diff --git a/runtime/json.cpp b/runtime/json.cpp
index 901b573..c72cc49 100644
--- a/runtime/json.cpp
+++ b/runtime/json.cpp
@@ -2,6 +2,7 @@
 #include <cstdlib>
 #include <format>
 #include <iostream>
+#include <memory>
 #include <string>
 #include <unordered_map>
 
@@ -20,12 +21,12 @@ auto id_tail_chars = "abcdefghijklmnopqrstuvwxyz"
                      "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                      "1234567890";
 
-auto Lexer::next() -> Tok
+auto Lexer::next() -> Res<Tok>
 {
-    if (done()) {
-        return TokTyp::Eof;
-    }
     auto pos = this->pos();
+    if (done()) {
+        return Tok(TokTyp::Eof, pos);
+    }
     if (test('"')) {
         step();
         auto value = std::string();
@@ -54,6 +55,16 @@ auto Lexer::next() -> Tok
             }
             step();
         }
+        if (!test('"')) {
+            return Err {
+                .pos = pos,
+                .msg
+                = std::format("malformed string, expected '\"', got '{}' token",
+                    this->cur()),
+            };
+        }
+        step();
+        return Tok(TokTyp::String, pos, intern_str(value));
     }
     auto step_n_ret = [&](auto tok) {
         step();
@@ -61,19 +72,19 @@ auto Lexer::next() -> Tok
     };
     switch (cur()) {
         case '0':
-            return step_n_ret(Tok(TokTyp::Float, 0.0));
+            return step_n_ret(Tok(TokTyp::Float, pos, 0.0));
         case '{':
-            return step_n_ret(TokTyp::LBrace);
+            return step_n_ret(Tok(TokTyp::LBrace, pos));
         case '}':
-            return step_n_ret(TokTyp::RBrace);
+            return step_n_ret(Tok(TokTyp::RBrace, pos));
         case '[':
-            return step_n_ret(TokTyp::LBracket);
+            return step_n_ret(Tok(TokTyp::LBracket, pos));
         case ']':
-            return step_n_ret(TokTyp::RBracket);
+            return step_n_ret(Tok(TokTyp::RBracket, pos));
         case ',':
-            return step_n_ret(TokTyp::Comma);
+            return step_n_ret(Tok(TokTyp::Comma, pos));
         case ':':
-            return step_n_ret(TokTyp::Colon);
+            return step_n_ret(Tok(TokTyp::Colon, pos));
     }
     if (test_in(id_start_chars)) {
         auto value = std::string();
@@ -82,12 +93,12 @@ auto Lexer::next() -> Tok
             step();
         }
         if (ident_tok_typs.find(value) == ident_tok_typs.end()) {
-            std::cerr << std::format("sliger::json::Lexer error: unknown "
-                                     "identifier \"{}\" at {}:{}",
-                value, pos.col, pos.line);
-            return TokTyp::Error;
+            return Err {
+                .pos = pos,
+                .msg = std::format("unknown identifier \"{}\"", value),
+            };
         }
-        return ident_tok_typs.at(value);
+        return Tok(ident_tok_typs.at(value), pos);
     }
     if (test_in("123456789")) {
         auto value_str = std::string();
@@ -96,11 +107,72 @@ auto Lexer::next() -> Tok
             step();
         }
         auto value = std::atof(value_str.c_str());
-        return Tok(TokTyp::Float, value);
+        return Tok(TokTyp::Float, pos, value);
     }
-    std::cerr << std::format(
-        "sliger::json::Lexer error: unknown character '{}' at {}:{}", cur(),
-        this->line, this->col);
+    auto ch = cur();
     step();
-    return TokTyp::Error;
+    return Err {
+        .pos = pos,
+        .msg = std::format("unknown character '{}'", ch),
+    };
+}
+
+auto Parser::parse_val() -> Res<std::unique_ptr<Value>>
+{
+    if (not this->cur.ok())
+        return this->cur.err();
+    auto cur = this->cur.val();
+    switch (cur.typ) {
+        case TokTyp::Eof:
+            return Err {
+                .pos = cur.pos,
+                .msg = "expected value, got eof",
+            };
+        case TokTyp::String: {
+            auto value = this->lexer.val(cur.val_id);
+            step();
+            return Res<std::unique_ptr<Value>>(std::make_unique<String>(value));
+        }
+        case TokTyp::Float: {
+            auto value = cur.float_val;
+            step();
+            return Res<std::unique_ptr<Value>>(std::make_unique<Number>(value));
+        }
+        case TokTyp::False: {
+            step();
+            return Res<std::unique_ptr<Value>>(std::make_unique<Bool>(false));
+        }
+        case TokTyp::True: {
+            step();
+            return Res<std::unique_ptr<Value>>(std::make_unique<Bool>(true));
+        }
+        case TokTyp::Null: {
+            step();
+            return Res<std::unique_ptr<Value>>(std::make_unique<Null>());
+        }
+        case TokTyp::LBrace: {
+            step();
+            ObjectFields fields;
+            while (curtyp() != TokTyp::RBrace) { }
+            if (curtyp() != TokTyp::RBrace) {
+                return Err {
+                    .pos = this->cur.val().pos,
+                    .msg
+                    = std::format("malformed object, expected '}', got '{}'",
+                        tok_typ_to_string(this->cur.val().typ)),
+                };
+            }
+        }
+        case TokTyp::RBrace:
+        case TokTyp::LBracket:
+        case TokTyp::RBracket:
+        case TokTyp::Comma:
+        case TokTyp::Colon:
+            return Err {
+                .pos = cur.pos,
+                .msg = std::format("expected value, got '{}' token",
+                    tok_typ_to_string(cur.typ)),
+            };
+            break;
+    }
 }
diff --git a/runtime/json.hpp b/runtime/json.hpp
index af472cf..0d39e52 100644
--- a/runtime/json.hpp
+++ b/runtime/json.hpp
@@ -2,14 +2,80 @@
 
