diff --git a/src/libexpr/json-to-value.cc b/src/libexpr/json-to-value.cc
index 47cab2bb5..19d9a9b90 100644
--- a/src/libexpr/json-to-value.cc
+++ b/src/libexpr/json-to-value.cc
@@ -1,232 +1,165 @@
 #include "json-to-value.hh"
 
-#include <cstring>
+#include <variant>
+#include <nlohmann/json.hpp>
+#include <nlohmann/detail/exceptions.hpp>
+
+using json = nlohmann::json;
 
 namespace nix {
 
+// for more information, refer to
+// https://github.com/nlohmann/json/blob/master/include/nlohmann/detail/input/json_sax.hpp
+class JSONSax : nlohmann::json_sax<json> {
+    class JSONState {
+    protected:
+        JSONState* parent;
+        Value * v;
+    public:
+        virtual JSONState* resolve(EvalState &)
+        {
+            throw std::logic_error("tried to close toplevel json parser state");
+        };
+        explicit JSONState(JSONState* p) : parent(p), v(nullptr) {};
+        explicit JSONState(Value* v) : v(v) {};
+        JSONState(JSONState& p) = delete;
+        Value& value(EvalState & state)
+        {
+            if (v == nullptr)
+                v = state.allocValue();
+            return *v;
+        };
+        virtual ~JSONState() {};
+        virtual void add() {};
+    };
 
