class json a = {ToJson : a -> string,
                FromJson : string -> a * string}

fun mkJson [a] (x : {ToJson : a -> string,
                     FromJson : string -> a * string}) = x

fun skipSpaces s =
    let
        val len = String.length s

        fun skip i =
            if i >= len then
                ""
            else
                let
                    val ch = String.sub s i
                in
                    if Char.isSpace ch then
                        skip (i+1)
                    else
                        String.substring s {Start = i, Len = len-i}
                end
    in
        skip 0
    end

fun toJson [a] (j : json a) : a -> string = j.ToJson

fun fromJson [a] (j : json a) (s : string) : a =
    let
        val (v, s') = j.FromJson (skipSpaces s)
    in
        if String.all Char.isSpace s' then
            v
        else
            error <xml>Extra content at end of JSON record: {[s']}</xml>
    end

fun escape s =
    let
        val len = String.length s

        fun esc i =
            if i >= len then
                "\""
            else
                let
                    val ch = String.sub s i
                in
                    (if ch = #"\"" || ch = #"\\" then
                         "\\" ^ String.str ch
                     else
                         String.str ch) ^ esc (i+1)
                end
    in
        "\"" ^ esc 0
    end

fun unescape s =
    let
        val len = String.length s

        fun findEnd i =
            if i >= len then
                error <xml>JSON unescape: string ends before quote: {[s]}</xml>
            else
                let
                    val ch = String.sub s i
                in
                    case ch of
                        #"\"" => i
                      | #"\\" =>
                        if i+1 >= len then
                            error <xml>JSON unescape: Bad escape sequence: {[s]}</xml>
                        else
                            findEnd (i+2)
                      | _ => findEnd (i+1)
                end

        val last = findEnd 1

        fun unesc i =
            if i >= last then
                ""
            else
                let
                    val ch = String.sub s i
                in
                    case ch of
                        #"\\" =>
                        if i+1 >= len then
                            error <xml>JSON unescape: Bad escape sequence: {[s]}</xml>
                        else
                            String.str (String.sub s (i+1)) ^ unesc (i+2)
                      | _ => String.str ch ^ unesc (i+1)
                end
    in
        if len = 0 || String.sub s 0 <> #"\"" then
            error <xml>JSON unescape: String doesn't start with double quote: {[s]}</xml>
        else
            (unesc 1, String.substring s {Start = last+1, Len = len-last-1})
    end

val json_string = {ToJson = escape,
                   FromJson = unescape}

fun numIn [a] (_ : read a) s : a * string =
    let
        val len = String.length s

        fun findEnd i =
            if i >= len then
                i
            else
                let
                    val ch = String.sub s i
                in
                    if Char.isDigit ch || ch = #"-" || ch = #"." || ch = #"E" || ch = #"e" then
                        findEnd (i+1)
                    else
                        i
                end

        val last = findEnd 0
    in
        (readError (String.substring s {Start = 0, Len = last}), String.substring s {Start = last, Len = len-last})
    end

fun json_num [a] (_ : show a) (_ : read a) : json a = {ToJson = show,
                                                       FromJson = numIn}

val json_int = json_num
val json_float = json_num

val json_bool = {ToJson = fn b => if b then "true" else "false",
                 FromJson = fn s => if String.isPrefix {Full = s, Prefix = "true"} then
                                        (True, String.substring s {Start = 4, Len = String.length s - 4})
                                    else if String.isPrefix {Full = s, Prefix = "false"} then
                                        (False, String.substring s {Start = 5, Len = String.length s - 5})
                                    else
                                        error <xml>JSON: bad boolean string: {[s]}</xml>}

fun json_list [a] (j : json a) : json (list a) =
    let
        fun toJ' (ls : list a) : string =
            case ls of
                [] => ""
              | x :: ls => "," ^ toJson j x ^ toJ' ls

        fun toJ (x : list a) : string =
            case x of
                [] => "[]"
              | x :: [] => "[" ^ toJson j x ^ "]"
              | x :: ls => "[" ^ toJson j x ^ toJ' ls ^ "]"

        fun fromJ (s : string) : list a * string =
            let
                fun fromJ' (s : string) : list a * string =
                    if String.length s = 0 then
                        error <xml>JSON list doesn't end with ']'</xml>
                    else
                        let
                            val ch = String.sub s 0
                        in
                            case ch of
                                #"]" => ([], String.substring s {Start = 1, Len = String.length s - 1})
                              | _ =>
                                let
                                    val (x, s') = j.FromJson s
                                    val s' = skipSpaces s'
                                    val s' = if String.length s' = 0 then
                                                 error <xml>JSON list doesn't end with ']'</xml>
                                             else if String.sub s' 0 = #"," then
                                                 skipSpaces (String.substring s' {Start = 1, Len = String.length s' - 1})
                                             else
                                                 s'

                                    val (ls, s'') = fromJ' s'
                                in
                                    (x :: ls, s'')
                                end
                        end
            in
                if String.length s = 0 || String.sub s 0 <> #"[" then
                    error <xml>JSON list doesn't start with '[': {[s]}</xml>
                else
                    fromJ' (skipSpaces (String.substring s {Start = 1, Len = String.length s - 1}))
            end
    in
        {ToJson = toJ,
         FromJson = fromJ}
    end