diff --git a/src/cargo/cargo.rs b/src/cargo/cargo.rs
index ea5953fdf89..a4816fedf97 100644
--- a/src/cargo/cargo.rs
+++ b/src/cargo/cargo.rs
@@ -9,6 +9,7 @@ import rustc::util::filesearch::{get_cargo_root, get_cargo_root_nearest,
                                  get_cargo_sysroot, libdir};
 import rustc::driver::diagnostic;
 
+import result::{ok, err};
 import std::fs;
 import std::io;
 import io::writer_util;
@@ -225,15 +226,15 @@ fn parse_source(name: str, j: json::json) -> source {
 fn try_parse_sources(filename: str, sources: map::hashmap<str, source>) {
     if !fs::path_exists(filename)  { ret; }
     let c = io::read_whole_file_str(filename);
-    let j = json::from_str(result::get(c));
-    alt j {
-        some(json::dict(_j)) {
-            _j.items { |k, v|
+    alt json::from_str(result::get(c)) {
+        ok(json::dict(j)) {
+            j.items { |k, v|
                 sources.insert(k, parse_source(k, v));
                 #debug("source: %s", k);
             }
         }
-        _ { fail "malformed sources.json"; }
+        ok(_) { fail "malformed sources.json"; }
+        err(e) { fail #fmt("%s:%u:%u: %s", filename, e.line, e.col, e.msg); }
     }
 }
 
@@ -278,7 +279,7 @@ fn load_one_source_package(&src: source, p: map::hashmap<str, json::json>) {
     let tags = [];
     alt p.find("tags") {
         some(json::list(js)) {
-            for j in *js {
+            for j in js {
                 alt j {
                     json::string(_j) { vec::grow(tags, 1u, _j); }
                     _ { }
@@ -316,10 +317,9 @@ fn load_source_packages(&c: cargo, &src: source) {
     let pkgfile = fs::connect(dir, "packages.json");
     if !fs::path_exists(pkgfile) { ret; }
     let pkgstr = io::read_whole_file_str(pkgfile);
-    let j = json::from_str(result::get(pkgstr));
-    alt j {
-        some(json::list(js)) {
-            for _j: json::json in *js {
+    alt json::from_str(result::get(pkgstr)) {
+        ok(json::list(js)) {
+            for _j: json::json in js {
                 alt _j {
                     json::dict(_p) {
                         load_one_source_package(src, _p);
@@ -331,8 +331,12 @@ fn load_source_packages(&c: cargo, &src: source) {
                 }
             }
         }
-        _ {
-            warn("Malformed source json: " + src.name);
+        ok(_) {
+            warn("Malformed source json: " + src.name +
+                 "(packages is not a list)");
+        }
+        err(e) {
+            warn(#fmt("%s:%u:%u: %s", src.name, e.line, e.col, e.msg));
         }
     };
 }
diff --git a/src/libstd/json.rs b/src/libstd/json.rs
index 8de15a60c87..d2bbc7dc5e7 100644
--- a/src/libstd/json.rs
+++ b/src/libstd/json.rs
@@ -1,18 +1,24 @@
 // Rust JSON serialization library
 // Copyright (c) 2011 Google Inc.
 
-import float;
+import result::{ok, err};
+import io;
+import io::{reader_util, writer_util};
 import map;
 
 export json;
+export to_writer;
 export to_str;
+export from_reader;
 export from_str;
+export eq;
 
 export num;
 export string;
 export boolean;
 export list;
 export dict;
+export null;
 
 /*
 Tag: json
@@ -27,233 +33,453 @@ enum json {
     /* Variant: boolean */
     boolean(bool),
     /* Variant: list */
-    list(@[json]),
+    list([json]),
     /* Variant: dict */
     dict(map::map<str,json>),
     /* Variant: null */
     null,
 }
 