-static void skipWhitespace(const char * & s)
-{
-    while (*s == ' ' || *s == '\t' || *s == '\n' || *s == '\r') s++;
-}
-
-
-/*
-  Parse an unicode escape sequence (4 hex characters following \u) in JSON string
-*/
-static string parseUnicodeEscapeSequence(const char * & s)
-{
-    int codepoint = 0;
-
-    const auto factors = { 12u, 8u, 4u, 0u };
-    for (const auto factor : factors)
-    {
-        if (!*s) throw JSONParseError("got end-of-string in JSON string while parsing \\u sequence");
-
-        if (*s >= '0' and *s <= '9') {
-            codepoint += static_cast<int>((static_cast<unsigned int>(*s) - 0x30u) << factor);
-        } else if (*s >= 'A' and *s <= 'F') {
-            codepoint += static_cast<int>((static_cast<unsigned int>(*s) - 0x37u) << factor);
-        } else if (*s >= 'a' and *s <= 'f') {
-            codepoint += static_cast<int>((static_cast<unsigned int>(*s) - 0x57u) << factor);
-        } else {
-            throw JSONParseError(format("illegal character '%1%' in \\u escape sequence.") % *s);
+    class JSONObjectState : public JSONState {
+        using JSONState::JSONState;
+        ValueMap attrs = ValueMap();
+        virtual JSONState* resolve(EvalState & state) override
+        {
+            Value& v = parent->value(state);
+            state.mkAttrs(v, attrs.size());
+            for (auto & i : attrs)
+                v.attrs->push_back(Attr(i.first, i.second));
+            return parent;
         }
-        s++;
-    }
-
-    if ((codepoint > 0xd7ff && codepoint < 0xe000) || codepoint > 0x10ffff) {
-        throw JSONParseError("Unicode escape sequence is not a Unicode scalar value");
-    }
-
-    // taken from cpptoml.h
-    std::string result;
-    // See Table 3-6 of the Unicode standard
-    if (codepoint <= 0x7f)
-    {
-        // 1-byte codepoints: 00000000 0xxxxxxx
-        // repr: 0xxxxxxx
-        result += static_cast<char>(codepoint & 0x7f);
-    }
-    else if (codepoint <= 0x7ff)
-    {
-        // 2-byte codepoints: 00000yyy yyxxxxxx
-        // repr: 110yyyyy 10xxxxxx
-        //
-        // 0x1f = 00011111
-        // 0xc0 = 11000000
-        //
-        result += static_cast<char>(0xc0 | ((codepoint >> 6) & 0x1f));
-        //
-        // 0x80 = 10000000
-        // 0x3f = 00111111
-        //
-        result += static_cast<char>(0x80 | (codepoint & 0x3f));
-    }
-    else if (codepoint <= 0xffff)
-    {
-        // 3-byte codepoints: zzzzyyyy yyxxxxxx
-        // repr: 1110zzzz 10yyyyyy 10xxxxxx
-        //
-        // 0xe0 = 11100000
-        // 0x0f = 00001111
-        //
-        result += static_cast<char>(0xe0 | ((codepoint >> 12) & 0x0f));
-        result += static_cast<char>(0x80 | ((codepoint >> 6) & 0x1f));
-        result += static_cast<char>(0x80 | (codepoint & 0x3f));
-    }
-    else
-    {
-        // 4-byte codepoints: 000uuuuu zzzzyyyy yyxxxxxx
-        // repr: 11110uuu 10uuzzzz 10yyyyyy 10xxxxxx
-        //
-        // 0xf0 = 11110000
-        // 0x07 = 00000111
-        //
-        result += static_cast<char>(0xf0 | ((codepoint >> 18) & 0x07));
-        result += static_cast<char>(0x80 | ((codepoint >> 12) & 0x3f));
-        result += static_cast<char>(0x80 | ((codepoint >> 6) & 0x3f));
-        result += static_cast<char>(0x80 | (codepoint & 0x3f));
-    }
-    return result;
-}
-
-
-static string parseJSONString(const char * & s)
-{
-    string res;
-    if (*s++ != '"') throw JSONParseError("expected JSON string");
-    while (*s != '"') {
-        if (!*s) throw JSONParseError("got end-of-string in JSON string");
-        if (*s == '\\') {
-            s++;
-            if (*s == '"') res += '"';
-            else if (*s == '\\') res += '\\';
-            else if (*s == '/') res += '/';
-            else if (*s == 'b') res += '\b';
-            else if (*s == 'f') res += '\f';
-            else if (*s == 'n') res += '\n';
-            else if (*s == 'r') res += '\r';
-            else if (*s == 't') res += '\t';
-            else if (*s == 'u') {
-                res += parseUnicodeEscapeSequence(++s);
-                // to neuter the outside s++
-                s--;
-            } else throw JSONParseError("invalid escaped character in JSON string");
-            s++;
-        } else
-            res += *s++;
-    }
-    s++;
-    return res;
-}
-
-
-static void parseJSON(EvalState & state, const char * & s, Value & v)
-{
-    skipWhitespace(s);
-
-    if (!*s) throw JSONParseError("expected JSON value");
-
-    if (*s == '[') {
-        s++;
-        ValueVector values;
-        values.reserve(128);
-        skipWhitespace(s);
-        while (1) {
-            if (values.empty() && *s == ']') break;
-            Value * v2 = state.allocValue();
-            parseJSON(state, s, *v2);
-            values.push_back(v2);
-            skipWhitespace(s);
-            if (*s == ']') break;
-            if (*s != ',') throw JSONParseError("expected ',' or ']' after JSON array element");
-            s++;
+        virtual void add() override { v = nullptr; };
+    public:
+        void key(string_t& name, EvalState & state)
+        {
+            attrs[state.symbols.create(name)] = &value(state);
         }
-        s++;
-        state.mkList(v, values.size());
-        for (size_t n = 0; n < values.size(); ++n)
-            v.listElems()[n] = values[n];
-    }
+    };
 
-    else if (*s == '{') {
-        s++;
-        ValueMap attrs;
-        while (1) {
-            skipWhitespace(s);
-            if (attrs.empty() && *s == '}') break;
-            string name = parseJSONString(s);
-            skipWhitespace(s);
-            if (*s != ':') throw JSONParseError("expected ':' in JSON object");
-            s++;
-            Value * v2 = state.allocValue();
-            parseJSON(state, s, *v2);
-            attrs[state.symbols.create(name)] = v2;
-            skipWhitespace(s);
-            if (*s == '}') break;
-            if (*s != ',') throw JSONParseError("expected ',' or '}' after JSON member");
-            s++;
+    class JSONListState : public JSONState {
+        ValueVector values = ValueVector();
+        virtual JSONState* resolve(EvalState & state) override
+        {
+            Value& v = parent->value(state);
+            state.mkList(v, values.size());
+            for (size_t n = 0; n < values.size(); ++n) {
+                v.listElems()[n] = values[n];
+            }
+            return parent;
         }
-        state.mkAttrs(v, attrs.size());
-        for (auto & i : attrs)
-            v.attrs->push_back(Attr(i.first, i.second));
-        v.attrs->sort();
-        s++;
-    }
-
-    else if (*s == '"') {
-        mkString(v, parseJSONString(s));
-    }
-
-    else if (isdigit(*s) || *s == '-' || *s == '.' ) {
-        // Buffer into a string first, then use built-in C++ conversions
-        std::string tmp_number;
-        ValueType number_type = tInt;
-
-        while (isdigit(*s) || *s == '-' || *s == '.' || *s == 'e' || *s == 'E') {
-            if (*s == '.' || *s == 'e' || *s == 'E')
-                number_type = tFloat;
-            tmp_number += *s++;
+        virtual void add() override {
+            values.push_back(v);
+            v = nullptr;
+        };
+    public:
+        JSONListState(JSONState* p, std::size_t reserve) : JSONState(p)
+        {
+            values.reserve(reserve);
         }
+    };
 
