diff --git a/examples/curl.zr b/examples/curl.zr
index 6034ddf..2f7540f 100644
--- a/examples/curl.zr
+++ b/examples/curl.zr
@@ -3,7 +3,7 @@ func main[argc: i64, argv: ptr] : i64
         io.println("ERROR: url is missing")
         return 1
 
-    let url: str = mem.read64(argv + 8)
+    let url: str = mem.read64(argv + 8) as str
 
     if str.len(url) <= 7
         io.println("ERROR: invalid url (Did you forget \"http://\"?)")
@@ -36,7 +36,7 @@ func main[argc: i64, argv: ptr] : i64
     req = str.concat(req, " HTTP/1.0\r\nHost: ")
     req = str.concat(req, host)
     req = str.concat(req, "\r\nConnection: close\r\n\r\n")
-    net.send(s, req, str.len(req))
+    net.send(s, req as ptr, str.len(req))
     mem.free(req)
 
     let header_buf: str = mem.alloc(8192)
@@ -45,7 +45,7 @@ func main[argc: i64, argv: ptr] : i64
     let end_index: i64 = -1
 
     while !found && header_size < 8192
-        let n: i64 = net.read(s, header_buf + header_size, 8192 - header_size)
+        let n: i64 = net.read(s, header_buf as ptr + header_size, 8192 - header_size)
         if n <= 0
             break
         let current_size: i64 = header_size + n
@@ -59,7 +59,7 @@ func main[argc: i64, argv: ptr] : i64
         header_size = current_size
 
     if end_index < header_size
-        io.print_sized(header_buf + end_index, header_size - end_index)
+        io.print_sized(header_buf as ptr + end_index, header_size as i64 - end_index)
     mem.free(header_buf)
 
     let buffer: ptr = mem.alloc(4096)
diff --git a/examples/puzzles/aoc2025_03.zr b/examples/puzzles/aoc2025_03.zr
index 6054439..61cc246 100644
--- a/examples/puzzles/aoc2025_03.zr
+++ b/examples/puzzles/aoc2025_03.zr
@@ -25,7 +25,7 @@ func part2_rec[bank: str, start: i64, remaining: i64] : i64
     let len: i64 = str.len(bank)
 
     for i in (start)..(len-remaining+1)
-        let v: i64 = bank[i] - '0'
+        let v: i64 = (bank[i] - '0') as i64
         if v > largest
             largest = v
             largest_idx = i
diff --git a/examples/tcp_server.zr b/examples/tcp_server.zr
index de94fb7..a45250f 100644
--- a/examples/tcp_server.zr
+++ b/examples/tcp_server.zr
@@ -2,7 +2,7 @@ func main[] : i64
     let s: i64 = must(net.listen?(net.pack_addr(127, 0, 0, 1), 8000))
 
     io.println("Listening on port 8000...")
-    let resp: str = mem.alloc(60000)
+    let resp: ptr = mem.alloc(60000)
     while true
         let conn: i64 = net.accept(s)
         if conn < 0
diff --git a/examples/udp_server.zr b/examples/udp_server.zr
index 5f2f345..1ec4c65 100644
--- a/examples/udp_server.zr
+++ b/examples/udp_server.zr
@@ -1,5 +1,5 @@
 func main[] : i64
-    let s: i64 = must(net.udp_listen?(net.pack_addr(127, 0, 0, 1), 8000))
+    let s: net.UDPSocket = must(net.create_udp_server?(net.pack_addr(127, 0, 0, 1), 8000))
 
     io.println("Listening on port 8000...")
     while true
diff --git a/src/analyzer.rs b/src/analyzer.rs
index 2abb0e7..e218125 100644
--- a/src/analyzer.rs
+++ b/src/analyzer.rs
@@ -5,25 +5,57 @@ use crate::{
     tokenizer::{ZernError, error},
 };
 