+type error = {
+    line: uint,
+    col: uint,
+    msg: str,
+};
+
+/*
+Function: to_writer
+
+Serializes a json value into a io::writer.
+*/
+fn to_writer(wr: io::writer, j: json) {
+    alt j {
+      num(n) { wr.write_str(float::to_str(n, 6u)); }
+      string(s) {
+        wr.write_char('"');
+        let escaped = "";
+        str::chars_iter(s) { |c|
+            alt c {
+              '"' { escaped += "\\\""; }
+              '\\' { escaped += "\\\\"; }
+              '\x08' { escaped += "\\b"; }
+              '\x0c' { escaped += "\\f"; }
+              '\n' { escaped += "\\n"; }
+              '\r' { escaped += "\\r"; }
+              '\t' { escaped += "\\t"; }
+              _ { escaped += str::from_char(c); }
+            }
+        };
+        wr.write_str(escaped);
+        wr.write_char('"');
+      }
+      boolean(b) {
+        wr.write_str(if b { "true" } else { "false" });
+      }
+      list(v) {
+        wr.write_char('[');
+        let first = true;
+        vec::iter(v) { |item|
+            if !first {
+                wr.write_str(", ");
+            }
+            first = false;
+            to_writer(wr, item);
+        };
+        wr.write_char(']');
+      }
+      dict(d) {
+        if d.size() == 0u {
+            wr.write_str("{}");
+            ret;
+        }
+
+        wr.write_str("{ ");
+        let first = true;
+        d.items { |key, value|
+            if !first {
+                wr.write_str(", ");
+            }
+            first = false;
+            to_writer(wr, string(key));
+            wr.write_str(": ");
+            to_writer(wr, value);
+        };
+        wr.write_str(" }");
+      }
+      null {
+        wr.write_str("null");
+      }
+    }
+}
+
 /*
 Function: to_str
 
 Serializes a json value into a string.
 */
 fn to_str(j: json) -> str {
-    alt j {
-        num(f) { float::to_str(f, 6u) }
-        string(s) { #fmt["\"%s\"", s] } // XXX: escape
-        boolean(true) { "true" }
-        boolean(false) { "false" }
-        list(@js) {
-            str::concat(["[",
-                    str::connect(
-                        vec::map::<json,str>(js, { |e| to_str(e) }),
-                        ", "),
-                    "]"])
-        }
-        dict(m) {
-            let parts = [];
-            m.items({ |k, v|
-                        vec::grow(parts, 1u,
-                                  str::concat(["\"", k, "\": ", to_str(v)])
-                        )
-            });
-            str::concat(["{ ", str::connect(parts, ", "), " }"])
-        }
-        null { "null" }
-    }
+    io::with_str_writer { |wr| to_writer(wr, j) }
 }
 
-fn rest(s: str) -> str {
-    assert(str::len(s) >= 1u);
-    str::slice(s, 1u, str::len(s))
-}
+type parser = {
+    rdr: io::reader,
+    mutable ch: char,
+    mutable line: uint,
+    mutable col: uint,
+};
 
-fn from_str_str(s: str) -> (option<json>, str) {
-    let pos = 0u;
-    let len = str::len(s);
-    let escape = false;
-    let res = "";
+impl parser for parser {
+    fn eof() -> bool { self.ch == -1 as char }
 
-    alt str::char_at(s, 0u) {
-        '"' { pos = 1u; }
-        _ { ret (none, s); }
+    fn bump() {
+        self.ch = self.rdr.read_char();
+
+        if self.ch == '\n' {
+            self.line += 1u;
+            self.col = 1u;
+        } else {
+            self.col += 1u;
+        }
     }
 
-    while (pos < len) {
-        let chr = str::char_range_at(s, pos);
-        let c = chr.ch;
-        pos = chr.next;
-        if (escape) {
-            res = res + str::from_char(c);
-            escape = false;
-            cont;
-        }
-        if (c == '\\') {
-            escape = true;
-            cont;
-        } else if (c == '"') {
-            ret (some(string(res)),
-                 str::slice(s, pos, str::len(s)));
-        }
-        res = res + str::from_char(c);
+    fn next_char() -> char {
+        self.bump();
+        self.ch
     }
 
