diff --git a/moon.mod.json b/moon.mod.json
index ca0a064..daf9820 100644
--- a/moon.mod.json
+++ b/moon.mod.json
@@ -7,5 +7,6 @@
   "license": "Apache-2.0",
   "keywords": ["Cirru"],
   "source": "src",
-  "description": "parser for Cirru Syntax"
+  "description": "parser for Cirru Syntax",
+  "preferred-target": "js"
 }
diff --git a/src/lib/lib.mbti b/src/lib/lib.mbti
index 6bfde94..75eba86 100644
--- a/src/lib/lib.mbti
+++ b/src/lib/lib.mbti
@@ -1,63 +1,44 @@
-package tiye/cirru-parser/lib
+// Generated using `moon info`, DON'T EDIT IT
+package "tiye/cirru-parser/lib"
+
+import(
+  "moonbitlang/core/json"
+)
 
 // Values
-fn format(Array[Cirru], CirruWriterOptions) -> String!FormatCirruError
+fn format_to_lisp(Array[Cirru]) -> String raise FormatError
 
-fn format_to_lisp(Array[Cirru]) -> String!FormatError
+// Errors
+type CirruParseError
+fn CirruParseError::to_string(Self) -> String
 
-fn parse(String) -> Array[Cirru]!CirruParseError
+pub(all) suberror FormatCirruError String
+impl Show for FormatCirruError
+
+type FormatError
 
 // Types and methods
 pub(all) enum Cirru {
   Leaf(String)
   List(Array[Cirru])
 }
-impl Cirru {
-  compare(Self, Self) -> Int
-  debug_write(Self, Buffer) -> Unit
-  is_comment(Self) -> Bool
-  is_empty(Self) -> Bool
-  is_nested(Self) -> Bool
-  length(Self) -> Int
-  op_equal(Self, Self) -> Bool
-  output(Self, Logger) -> Unit
-  to_json(Self) -> Json
-  to_string(Self) -> String
-}
+fn Cirru::format(Array[Self], CirruWriterOptions) -> String raise FormatCirruError
+fn Cirru::is_comment(Self) -> Bool
+fn Cirru::is_empty(Self) -> Bool
+fn Cirru::is_nested(Self) -> Bool
+fn Cirru::length(Self) -> Int
+fn Cirru::parse(String) -> Array[Self] raise CirruParseError
+impl Compare for Cirru
 impl Eq for Cirru
 impl Hash for Cirru
-impl @moonbitlang/core/json.FromJson for Cirru
-
-type CirruLexItem
-
-type CirruLexState
-impl CirruLexState {
-  to_string(Self) -> String
-}
-impl Show for CirruLexState
-
-type CirruParseError
-impl CirruParseError {
-  to_string(Self) -> String
-}
+impl Show for Cirru
+impl ToJson for Cirru
+impl @json.FromJson for Cirru
 
 pub(all) struct CirruWriterOptions {
   use_inline : Bool
 }
 
-pub(all) type! FormatCirruError String
-impl FormatCirruError {
-  to_string(Self) -> String
-}
-
-type FormatError
-
-type WriterNode
-impl WriterNode {
-  op_equal(Self, Self) -> Bool
-}
-impl Eq for WriterNode
-
 // Type aliases
 
 // Traits
diff --git a/src/lib/parser.mbt b/src/lib/parser.mbt
index 4e9e6ed..c8688d9 100644
--- a/src/lib/parser.mbt
+++ b/src/lib/parser.mbt
@@ -1,6 +1,6 @@
 ///|
 fn build_exprs(
-  tokens : Array[CirruLexItem]
+  tokens : Array[CirruLexItem],
 ) -> Array[Cirru] raise CirruParseError {
   let acc : Array[Cirru] = Array::new()
   let mut idx = 0
diff --git a/src/lib/parser_test.mbt b/src/lib/parser_test.mbt
index b70b01f..d540957 100644
--- a/src/lib/parser_test.mbt
+++ b/src/lib/parser_test.mbt
@@ -16,14 +16,14 @@ test "existed demos" {
   for demo in demos {
     let demo_file = fsReadSync("./test/cirru/\{demo}.cirru")
     let json_file = fsReadSync("./test/data/\{demo}.json")
-    let tree = Cirru::parse?(demo_file).unwrap().to_json().stringify()
-    let defined = @json.parse?(json_file).unwrap()
+    let tree = (try? Cirru::parse(demo_file)).unwrap().to_json().stringify()
+    let defined = (try? @json.parse(json_file)).unwrap()
     let defined_str = defined.stringify()
     assert_eq(tree, defined_str)
     let formatted = Cirru::format(@json.from_json(defined), {
       use_inline: false,
     })
-    let ret = Cirru::parse?(formatted).unwrap().to_json().stringify()
+    let ret = (try? Cirru::parse(formatted)).unwrap().to_json().stringify()
     assert_eq(ret, defined_str)
   }
 }
diff --git a/src/lib/primes.mbt b/src/lib/primes.mbt
index 6f7e41a..1a932cf 100644
--- a/src/lib/primes.mbt
+++ b/src/lib/primes.mbt
@@ -64,11 +64,6 @@ pub impl @json.FromJson for Cirru with from_json(json, path) {
   }
 }
 