+pub type Type = String;
+
+pub struct StructField {
+    pub offset: usize,
+    pub field_type: Type,
+}
+
+#[derive(Clone)]
+pub struct FnType {
+    pub return_type: Type,
+    pub params: Option<Vec<Type>>,
+}
+
+impl FnType {
+    fn new(return_type: &str, params: Vec<&str>) -> FnType {
+        FnType {
+            return_type: return_type.to_string(),
+            params: Some(params.iter().map(|x| x.to_string()).collect()),
+        }
+    }
+
+    fn new_variadic(return_type: &str) -> FnType {
+        FnType {
+            return_type: return_type.to_string(),
+            params: None,
+        }
+    }
+}
+
 pub struct Analyzer {
-    pub functions: HashMap<String, i32>,
+    pub functions: HashMap<String, FnType>,
     pub constants: HashMap<String, u64>,
-    pub structs: HashMap<String, HashMap<String, usize>>,
+    pub structs: HashMap<String, HashMap<String, StructField>>,
 }
 
 impl Analyzer {
     pub fn new() -> Analyzer {
         Analyzer {
             functions: HashMap::from([
-                ("_builtin_heap_head".into(), 0),
-                ("_builtin_heap_tail".into(), 0),
-                ("_builtin_err_code".into(), 0),
-                ("_builtin_err_msg".into(), 0),
-                ("_builtin_read64".into(), 1),
-                ("_builtin_set64".into(), 2),
-                ("_builtin_syscall".into(), -1),
-                ("io.printf".into(), -1),
-                ("_builtin_environ".into(), 0),
+                ("_builtin_heap_head".into(), FnType::new("ptr", vec![])),
+                ("_builtin_heap_tail".into(), FnType::new("ptr", vec![])),
+                ("_builtin_err_code".into(), FnType::new("ptr", vec![])),
+                ("_builtin_err_msg".into(), FnType::new("ptr", vec![])),
+                ("_builtin_read64".into(), FnType::new("i64", vec!["ptr"])),
+                (
+                    "_builtin_set64".into(),
+                    FnType::new("void", vec!["ptr", "i64"]),
+                ),
+                ("_builtin_syscall".into(), FnType::new_variadic("i64")),
+                ("io.printf".into(), FnType::new_variadic("void")),
+                ("_builtin_environ".into(), FnType::new("ptr", vec![])),
             ]),
             constants: HashMap::new(),
             structs: HashMap::new(),
@@ -34,7 +66,7 @@ impl Analyzer {
         if let Stmt::Function {
             name,
             params,
-            return_type: _,
+            return_type,
             body: _,
             exported: _,
         } = stmt
@@ -42,8 +74,13 @@ impl Analyzer {
             if self.functions.contains_key(&name.lexeme) {
                 return error!(name.loc, format!("tried to redefine '{}'", name.lexeme));
             }
-            self.functions
-                .insert(name.lexeme.clone(), params.len() as i32);
+            self.functions.insert(
+                name.lexeme.clone(),
+                FnType {
+                    return_type: return_type.lexeme.clone(),
+                    params: Some(params.iter().map(|x| x.var_type.lexeme.clone()).collect()),
+                },
+            );
         }
         Ok(())
     }
@@ -83,6 +120,7 @@ impl Analyzer {
                 }
             }
             Stmt::If {
+                keyword: _,
                 condition,
                 then_branch,
                 else_branch,
@@ -91,7 +129,11 @@ impl Analyzer {
                 self.analyze_stmt(then_branch)?;
                 self.analyze_stmt(else_branch)?;
             }
-            Stmt::While { condition, body } => {
+            Stmt::While {
+                keyword: _,
+                condition,
+                body,
+            } => {
                 self.analyze_expr(condition)?;
                 self.analyze_stmt(body)?;
             }
@@ -108,7 +150,7 @@ impl Analyzer {
 
                 self.analyze_stmt(body)?;
             }
-            Stmt::Return(expr) => {
+            Stmt::Return { expr, keyword: _ } => {
                 self.analyze_expr(expr)?;
             }
             Stmt::For {
@@ -127,14 +169,21 @@ impl Analyzer {
                 if self.functions.contains_key(&name.lexeme) {
                     return error!(name.loc, format!("tried to redefine '{}'", name.lexeme));
                 }
-                self.functions.insert(name.lexeme.clone(), -1);
+                self.functions
+                    .insert(name.lexeme.clone(), FnType::new_variadic("any"));
             }
             Stmt::Struct { name, fields } => {
-                let mut fields_map: HashMap<String, usize> = HashMap::new();
+                let mut fields_map: HashMap<String, StructField> = HashMap::new();
 
                 let mut offset: usize = 0;
                 for field in fields {
-                    fields_map.insert(field.var_name.lexeme.clone(), offset);
+                    fields_map.insert(
+                        field.var_name.lexeme.clone(),
+                        StructField {
+                            offset,
+                            field_type: field.var_type.lexeme.clone(),
+                        },
+                    );
                     offset += 8;
                 }
 