 #include <concepts>
 #include <cstddef>
-#include <cstdint>
 #include <memory>
 #include <optional>
 #include <string>
+#include <unordered_map>
+#include <unordered_set>
 #include <vector>
 
 namespace sliger::json {
 
+struct Pos {
+    int line;
+    int col;
+};
+
+struct Err {
+    Pos pos;
+    std::string msg;
+};
+
+template <typename T> class Res {
+public:
+    Res(T value)
+        : maybe_value(std::move(value))
+        , maybe_error()
+    {
+    }
+    Res(Err error)
+        : maybe_value()
+        , maybe_error(std::move(error))
+    {
+    }
+
+    inline auto ok() const -> bool { return this->maybe_value.has_value(); }
+
+    inline auto val() const& -> const T& { return this->maybe_value.value(); }
+    inline auto val() & -> T& { return this->maybe_value.value(); }
+    inline auto val() && -> T&& { return std::move(this->maybe_value.value()); }
+
+    inline auto err() const& -> const Err& { return this->maybe_error.value(); }
+    inline auto err() & -> Err& { return this->maybe_error.value(); }
+    inline auto err() && -> Err&&
+    {
+        return std::move(this->maybe_error.value());
+    }
+
+private:
+    std::optional<T> maybe_value;
+    std::optional<Err> maybe_error;
+};
+
+template <> class Res<void> {
+public:
+    Res()
+        : maybe_error()
+    {
+    }
+    Res(Err error)
+        : maybe_error(std::move(error))
+    {
+    }
+
+    inline auto ok() const -> bool { return not this->maybe_error.has_value(); }
+
+    inline auto err() const& -> const Err& { return this->maybe_error.value(); }
+    inline auto err() & -> Err& { return this->maybe_error.value(); }
+    inline auto err() && -> Err&&
+    {
+        return std::move(this->maybe_error.value());
+    }
+
+private:
+    std::optional<Err> maybe_error;
+};
+
 enum class Type {
     Null,
     String,
@@ -36,40 +102,58 @@ struct Null final : public Value {
 };
 
 struct String final : public Value {
+    String(std::string value)
+        : value(std::move(value))
+    {
+    }
     auto type() const -> Type override { return Type::String; }
 
     std::string value;
 };
 
 struct Number final : public Value {
+    Number(double value)
+        : value(value)
+    {
+    }
     auto type() const -> Type override { return Type::Number; }
 
     double value;
 };
 
 struct Bool final : public Value {
+    Bool(bool value)
+        : value(value)
+    {
+    }
     auto type() const -> Type override { return Type::Bool; }
 
     bool value;
 };
 
