JsonReadable = concept x
    var j: JsonNode
    parseFromJson(j, type(x)) is type(x)

JsonWritable = concept x
    renderToJson(x) is JsonNode

proc ok(j: JsonNode): Handler {.
raises: [], tags: []
.}

proc ok[A: JsonWritable](a: A): Handler

proc jsonBody(p: proc (j: JsonNode): Handler): Handler {.
raises: [], tags: []
.}

proc jsonBody[A: JsonReadable](p: proc (a: A): Handler): Handler