-///|
-impl Default for Cirru with default() -> Cirru {
-  List(Array::new())
-}
-
 ///|
 pub impl Show for Cirru with output(self : Cirru, logger : &Logger) -> Unit {
   match self {
@@ -97,7 +92,7 @@ pub impl Compare for Cirru with compare(self, other) -> Int {
   match (self, other) {
     (Leaf(a), Leaf(b)) => a.compare(b)
     (List(xs), List(ys)) => {
-      let size = @math.minimum(xs.length(), ys.length())
+      let size = @cmp.minimum(xs.length(), ys.length())
       for i = 0; i < size; i = i + 1 {
         let x = xs[i]
         let y = ys[i]
@@ -148,7 +143,7 @@ pub fn Cirru::is_nested(self : Cirru) -> Bool {
 ///|
 pub fn Cirru::is_comment(self : Cirru) -> Bool {
   match self {
-    Leaf(s) => s.char_at(0) == ';'
+    Leaf(s) => s.get_char(0).unwrap() == ';'
     _ => false
   }
 }
@@ -165,7 +160,7 @@ priv enum CirruLexState {
   Indent
   /// Working on a string
   Str
-} derive(Show)
+}
 
 ///| lexer is a simpler state machine to tokenize Cirru code
 priv enum CirruLexItem {
@@ -198,7 +193,7 @@ fn CirruLexItem::is_normal_str(tok : String) -> Bool {
   }
   let mut i = 0
   while i < size {
-    let c = tok.char_at(i)
+    let c = tok.get_char(i).unwrap()
     match c {
       'A'..='Z' => ()
       'a'..='z' => ()
@@ -236,7 +231,7 @@ fn escape_cirru_leaf(s : String) -> String {
     let size = s.length()
     let mut i = 0
     while i < size {
-      let c = s.char_at(i)
+      let c = s.get_char(i).unwrap()
       match c {
         '\n' => out += "\\n"
         '\t' => out += "\\t"
diff --git a/src/lib/s_expr.mbt b/src/lib/s_expr.mbt
index 145e5cd..ad3ce83 100644
--- a/src/lib/s_expr.mbt
+++ b/src/lib/s_expr.mbt
@@ -49,7 +49,7 @@ fn format_lispy_expr(self : Cirru, indent : Int) -> String raise FormatError {
       if token.is_empty() {
         raise FormatError("empty string is invalid")
       } else {
-        let s0 = token.char_at(0)
+        let s0 = token.get_char(0).unwrap()
         if s0 == '|' || s0 == '"' {
           "\"" + escape_string(token.substring(start=1)) + "\""
         } else if token.contains(" ") ||
diff --git a/src/lib/writer.mbt b/src/lib/writer.mbt
index 49b22f8..5a1c998 100644
--- a/src/lib/writer.mbt
+++ b/src/lib/writer.mbt
@@ -172,7 +172,7 @@ fn generate_tree(
   insist_head : Bool,
   options : CirruWriterOptions,
   base_level : Int,
-  in_tail : Bool
+  in_tail : Bool,
 ) -> String raise FormatCirruError {
   let mut prev_kind : WriterNode = Nil
   let mut level = base_level
@@ -299,7 +299,7 @@ fn generate_tree(
 ///|
 fn generate_statements(
   ys : Array[Cirru],
-  options : CirruWriterOptions
+  options : CirruWriterOptions,
 ) -> String raise FormatCirruError {
   let mut zs = ""
   for y in ys {
@@ -318,7 +318,7 @@ fn generate_statements(
 ///| format Cirru code, use options to control `use_inline` option
 pub fn Cirru::format(
   xs : Array[Cirru],
-  options : CirruWriterOptions
+  options : CirruWriterOptions,
 ) -> String raise FormatCirruError {
   generate_statements(xs, options)
 }
diff --git a/src/main/main.mbt b/src/main/main.mbt
index 046a915..5327d51 100644
--- a/src/main/main.mbt
+++ b/src/main/main.mbt
@@ -13,11 +13,12 @@ pub fn main_parser() -> Unit {
 
   let demo_file = fsReadSync("./test/cirru/unfolding.cirru")
   let json_file = fsReadSync("./test/data/unfolding.json")
-  let tree = Cirru::parse?(demo_file)
+  let tree = try? Cirru::parse(demo_file)
   match tree {
     Ok(tree) => {
       println(tree.to_json().stringify())
-      match @json.parse?(json_file) {
+      let json_result = try? @json.parse(json_file)
+      match json_result {
         Ok(json) => println(json.stringify())
         Err(err) => println(err.to_string())
       }
@@ -32,13 +33,13 @@ pub fn main_writer() -> Unit {
   let demo = "unfolding"
   let demo_file = fsReadSync("./test/cirru/\{demo}.cirru")
   let json_file = fsReadSync("./test/data/\{demo}.json")
-  let defined = @json.parse?(json_file).unwrap()
-  let tree : Array[Cirru] = @json.from_json?(defined).unwrap()
+  let defined = (try? @json.parse(json_file)).unwrap()
+  let tree : Array[Cirru] = (try? @json.from_json(defined)).unwrap()
   println("TREE:")
   for item in tree {
     println(item.to_json().stringify())
   }
-  let ret = Cirru::format?(tree, { use_inline: false }).unwrap()
+  let ret = (try? Cirru::format(tree, { use_inline: false })).unwrap()
   println("Generated:")
   println(ret)
   println("Expected:")
diff --git a/src/main/main.mbti b/src/main/main.mbti
index f49045a..21245fb 100644
--- a/src/main/main.mbti
+++ b/src/main/main.mbti
@@ -1,4 +1,5 @@
-package tiye/cirru-parser/main
+// Generated using `moon info`, DON'T EDIT IT
+package "tiye/cirru-parser/main"
 
 // Values
 fn fsReadSync(String) -> String
@@ -7,6 +8,8 @@ fn main_parser() -> Unit
 
 fn main_writer() -> Unit
 
+// Errors
+
 // Types and methods
 
 // Type aliases