ThothTest.fs

open System
open Thoth.Json.Net

type DecodeBuilder() =
    member _.Bind(decoder, f) : Decoder<_> =
        Decode.andThen f decoder
    member _.Return(value) =
        Decode.succeed value
    member _.ReturnFrom(decoder : Decoder<_>) =
        decoder

let decode = DecodeBuilder()

let decodeReverse =
    decode {
        let! str = Decode.string
        return str
            |> Seq.rev
            |> Seq.toArray
            |> String
    }

let decodeRot13 =

    let rot13 c =
        if 'a' <= c && c <= 'm' || 'A' <= c && c <= 'M' then
            char (int c + 13)
        elif 'n' <= c && c <= 'z' || 'N' <= c && c <= 'Z' then
            char (int c - 13)
        else c
        
    decode {
        let! str = Decode.string
        return str
            |> Seq.map rot13
            |> Seq.toArray
            |> String
    }

let customDecoders =
    Map [
        "Reverse", decodeReverse
        "Rot13", decodeRot13
    ]

let decodeByType =
    decode {
        let! typ = Decode.field "type" Decode.string
        return! Decode.field "value" customDecoders.[typ]
    }

Decode.fromString
    (Decode.array decodeByType)
    """
    [
        {
            "type" : "Reverse",
            "value" : "edcba"
        },
        {
            "type" : "Rot13",
            "value" : "qrpbqr guvf"
        }
    ]
    """
    |> printfn "%A"