-    ret (none, s);
-}
-
-fn from_str_list(s: str) -> (option<json>, str) {
-    if str::char_at(s, 0u) != '[' { ret (none, s); }
-    let s0 = str::trim_left(rest(s));
-    let vals = [];
-    if str::is_empty(s0) { ret (none, s0); }
-    if str::char_at(s0, 0u) == ']' { ret (some(list(@[])), rest(s0)); }
-    while str::is_not_empty(s0) {
-        s0 = str::trim_left(s0);
-        let (next, s1) = from_str_helper(s0);
-        s0 = s1;
-        alt next {
-            some(j) { vec::grow(vals, 1u, j); }
-            none { ret (none, s0); }
-        }
-        s0 = str::trim_left(s0);
-        if str::is_empty(s0) { ret (none, s0); }
-        alt str::char_at(s0, 0u) {
-            ',' { }
-            ']' { ret (some(list(@vals)), rest(s0)); }
-            _ { ret (none, s0); }
-        }
-        s0 = rest(s0);
+    fn error<T>(msg: str) -> result::t<T, error> {
+        err({ line: self.line, col: self.col, msg: msg })
     }
-    ret (none, s0);
-}
 
-fn from_str_dict(s: str) -> (option<json>, str) {
-    if str::char_at(s, 0u) != '{' { ret (none, s); }
-    let s0 = str::trim_left(rest(s));
-    let vals = map::new_str_hash::<json>();
-    if str::is_empty(s0) { ret (none, s0); }
-    if str::char_at(s0, 0u) == '}' { ret (some(dict(vals)), rest(s0)); }
-    while str::is_not_empty(s0) {
-        s0 = str::trim_left(s0);
-        let (next, s1) = from_str_helper(s0);    // key
-        let key = "";
-        s0 = s1;
-        alt next {
-            some(string(k)) { key = k; }
-            _ { ret (none, s0); }
+    fn parse() -> result::t<json, error> {
+        alt self.parse_value() {
+          ok(value) {
+            // Make sure there is no trailing characters.
+            if self.eof() {
+                ok(value)
+            } else {
+                self.error("trailing characters")
+            }
+          }
+          e { e }
         }
-        s0 = str::trim_left(s0);
-        if str::is_empty(s0) { ret (none, s0); }
-        if str::char_at(s0, 0u) != ':' { ret (none, s0); }
-        s0 = str::trim_left(rest(s0));
-        let (next, s1) = from_str_helper(s0);    // value
-        s0 = s1;
-        alt next {
-            some(j) { vals.insert(key, j); }
-            _ { ret (none, s0); }
-        }
-        s0 = str::trim_left(s0);
-        if str::is_empty(s0) { ret (none, s0); }
-        alt str::char_at(s0, 0u) {
-            ',' { }
-            '}' { ret (some(dict(vals)), rest(s0)); }
-            _ { ret (none, s0); }
-        }
-        s0 = str::trim_left(rest(s0));
     }
-    (none, s)
-}
 
-fn from_str_float(s: str) -> (option<json>, str) {
-    let pos = 0u;
-    let len = str::len(s);
-    let res = 0f;
-    let neg = 1f;
+    fn parse_value() -> result::t<json, error> {
+        self.parse_whitespace();
 
-    alt str::char_at(s, 0u) {
-        '-' {
+        if self.eof() { ret self.error("EOF while parsing value"); }
+
+        alt self.ch {
+          'n' { self.parse_ident("ull", null) }
+          't' { self.parse_ident("rue", boolean(true)) }
+          'f' { self.parse_ident("alse", boolean(false)) }
+          '0' to '9' | '-' { self.parse_number() }
+          '"' {
+              alt self.parse_str() {
+                ok(s) { ok(string(s)) }
+                err(e) { err(e) }
+              }
+          }
+          '[' { self.parse_list() }
+          '{' { self.parse_object() }
+          _ { self.error("invalid syntax") }
+        }
+    }
+
+    fn parse_whitespace() {
+        while char::is_whitespace(self.ch) { self.bump(); }
+    }
+
+    fn parse_ident(ident: str, value: json) -> result::t<json, error> {
+        if str::all(ident, { |c| c == self.next_char() }) {
+            self.bump();
+            ok(value)
+        } else {
+            self.error("invalid syntax")
+        }
+    }
+
+    fn parse_number() -> result::t<json, error> {
+        let neg = 1f;
+
+        if self.ch == '-' {
+            self.bump();
             neg = -1f;
-            pos = 1u;
         }
-        '+' {
-            pos = 1u;
-        }
-        '0' to '9' | '.' { }
-        _ { ret (none, s); }
-    }
 