@@ -172,11 +221,18 @@ impl Analyzer {
                 if let Expr::Variable(callee_name) = *callee.clone() {
                     if self.functions.contains_key(&callee_name.lexeme) {
                         // its a function (defined/builtin/extern)
-                        if let Some(arity) = self.functions.get(&callee_name.lexeme) {
-                            if *arity >= 0 && *arity != args.len() as i32 {
+                        if let Some(fn_type) = self.functions.get(&callee_name.lexeme) {
+                            // if its None, its variadic
+                            if let Some(params) = &fn_type.params
+                                && params.len() != args.len()
+                            {
                                 return error!(
                                     &paren.loc,
-                                    format!("expected {} arguments, got {}", arity, args.len())
+                                    format!(
+                                        "expected {} arguments, got {}",
+                                        params.len(),
+                                        args.len()
+                                    )
                                 );
                             }
                         } else {
@@ -203,7 +259,11 @@ impl Analyzer {
                     self.analyze_expr(expr)?;
                 }
             }
-            Expr::Index { expr, index } => {
+            Expr::Index {
+                expr,
+                bracket: _,
+                index,
+            } => {
                 self.analyze_expr(expr)?;
                 self.analyze_expr(index)?;
             }
@@ -214,6 +274,9 @@ impl Analyzer {
             Expr::MemberAccess { left, field: _ } => {
                 self.analyze_expr(left)?;
             }
+            Expr::Cast { expr, type_name: _ } => {
+                self.analyze_expr(expr)?;
+            }
         }
         Ok(())
     }
diff --git a/src/codegen_x86_64.rs b/src/codegen_x86_64.rs
index 94f4998..dd6ad99 100644
--- a/src/codegen_x86_64.rs
+++ b/src/codegen_x86_64.rs
@@ -69,25 +69,22 @@ macro_rules! emit {
 
 static REGISTERS: [&str; 6] = ["rdi", "rsi", "rdx", "rcx", "r8", "r9"];
 
-// TODO: currently they are all just 64 bit values
-static BUILTIN_TYPES: [&str; 7] = ["void", "u8", "i64", "str", "bool", "ptr", "any"];
-
-pub struct CodegenX86_64 {
+pub struct CodegenX86_64<'a> {
     output: String,
     data_section: String,
     label_counter: usize,
     data_counter: usize,
-    pub analyzer: Analyzer,
+    pub analyzer: &'a Analyzer,
 }
 
-impl CodegenX86_64 {
-    pub fn new() -> CodegenX86_64 {
+impl<'a> CodegenX86_64<'a> {
+    pub fn new(analyzer: &'a Analyzer) -> CodegenX86_64<'a> {
         CodegenX86_64 {
             output: String::new(),
             data_section: String::new(),
             label_counter: 0,
             data_counter: 1,
-            analyzer: Analyzer::new(),
+            analyzer,
         }
     }
 
@@ -248,10 +245,6 @@ _builtin_environ:
                     },
                 };
 