+using ArrayValues = std::vector<std::unique_ptr<Value>>;
 struct Array final : public Value {
+    Array(ArrayValues values)
+        : values(std::move(values))
+    {
+    }
     auto type() const -> Type override { return Type::Array; }
 
-    std::vector<std::unique_ptr<Value>> values;
+    ArrayValues values;
 };
 
+using ObjectFields = std::unordered_map<std::string, std::unique_ptr<Value>>;
 struct Object final : public Value {
+    Object(ObjectFields fields)
+        : fields(std::move(fields))
+    {
+    }
     auto type() const -> Type override { return Type::Object; }
-};
 
-struct Pos {
-    int line;
-    int col;
+    ObjectFields fields;
 };
 
 enum class TokTyp {
-    Error,
     Eof,
     String,
     Float,
@@ -84,29 +168,64 @@ enum class TokTyp {
     Colon,
 };
 
+inline auto tok_typ_to_string(TokTyp typ) -> std::string
+{
+    switch (typ) {
+        case TokTyp::Eof:
+            return "Eof";
+        case TokTyp::String:
+            return "String";
+        case TokTyp::Float:
+            return "Float";
+        case TokTyp::False:
+            return "False";
+        case TokTyp::True:
+            return "True";
+        case TokTyp::Null:
+            return "Null";
+        case TokTyp::LBrace:
+            return "LBrace";
+        case TokTyp::RBrace:
+            return "RBrace";
+        case TokTyp::LBracket:
+            return "LBracket";
+        case TokTyp::RBracket:
+            return "RBracket";
+        case TokTyp::Comma:
+            return "Comma";
+        case TokTyp::Colon:
+            return "Colon";
+    }
+}
+
 struct Tok {
-    Tok(TokTyp typ)
+    Tok(TokTyp typ, Pos pos)
         : typ(typ)
+        , pos(pos)
         , val_id(0)
     {
     }
-    Tok(TokTyp typ, size_t val_id)
+    Tok(TokTyp typ, Pos pos, size_t val_id)
         : typ(typ)
+        , pos(pos)
         , val_id(val_id)
     {
     }
-    Tok(TokTyp typ, double float_val)
+    Tok(TokTyp typ, Pos pos, double float_val)
         : typ(typ)
+        , pos(pos)
         , float_val(float_val)
     {
     }
-    Tok(TokTyp typ, bool bool_val)
+    Tok(TokTyp typ, Pos pos, bool bool_val)
         : typ(typ)
+        , pos(pos)
         , bool_val(bool_val)
     {
     }
 
     TokTyp typ;
+    Pos pos;
     union {
         size_t val_id;
         double float_val;
@@ -121,13 +240,28 @@ public:
     {
     }
 
-    auto next() -> Tok;
+    auto next() -> Res<Tok>;
 
     inline auto pos() const -> Pos { return { this->line, this->col }; }
 
+    inline auto val(size_t val_id) const -> const std::string&
+    {
+        return this->strs_vals.at(val_id);
+    }
+
 private:
     inline void step() { this->i += 1; }
 
+    inline auto intern_str(std::string val) -> size_t
+    {
+        if (this->strs_val_id_map.contains(val))
+            return strs_val_id_map.at(val);
+        auto id = this->strs_vals.size();
+        this->strs_vals.push_back(val);
+        this->strs_val_id_map.insert_or_assign(val, id);
+        return id;
+    }
+
     inline auto test_in(std::string_view chs) const -> bool
     {
         for (auto ch : chs)
@@ -143,6 +277,8 @@ private:
     size_t i = 0;
     int line = 1;
     int col = 1;
+    std::unordered_map<std::string, size_t> strs_val_id_map;
+    std::vector<std::string> strs_vals;
 };
 
 class Parser {
@@ -153,9 +289,20 @@ public:
     {
     }
 
+    auto parse_val() -> Res<std::unique_ptr<Value>>;
+
 private:
+    inline auto step() -> Res<void>
+    {
+        auto tok = this->lexer.next();
+        if (not tok.ok())
+            return tok.err();
+        return {};
+    }
+    inline auto curtyp() const -> TokTyp { return this->cur.val().typ; }
+
     Lexer lexer;
-    Tok cur;
+    Res<Tok> cur;
 };
 
 struct Jsonable {