-    while (pos < len) {
-        let opos = pos;
-        let chr = str::char_range_at(s, pos);
-        let c = chr.ch;
-        pos = chr.next;
-        alt c {
-            '0' to '9' {
-                res = res * 10f;
-                res += ((c as int) - ('0' as int)) as float;
+        let res =  alt self.parse_integer() {
+          ok(res) { res }
+          err(e) { ret err(e); }
+        };
+
+        if self.ch == '.' {
+            alt self.parse_decimal(res) {
+              ok(r) { res = r; }
+              err(e) { ret err(e); }
             }
-            '.' { break; }
-            _ { ret (some(num(neg * res)),
-                     str::slice(s, opos, str::len(s))); }
         }
+
+        if self.ch == 'e' || self.ch == 'E' {
+            alt self.parse_exponent(res) {
+              ok(r) { res = r; }
+              err(e) { ret err(e); }
+            }
+        }
+
+        ok(num(neg * res))
     }
 
-    if pos == len {
-        ret (some(num(neg * res)),
-             str::slice(s, pos, str::len(s)));
+    fn parse_integer() -> result::t<float, error> {
+        let res = 0f;
+
+        alt self.ch {
+          '0' {
+            self.bump();
+
+            // There can be only one leading '0'.
+            alt self.ch {
+              '0' to '9' { ret self.error("invalid number"); }
+              _ {}
+            }
+          }
+          '1' to '9' {
+            while !self.eof() {
+                alt self.ch {
+                  '0' to '9' {
+                    res *= 10f;
+                    res += ((self.ch as int) - ('0' as int)) as float;
+
+                    self.bump();
+                  }
+                  _ { break; }
+                }
+            }
+          }
+          _ { ret self.error("invalid number"); }
+        }
+
+        ok(res)
     }
 
-    let dec = 1f;
-    while (pos < len) {
-        let opos = pos;
-        let chr = str::char_range_at(s, pos);
-        let c = chr.ch;
-        pos = chr.next;
-        alt c {
-            '0' to '9' {
+    fn parse_decimal(res: float) -> result::t<float, error> {
+        self.bump();
+
+        // Make sure a digit follows the decimal place.
+        alt self.ch {
+          '0' to '9' {}
+          _ { ret self.error("invalid number"); }
+        }
+
+        let res = res;
+        let dec = 1f;
+        while !self.eof() {
+            alt self.ch {
+              '0' to '9' {
                 dec /= 10f;
-                res += (((c as int) - ('0' as int)) as float) * dec;
+                res += (((self.ch as int) - ('0' as int)) as float) * dec;
+
+                self.bump();
+              }
+              _ { break; }
             }
-            _ { ret (some(num(neg * res)),
-                     str::slice(s, opos, str::len(s))); }
         }
-    }
-    ret (some(num(neg * res)), str::slice(s, pos, str::len(s)));
-}
 