-                if !self.is_valid_type_name(&var_type) {
-                    return error!(&name.loc, "unrecognized type: ".to_owned() + &var_type);
-                }
-
                 self.compile_expr(env, initializer)?;
                 let offset = env.define_var(name.lexeme.clone(), var_type);
                 emit!(&mut self.output, "    mov QWORD [rbp-{}], rax", offset);
@@ -267,6 +260,7 @@ _builtin_environ:
                 env.pop_scope();
             }
             Stmt::If {
+                keyword: _,
                 condition,
                 then_branch,
                 else_branch,
@@ -283,7 +277,11 @@ _builtin_environ:
                 self.compile_stmt(env, else_branch)?;
                 emit!(&mut self.output, "{}:", end_label);
             }
-            Stmt::While { condition, body } => {
+            Stmt::While {
+                keyword: _,
+                condition,
+                body,
+            } => {
                 let old_loop_begin_label = env.loop_begin_label.clone();
                 let old_loop_end_label = env.loop_end_label.clone();
                 let old_loop_continue_label = env.loop_continue_label.clone();
@@ -306,7 +304,7 @@ _builtin_environ:
             Stmt::Function {
                 name,
                 params,
-                return_type,
+                return_type: _,
                 body,
                 exported,
             } => {
@@ -319,21 +317,7 @@ _builtin_environ:
                 emit!(&mut self.output, "    mov rbp, rsp");
                 emit!(&mut self.output, "    sub rsp, 256"); // TODO
 
-                if !self.is_valid_type_name(&return_type.lexeme) {
-                    return error!(
-                        &return_type.loc,
-                        "unrecognized type: ".to_owned() + &return_type.lexeme
-                    );
-                }
-
                 for (i, param) in params.iter().enumerate() {
-                    if !self.is_valid_type_name(&param.var_type.lexeme) {
-                        return error!(
-                            &param.var_name.loc,
-                            "unrecognized type: ".to_owned() + &param.var_type.lexeme
-                        );
-                    }
-
                     let offset = env
                         .define_var(param.var_name.lexeme.clone(), param.var_type.lexeme.clone());
                     if let Some(reg) = REGISTERS.get(i) {
@@ -360,7 +344,7 @@ _builtin_environ:
                     emit!(&mut self.output, "    ret");
                 }
             }
-            Stmt::Return(expr) => {
+            Stmt::Return { expr, keyword: _ } => {
                 self.compile_expr(env, expr)?;
                 emit!(&mut self.output, "    mov rsp, rbp");
                 emit!(&mut self.output, "    pop rbp");
@@ -578,15 +562,9 @@ _builtin_environ:
                         self.analyzer.constants[&name.lexeme]
                     );
                 } else {
-                    // TODO: move to analyzer
                     let var = match env.get_var(&name.lexeme) {
                         Some(x) => x,
-                        None => {
-                            return error!(
-                                name.loc,
-                                format!("undefined variable: {}", &name.lexeme)
-                            );
-                        }
+                        None => unreachable!("this should be caught in the typechecker"),
                     };
                     emit!(
                         &mut self.output,
@@ -600,15 +578,9 @@ _builtin_environ:
 
                 match left.as_ref() {
                     Expr::Variable(name) => {
-                        // TODO: move to analyzer
                         let var = match env.get_var(&name.lexeme) {
                             Some(x) => x,
-                            None => {
-                                return error!(
-                                    name.loc,
-                                    format!("undefined variable: {}", &name.lexeme)
-                                );
-                            }
+                            None => unreachable!(),
                         };
                         emit!(
                             &mut self.output,
@@ -616,7 +588,11 @@ _builtin_environ:
                             var.stack_offset,
                         );
                     }
-                    Expr::Index { expr, index } => {
+                    Expr::Index {
+                        expr,
+                        bracket: _,
+                        index,
+                    } => {
                         emit!(&mut self.output, "    push rax");
                         self.compile_expr(env, expr)?;
                         emit!(&mut self.output, "    push rax");
@@ -710,7 +686,11 @@ _builtin_environ:
                 }
                 emit!(&mut self.output, "    pop rax");
             }
-            Expr::Index { expr, index } => {
+            Expr::Index {
+                expr,
+                bracket: _,
+                index,
+            } => {
                 self.compile_expr(env, expr)?;
                 emit!(&mut self.output, "    push rax");
                 self.compile_expr(env, index)?;
@@ -761,20 +741,13 @@ _builtin_environ:
                 self.compile_expr(env, left)?;
                 emit!(&mut self.output, "    mov rax, QWORD [rax+{}]", offset);
             }
+            Expr::Cast { expr, type_name: _ } => {
+                self.compile_expr(env, expr)?;
+            }
         }
         Ok(())
     }
 