-        try {
-            if (number_type == tFloat)
-                mkFloat(v, stod(tmp_number));
-            else
-                mkInt(v, stol(tmp_number));
-        } catch (std::invalid_argument & e) {
-            throw JSONParseError("invalid JSON number");
-        } catch (std::out_of_range & e) {
-            throw JSONParseError("out-of-range JSON number");
-        }
+    EvalState & state;
+    JSONState* rs;
+
+    template<typename T, typename... Args> inline bool handle_value(T f, Args... args)
+    {
+        f(rs->value(state), args...);
+        rs->add();
+        return true;
     }
 
-    else if (strncmp(s, "true", 4) == 0) {
-        s += 4;
-        mkBool(v, true);
+public:
+    JSONSax(EvalState & state, Value & v) : state(state), rs(new JSONState(&v)) {};
+    ~JSONSax() { delete rs; };
+
+    bool null()
+    {
+        return handle_value(mkNull);
     }
 
-    else if (strncmp(s, "false", 5) == 0) {
-        s += 5;
-        mkBool(v, false);
+    bool boolean(bool val)
+    {
+        return handle_value(mkBool, val);
     }
 
-    else if (strncmp(s, "null", 4) == 0) {
-        s += 4;
-        mkNull(v);
+    bool number_integer(number_integer_t val)
+    {
+        return handle_value(mkInt, val);
     }
 
-    else throw JSONParseError("unrecognised JSON value");
-}
+    bool number_unsigned(number_unsigned_t val)
+    {
+        return handle_value(mkInt, val);
+    }
 
+    bool number_float(number_float_t val, const string_t& s)
+    {
+        return handle_value(mkFloat, val);
+    }
+
+    bool string(string_t& val)
+    {
+        return handle_value<void(Value&, const char*)>(mkString, val.c_str());
+    }
+
+    bool start_object(std::size_t len)
+    {
+        JSONState* old = rs;
+        rs = new JSONObjectState(old);
+        return true;
+    }
+
+    bool key(string_t& name)
+    {
+        dynamic_cast<JSONObjectState*>(rs)->key(name, state);
+        return true;
+    }
+
+    bool end_object() {
+        JSONState* old = rs;
+        rs = old->resolve(state);
+        delete old;
+        rs->add();
+        return true;
+    }
+
+    bool end_array() {
+        return end_object();
+    }
+
+    bool start_array(size_t len) {
+        JSONState* old = rs;
+        rs = new JSONListState(old, len != std::numeric_limits<size_t>::max() ? len : 128);
+        return true;
+    }
+
+    bool parse_error(std::size_t, const std::string&, const nlohmann::detail::exception& ex) {
+        throw JSONParseError(ex.what());
+    }
+};
 
 void parseJSON(EvalState & state, const string & s_, Value & v)
 {
-    const char * s = s_.c_str();
-    parseJSON(state, s, v);
-    skipWhitespace(s);
-    if (*s) throw JSONParseError(format("expected end-of-string while parsing JSON value: %1%") % s);
+    JSONSax parser(state, v);
+    bool res = json::sax_parse(s_, &parser);
+    if (!res)
+        throw JSONParseError("Invalid JSON Value");
 }
 
-
 }