-fn from_str_bool(s: str) -> (option<json>, str) {
-    if (str::starts_with(s, "true")) {
-        (some(boolean(true)), str::slice(s, 4u, str::len(s)))
-    } else if (str::starts_with(s, "false")) {
-        (some(boolean(false)), str::slice(s, 5u, str::len(s)))
-    } else {
-        (none, s)
+        ok(res)
+    }
+
+    fn parse_exponent(res: float) -> result::t<float, error> {
+        self.bump();
+
+        let res = res;
+        let exp = 0u;
+        let neg_exp = false;
+
+        alt self.ch {
+          '+' { self.bump(); }
+          '-' { self.bump(); neg_exp = true; }
+          _ {}
+        }
+
+        // Make sure a digit follows the exponent place.
+        alt self.ch {
+          '0' to '9' {}
+          _ { ret self.error("invalid number"); }
+        }
+
+        while !self.eof() {
+            alt self.ch {
+              '0' to '9' {
+                exp *= 10u;
+                exp += (self.ch as uint) - ('0' as uint);
+
+                self.bump();
+              }
+              _ { break; }
+            }
+        }
+
+        let exp = float::pow_with_uint(10u, exp);
+        if neg_exp {
+            res /= exp;
+        } else {
+            res *= exp;
+        }
+
+        ok(res)
+    }
+
+    fn parse_str() -> result::t<str, error> {
+        let escape = false;
+        let res = "";
+
+        while !self.eof() {
+            self.bump();
+
+            if (escape) {
+                alt self.ch {
+                  '"' { str::push_char(res, '"'); }
+                  '\\' { str::push_char(res, '\\'); }
+                  '/' { str::push_char(res, '/'); }
+                  'b' { str::push_char(res, '\x08'); }
+                  'f' { str::push_char(res, '\x0c'); }
+                  'n' { str::push_char(res, '\n'); }
+                  'r' { str::push_char(res, '\r'); }
+                  't' { str::push_char(res, '\t'); }
+                  'u' {
+                      // Parse \u1234.
+                      let i = 0u;
+                      let n = 0u;
+                      while i < 4u {
+                          alt self.next_char() {
+                            '0' to '9' {
+                              n = n * 10u +
+                                  (self.ch as uint) - ('0' as uint);
+                            }
+                            _ { ret self.error("invalid \\u escape"); }
+                          }
+                      }
+
+                      // Error out if we didn't parse 4 digits.
+                      if i != 4u {
+                          ret self.error("invalid \\u escape");
+                      }
+
+                      str::push_char(res, n as char);
+                  }
+                  _ { ret self.error("invalid escape"); }
+                }
+                escape = false;
+            } else if self.ch == '\\' {
+                escape = true;
+            } else {
+                if self.ch == '"' {
+                    self.bump();
+                    ret ok(res);
+                }
+                str::push_char(res, self.ch);
+            }
+        }
+
+        self.error("EOF while parsing string")
+    }
+
+    fn parse_list() -> result::t<json, error> {
+        self.bump();
+        self.parse_whitespace();
+
+        let values = [];
+
+        if self.ch == ']' {
+            self.bump();
+            ret ok(list(values));
+        }
+
+        while true {
+            alt self.parse_value() {
+              ok(v) { vec::push(values, v); }
+              e { ret e; }
+            }
+
+            self.parse_whitespace();
+            if self.eof() { break; }
+
+            alt self.ch {
+              ',' { self.bump(); }
+              ']' { self.bump(); ret ok(list(values)); }
+              _ { ret self.error("expecting ',' or ']'"); }
+            }
+        }
+
+        ret self.error("EOF while parsing list");
+    }
+
+    fn parse_object() -> result::t<json, error> {
+        self.bump();
+        self.parse_whitespace();
+
+        let values = map::new_str_hash();
+
+        if self.ch == '}' {
+          self.bump();
+          ret ok(dict(values));
+        }
+
+        while !self.eof() {
+            self.parse_whitespace();
+
+            if self.ch != '"' {
+                ret self.error("key must be a string");
+            }
+
+            let key = alt self.parse_str() {
+              ok(key) { key }
+              err(e) { ret err(e); }
+            };
+
+            self.parse_whitespace();
+
+            if self.ch != ':' {
+                if self.eof() { break; }
+                ret self.error("expecting ':'");
+            }
+            self.bump();
+
+            alt self.parse_value() {
+              ok(value) { values.insert(key, value); }
+              e { ret e; }
+            }
+            self.parse_whitespace();
+
+            alt self.ch {
+              ',' { self.bump(); }
+              '}' { self.bump(); ret ok(dict(values)); }
+              _ {
+                  if self.eof() { break; }
+                  ret self.error("expecting ',' or '}'");
+              }
+            }
+        }
+
+        ret self.error("EOF while parsing object");
     }
 }
 
-fn from_str_null(s: str) -> (option<json>, str) {
-    if (str::starts_with(s, "null")) {
-        (some(null), str::slice(s, 4u, str::len(s)))
-    } else {
-        (none, s)
-    }
-}
+/*
+Function: from_reader
 
-fn from_str_helper(s: str) -> (option<json>, str) {
-    let s = str::trim_left(s);
-    if str::is_empty(s) { ret (none, s); }
-    let start = str::char_at(s, 0u);
-    alt start {
-        '"' { from_str_str(s) }
-        '[' { from_str_list(s) }
-        '{' { from_str_dict(s) }
-        '0' to '9' | '-' | '+' | '.' { from_str_float(s) }
-        't' | 'f' { from_str_bool(s) }
-        'n' { from_str_null(s) }
-        _ { ret (none, s); }
-    }
+Deserializes a json value from an io::reader.
+*/
+
+fn from_reader(rdr: io::reader) -> result::t<json, error> {
+    let parser = {
+        rdr: rdr,
+        mutable ch: rdr.read_char(),
+        mutable line: 1u,
+        mutable col: 1u,
+    };
+
+    parser.parse()
 }
 