-    fn is_valid_type_name(&self, name: &str) -> bool {
-        if BUILTIN_TYPES.contains(&name) {
-            return true;
-        }
-        if self.analyzer.structs.contains_key(name) {
-            return true;
-        }
-        false
-    }
-
     fn get_field_offset(
         &self,
         env: &mut Env,
@@ -803,11 +776,11 @@ _builtin_environ:
             }
         };
 
-        let offset = match fields.get(&field.lexeme) {
-            Some(o) => *o,
+        let field = match fields.get(&field.lexeme) {
+            Some(o) => o,
             None => return error!(&field.loc, format!("unknown field: {}", &field.lexeme)),
         };
 
-        Ok(offset)
+        Ok(field.offset)
     }
 }
diff --git a/src/main.rs b/src/main.rs
index 5220485..da7fdd3 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2,6 +2,7 @@ mod analyzer;
 mod codegen_x86_64;
 mod parser;
 mod tokenizer;
+mod typechecker;
 
 use std::{
     fs,
@@ -13,55 +14,15 @@ use tokenizer::ZernError;
 
 use clap::Parser;
 
-macro_rules! compile_std {
-    ($codegen:expr, $( $name:literal ),* $(,)? ) => {
-        $(
-            compile_file_to(
-                $codegen,
-                $name,
-                include_str!(concat!("std/", $name)).into()
-            )?;
-        )*
+macro_rules! parse_std_file {
+    ($statements:expr, $filename:expr) => {
+        let source: String = include_str!($filename).into();
+        let tokenizer = tokenizer::Tokenizer::new($filename.to_owned(), source);
+        let parser = parser::Parser::new(tokenizer.tokenize()?);
+        $statements.extend(parser.parse()?);
     };
 }
 
-fn compile_file_to(
-    codegen: &mut codegen_x86_64::CodegenX86_64,
-    filename: &str,
-    source: String,
-) -> Result<(), ZernError> {
-    let tokenizer = tokenizer::Tokenizer::new(filename.to_owned(), source);
-    let tokens = tokenizer.tokenize()?;
-
-    let parser = parser::Parser::new(tokens);
-    let statements = parser.parse()?;
-
-    for stmt in &statements {
-        codegen.analyzer.register_function(stmt)?;
-    }
-
-    for stmt in &statements {
-        codegen.analyzer.analyze_stmt(stmt)?;
-    }
-
-    for stmt in statements {
-        // top level statements are all function/const/extern declarations so a new env for each
-        codegen.compile_stmt(&mut codegen_x86_64::Env::new(), &stmt)?;
-    }
-    Ok(())
-}
-
-fn run_command(cmd: String) {
-    if !Command::new("sh")
-        .args(["-c", &cmd])
-        .status()
-        .unwrap()
-        .success()
-    {
-        process::exit(1);
-    }
-}
-
 fn compile_file(args: Args) -> Result<(), ZernError> {
     let source = match fs::read_to_string(&args.path) {
         Ok(x) => x,
@@ -73,10 +34,34 @@ fn compile_file(args: Args) -> Result<(), ZernError> {
 
     let filename = Path::new(&args.path).file_name().unwrap().to_str().unwrap();
 
-    let mut codegen = codegen_x86_64::CodegenX86_64::new();
+    let mut statements = Vec::new();
+
+    parse_std_file!(statements, "std/syscalls.zr");
+    parse_std_file!(statements, "std/std.zr");
+    parse_std_file!(statements, "std/net.zr");
+
+    let tokenizer = tokenizer::Tokenizer::new(filename.to_owned(), source);
+    let parser = parser::Parser::new(tokenizer.tokenize()?);
+    statements.extend(parser.parse()?);
+
+    let mut analyzer = analyzer::Analyzer::new();
+    for stmt in &statements {
+        analyzer.register_function(stmt)?;
+    }
+    for stmt in &statements {
+        analyzer.analyze_stmt(stmt)?;
+    }
+
+    let mut typechecker = typechecker::TypeChecker::new(&analyzer);
+    for stmt in &statements {
+        typechecker.typecheck_stmt(&mut typechecker::Env::new(), stmt)?;
+    }
+
+    let mut codegen = codegen_x86_64::CodegenX86_64::new(&analyzer);
     codegen.emit_prologue(args.use_gcc)?;
-    compile_std!(&mut codegen, "syscalls.zr", "std.zr", "net.zr");
-    compile_file_to(&mut codegen, filename, source)?;
+    for stmt in statements {
+        codegen.compile_stmt(&mut codegen_x86_64::Env::new(), &stmt)?;
+    }
 
