield KeyValuePair(a1, sprintf "%A" a2); yield KeyValuePair(b1, sprintf "%A" b2)}
    member _.Combine(KeyValue(a1, a2), b) =
        seq {yield KeyValuePair(a1, sprintf "%A" a2); yield! seq { for KeyValue(b1, b2) in b -> KeyValuePair(b1, sprintf "%A" b2) }}
    member _.Delay f = f()
    member _.Quote q = q
    member _.Run(q: Quotations.Expr<'T>) =
        findQuotes q
        |> toJson
    member _.Zero() = ()

let json = JsonBuilder()

// {"a":"b","c":1,"d":[1.2,3.4,5.6]}
let a = json {
    "a" ==> "b"
    "c" ==> 1
    "d" ==> [|1.2; 3.4; 5.6|]
}

// [{"abc":"def","123":456},{"abc":"def","123":456}]
let b = json {
    [| json
        { "abc" ==> "def"
          "123" ==> 456 }
       json
        { "abc" ==> "def"
          "123" ==> 456 } |]
}