 /*
@@ -261,62 +487,275 @@ Function: from_str
 
 Deserializes a json value from a string.
 */
-fn from_str(s: str) -> option<json> {
-    let (j, _) = from_str_helper(s);
-    j
+fn from_str(s: str) -> result::t<json, error> {
+    from_reader(io::string_reader(s))
+}
+
+/*
+Function: eq
+
+Test if two json values are equal.
+*/
+fn eq(value0: json, value1: json) -> bool {
+    alt (value0, value1) {
+      (num(f0), num(f1)) { f0 == f1 }
+      (string(s0), string(s1)) { s0 == s1 }
+      (boolean(b0), boolean(b1)) { b0 == b1 }
+      (list(l0), list(l1)) { vec::all2(l0, l1, eq) }
+      (dict(d0), dict(d1)) {
+          if d0.size() == d1.size() {
+              let equal = true;
+              d0.items { |k, v0|
+                  alt d1.find(k) {
+                    some(v1) {
+                        if !eq(v0, v1) { equal = false; } }
+                    none { equal = false; }
+                  }
+              };
+              equal
+          } else {
+              false
+          }
+      }
+      (null, null) { true }
+      _ { false }
+    }
 }
 
 #[cfg(test)]
 mod tests {
-    #[test]
-    fn test_from_str_null() {
-        assert(from_str("null") == some(null));
+    fn mk_dict(items: [(str, json)]) -> json {
+        let d = map::new_str_hash();
+
+        vec::iter(items) { |item|
+            let (key, value) = item;
+            d.insert(key, value);
+        };
+
+        dict(d)
     }
 
     #[test]
-    fn test_from_str_num() {
-        assert(from_str("3") == some(num(3f)));
-        assert(from_str("3.1") == some(num(3.1f)));
-        assert(from_str("-1.2") == some(num(-1.2f)));
-        assert(from_str(".4") == some(num(0.4f)));
+    fn test_write_null() {
+        assert to_str(null) == "null";
     }
 
     #[test]
-    fn test_from_str_str() {
-        assert(from_str("\"foo\"") == some(string("foo")));
-        assert(from_str("\"\\\"\"") == some(string("\"")));
-        assert(from_str("\"lol") == none);
+    fn test_write_num() {
+        assert to_str(num(3f)) == "3";
+        assert to_str(num(3.1f)) == "3.1";
+        assert to_str(num(-1.5f)) == "-1.5";
+        assert to_str(num(0.5f)) == "0.5";
     }
 
     #[test]
-    fn test_from_str_bool() {
-        assert(from_str("true") == some(boolean(true)));
-        assert(from_str("false") == some(boolean(false)));
-        assert(from_str("truz") == none);
+    fn test_write_str() {
+        assert to_str(string("")) == "\"\"";
+        assert to_str(string("foo")) == "\"foo\"";
     }
 
     #[test]
-    fn test_from_str_list() {
-        assert(from_str("[]") == some(list(@[])));
-        assert(from_str("[true]") == some(list(@[boolean(true)])));
-        assert(from_str("[null]") == some(list(@[null])));
-        assert(from_str("[3, 1]") == some(list(@[num(3f), num(1f)])));
-        assert(from_str("[2, [4, 1]]") ==
-               some(list(@[num(2f), list(@[num(4f), num(1f)])])));
-        assert(from_str("[2, ]") == none);
-        assert(from_str("[5, ") == none);
-        assert(from_str("[6 7]") == none);
-        assert(from_str("[3") == none);
+    fn test_write_bool() {
+        assert to_str(boolean(true)) == "true";
+        assert to_str(boolean(false)) == "false";
     }
 