     if !args.output_asm {
         let out = args.out.unwrap_or_else(|| "out".into());
@@ -116,6 +101,17 @@ fn compile_file(args: Args) -> Result<(), ZernError> {
     Ok(())
 }
 
+fn run_command(cmd: String) {
+    if !Command::new("sh")
+        .args(["-c", &cmd])
+        .status()
+        .unwrap()
+        .success()
+    {
+        process::exit(1);
+    }
+}
+
 #[derive(Parser, Debug)]
 #[command(version, about, long_about = None)]
 struct Args {
diff --git a/src/parser.rs b/src/parser.rs
index da88eaf..142f52c 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -20,11 +20,13 @@ pub enum Stmt {
     },
     Block(Vec<Stmt>),
     If {
+        keyword: Token,
         condition: Expr,
         then_branch: Box<Stmt>,
         else_branch: Box<Stmt>,
     },
     While {
+        keyword: Token,
         condition: Expr,
         body: Box<Stmt>,
     },
@@ -41,7 +43,10 @@ pub enum Stmt {
         body: Box<Stmt>,
         exported: bool,
     },
-    Return(Expr),
+    Return {
+        expr: Expr,
+        keyword: Token,
+    },
     Break,
     Continue,
     Extern(Token),
@@ -83,6 +88,7 @@ pub enum Expr {
     ArrayLiteral(Vec<Expr>),
     Index {
         expr: Box<Expr>,
+        bracket: Token,
         index: Box<Expr>,
     },
     AddrOf {
@@ -94,6 +100,10 @@ pub enum Expr {
         left: Box<Expr>,
         field: Token,
     },
+    Cast {
+        expr: Box<Expr>,
+        type_name: Token,
+    },
 }
 
 pub struct Parser {
@@ -257,7 +267,11 @@ impl Parser {
         } else if self.match_token(&[TokenType::KeywordFor]) {
             self.for_statement()
         } else if self.match_token(&[TokenType::KeywordReturn]) {
-            Ok(Stmt::Return(self.expression()?))
+            let keyword = self.previous().clone();
+            Ok(Stmt::Return {
+                expr: self.expression()?,
+                keyword,
+            })
         } else if self.match_token(&[TokenType::KeywordBreak]) {
             Ok(Stmt::Break)
         } else if self.match_token(&[TokenType::KeywordContinue]) {
@@ -268,6 +282,7 @@ impl Parser {
     }
 
     fn if_statement(&mut self) -> Result<Stmt, ZernError> {
+        let keyword = self.previous().clone();
         let condition = self.expression()?;
         let then_branch = self.block()?;
         let else_branch = if self.match_token(&[TokenType::KeywordElse]) {
@@ -280,6 +295,7 @@ impl Parser {
             Box::new(Stmt::Block(vec![]))
         };
         Ok(Stmt::If {
+            keyword,
             condition,
             then_branch: Box::new(then_branch),
             else_branch,
@@ -287,9 +303,11 @@ impl Parser {
     }
 
     fn while_statement(&mut self) -> Result<Stmt, ZernError> {
+        let keyword = self.previous().clone();
         let condition = self.expression()?;
         let body = self.block()?;
         Ok(Stmt::While {
+            keyword,
             condition,
             body: Box::new(body),
         })
@@ -438,7 +456,7 @@ impl Parser {
     }
 
     fn factor(&mut self) -> Result<Expr, ZernError> {
-        let mut expr = self.unary()?;
+        let mut expr = self.cast()?;
 
         while self.match_token(&[
             TokenType::Star,
@@ -459,6 +477,20 @@ impl Parser {
         Ok(expr)
     }
 