     #[test]
-    fn test_from_str_dict() {
-        assert(from_str("{}") != none);
-        assert(from_str("{\"a\": 3}") != none);
-        assert(from_str("{\"a\": null}") != none);
-        assert(from_str("{\"a\": }") == none);
-        assert(from_str("{\"a\" }") == none);
-        assert(from_str("{\"a\"") == none);
-        assert(from_str("{") == none);
+    fn test_write_list() {
+        assert to_str(list([])) == "[]";
+        assert to_str(list([boolean(true)])) == "[true]";
+        assert to_str(list([
+            boolean(false),
+            null,
+            list([string("foo\nbar"), num(3.5f)])
+        ])) == "[false, null, [\"foo\\nbar\", 3.5]]";
+    }
+
+    #[test]
+    fn test_write_dict() {
+        assert to_str(mk_dict([])) == "{}";
+        assert to_str(mk_dict([("a", boolean(true))])) == "{ \"a\": true }";
+        assert to_str(mk_dict([
+            ("a", boolean(true)),
+            ("b", list([
+                mk_dict([("c", string("\x0c\r"))]),
+                mk_dict([("d", string(""))])
+            ]))
+        ])) ==
+            "{ " +
+                "\"a\": true, " +
+                "\"b\": [" +
+                    "{ \"c\": \"\\f\\r\" }, " +
+                    "{ \"d\": \"\" }" +
+                "]" +
+            " }";
+    }
+
+    #[test]
+    fn test_trailing_characters() {
+        assert from_str("nulla") ==
+            err({line: 1u, col: 5u, msg: "trailing characters"});
+        assert from_str("truea") ==
+            err({line: 1u, col: 5u, msg: "trailing characters"});
+        assert from_str("falsea") ==
+            err({line: 1u, col: 6u, msg: "trailing characters"});
+        assert from_str("1a") ==
+            err({line: 1u, col: 2u, msg: "trailing characters"});
+        assert from_str("[]a") ==
+            err({line: 1u, col: 3u, msg: "trailing characters"});
+        assert from_str("{}a") ==
+            err({line: 1u, col: 3u, msg: "trailing characters"});
+    }
+
+    #[test]
+    fn test_read_identifiers() {
+        assert from_str("n") ==
+            err({line: 1u, col: 2u, msg: "invalid syntax"});
+        assert from_str("nul") ==
+            err({line: 1u, col: 4u, msg: "invalid syntax"});
+
+        assert from_str("t") ==
+            err({line: 1u, col: 2u, msg: "invalid syntax"});
+        assert from_str("truz") ==
+            err({line: 1u, col: 4u, msg: "invalid syntax"});
+
+        assert from_str("f") ==
+            err({line: 1u, col: 2u, msg: "invalid syntax"});
+        assert from_str("faz") ==
+            err({line: 1u, col: 3u, msg: "invalid syntax"});
+
+        assert from_str("null") == ok(null);
+        assert from_str("true") == ok(boolean(true));
+        assert from_str("false") == ok(boolean(false));
+    }
+
+    #[test]
+    fn test_read_num() {
+        assert from_str("+") ==
+            err({line: 1u, col: 1u, msg: "invalid syntax"});
+        assert from_str(".") ==
+            err({line: 1u, col: 1u, msg: "invalid syntax"});
+
+        assert from_str("-") ==
+            err({line: 1u, col: 2u, msg: "invalid number"});
+        assert from_str("00") ==
+            err({line: 1u, col: 2u, msg: "invalid number"});
+        assert from_str("1.") ==
+            err({line: 1u, col: 3u, msg: "invalid number"});
+        assert from_str("1e") ==
+            err({line: 1u, col: 3u, msg: "invalid number"});
+        assert from_str("1e+") ==
+            err({line: 1u, col: 4u, msg: "invalid number"});
+
+        assert from_str("3") == ok(num(3f));
+        assert from_str("3.1") == ok(num(3.1f));
+        assert from_str("-1.2") == ok(num(-1.2f));
+        assert from_str("0.4") == ok(num(0.4f));
+        assert from_str("0.4e5") == ok(num(0.4e5f));
+        assert from_str("0.4e+15") == ok(num(0.4e15f));
+        assert from_str("0.4e-01") == ok(num(0.4e-01f));
+    }
+
+    #[test]
+    fn test_read_str() {
+        assert from_str("\"") ==
+            err({line: 1u, col: 2u, msg: "EOF while parsing string"});
+        assert from_str("\"lol") ==
+            err({line: 1u, col: 5u, msg: "EOF while parsing string"});
+
+        assert from_str("\"\"") == ok(string(""));
+        assert from_str("\"foo\"") == ok(string("foo"));
+        assert from_str("\"\\\"\"") == ok(string("\""));
+        assert from_str("\"\\b\"") == ok(string("\x08"));
+        assert from_str("\"\\n\"") == ok(string("\n"));
+        assert from_str("\"\\r\"") == ok(string("\r"));
+        assert from_str("\"\\t\"") == ok(string("\t"));
+    }
+
+    #[test]
+    fn test_read_list() {
+        assert from_str("[") ==
+            err({line: 1u, col: 2u, msg: "EOF while parsing value"});
+        assert from_str("[1") ==
+            err({line: 1u, col: 3u, msg: "EOF while parsing list"});
+        assert from_str("[1,") ==
+            err({line: 1u, col: 4u, msg: "EOF while parsing value"});
+        assert from_str("[1,]") ==
+            err({line: 1u, col: 4u, msg: "invalid syntax"});
+        assert from_str("[6 7]") ==
+            err({line: 1u, col: 4u, msg: "expecting ',' or ']'"});
+
+        assert from_str("[]") == ok(list([]));
+        assert from_str("[ ]") == ok(list([]));
+        assert from_str("[true]") == ok(list([boolean(true)]));
+        assert from_str("[ false ]") == ok(list([boolean(false)]));
+        assert from_str("[null]") == ok(list([null]));
+        assert from_str("[3, 1]") == ok(list([num(3f), num(1f)]));
+        assert from_str("[2, [4, 1]]") ==
+               ok(list([num(2f), list([num(4f), num(1f)])]));
+    }
+
+    #[test]
+    fn test_read_dict() {
+        assert from_str("{") ==
+            err({line: 1u, col: 2u, msg: "EOF while parsing object"});
+        assert from_str("{ ") ==
+            err({line: 1u, col: 3u, msg: "EOF while parsing object"});
+        assert from_str("{1") ==
+            err({line: 1u, col: 2u, msg: "key must be a string"});
+        assert from_str("{ \"a\"") ==
+            err({line: 1u, col: 6u, msg: "EOF while parsing object"});
+        assert from_str("{\"a\"") ==
+            err({line: 1u, col: 5u, msg: "EOF while parsing object"});
+        assert from_str("{\"a\" ") ==
+            err({line: 1u, col: 6u, msg: "EOF while parsing object"});
+
+        assert from_str("{\"a\" 1") ==
+            err({line: 1u, col: 6u, msg: "expecting ':'"});
+        assert from_str("{\"a\":") ==
+            err({line: 1u, col: 6u, msg: "EOF while parsing value"});
+        assert from_str("{\"a\":1") ==
+            err({line: 1u, col: 7u, msg: "EOF while parsing object"});
+        assert from_str("{\"a\":1 1") ==
+            err({line: 1u, col: 8u, msg: "expecting ',' or '}'"});
+        assert from_str("{\"a\":1,") ==
+            err({line: 1u, col: 8u, msg: "EOF while parsing object"});
+
+        assert eq(result::get(from_str("{}")), mk_dict([]));
+        assert eq(result::get(from_str("{\"a\": 3}")),
+                  mk_dict([("a", num(3.0f))]));
+
+        assert eq(result::get(from_str("{ \"a\": null, \"b\" : true }")),
+                  mk_dict([("a", null), ("b", boolean(true))]));
+        assert eq(result::get(from_str("{\"a\" : 1.0 ,\"b\": [ true ]}")),
+                  mk_dict([
+                      ("a", num(1.0)),
+                      ("b", list([boolean(true)]))
+                  ]));
+        assert eq(result::get(from_str(
+                      "{" +
+                          "\"a\": 1.0, " +
+                          "\"b\": [" +
+                              "true," +
+                              "\"foo\\nbar\", " +
+                              "{ \"c\": {\"d\": null} } " +
+                          "]" +
+                      "}")),
+                  mk_dict([
+                      ("a", num(1.0f)),
+                      ("b", list([
+                          boolean(true),
+                          string("foo\nbar"),
+                          mk_dict([
+                              ("c", mk_dict([("d", null)]))
+                          ])
+                      ]))
+                  ]));
+    }
+
+    #[test]
+    fn test_multiline_errors() {
+        assert from_str("{\n  \"foo\":\n \"bar\"") ==
+            err({line: 3u, col: 8u, msg: "EOF while parsing object"});
     }
 }