+    fn cast(&mut self) -> Result<Expr, ZernError> {
+        let mut expr = self.unary()?;
+
+        while self.match_token(&[TokenType::KeywordAs]) {
+            let type_name = self.consume(TokenType::Identifier, "expected type after 'as'")?;
+            expr = Expr::Cast {
+                expr: Box::new(expr),
+                type_name,
+            };
+        }
+
+        Ok(expr)
+    }
+
     fn unary(&mut self) -> Result<Expr, ZernError> {
         if self.match_token(&[TokenType::Xor]) {
             let op = self.previous().clone();
@@ -504,9 +536,10 @@ impl Parser {
                 };
             } else if self.match_token(&[TokenType::LeftBracket]) {
                 let index = self.expression()?;
-                self.consume(TokenType::RightBracket, "expected ']' after index")?;
+                let bracket = self.consume(TokenType::RightBracket, "expected ']' after index")?;
                 expr = Expr::Index {
                     expr: Box::new(expr),
+                    bracket,
                     index: Box::new(index),
                 }
             } else if self.match_token(&[TokenType::Arrow]) {
diff --git a/src/std/net.zr b/src/std/net.zr
index f47df90..61bdfde 100644
--- a/src/std/net.zr
+++ b/src/std/net.zr
@@ -101,7 +101,7 @@ func net.create_udp_client?[packed_host: i64, port: i64] : net.UDPSocket
     if s->fd < 0
         mem.free(s)
         err.set(ERR_SYSCALL_FAILED, "net.create_udp_client?: failed to create a socket")
-        return 0
+        return 0 as net.UDPSocket
 
     s->addr = mem.alloc(16)
     mem.zero(s->addr, 16)
@@ -115,16 +115,16 @@ func net.create_udp_client?[packed_host: i64, port: i64] : net.UDPSocket
     s->addr[7] = packed_host & 255
     return s
 
-func net.udp_listen?[packed_host: i64, port: i64] : net.UDPSocket
+func net.create_udp_server?[packed_host: i64, port: i64] : net.UDPSocket
     err.clear()
     let s: net.UDPSocket = net.create_udp_client?(packed_host, port)
     if err.check()
-        return 0
+        return 0 as net.UDPSocket
 
     if _builtin_syscall(SYS_bind, s->fd, s->addr, 16) < 0
         net.UDPSocket.close_and_free(s)
-        err.set(ERR_SYSCALL_FAILED, "net.udp_listen?: failed to bind")
-        return 0
+        err.set(ERR_SYSCALL_FAILED, "net.create_udp_server?: failed to bind")
+        return 0 as net.UDPSocket
 
     return s
 
@@ -166,7 +166,7 @@ func net.encode_dns_name[domain: str] : io.Buffer
             let part_len: i64 = i - part_start
             buf->data[out_pos] = part_len & 0xff
             out_pos = out_pos + 1
-            mem.copy(domain + part_start, buf->data + out_pos, part_len)
+            mem.copy(domain as ptr + part_start, buf->data + out_pos, part_len)
             out_pos = out_pos + part_len
             part_start = i + 1
         i = i + 1
@@ -221,6 +221,6 @@ func net.resolve?[domain: str] : i64
 
     pos = pos + 12 // skip name(2), type(2), class(2), ttl(4), rdlength(2)
 
-    let out: i64 = net.pack_addr(pkt->data[pos], pkt->data[pos+1], pkt->data[pos+2], pkt->data[pos+3])
+    let out: i64 = net.pack_addr(pkt->data[pos] as i64, pkt->data[pos+1] as i64, pkt->data[pos+2] as i64, pkt->data[pos+3] as i64)
     net.UDPPacket.free(pkt)
     return out
diff --git a/src/std/std.zr b/src/std/std.zr
index be155bf..45651f9 100644
--- a/src/std/std.zr
+++ b/src/std/std.zr
@@ -10,13 +10,13 @@ func err.code[] : i64
     return mem.read64(_builtin_err_code())
 
 func err.msg[] : str
-    return mem.read64(_builtin_err_msg())
+    return mem.read64(_builtin_err_msg()) as str
 
 func err.check[] : bool
     return err.code() != 0
 
 func err.clear[] : void
-    err.set(0, 0)
+    err.set(0, 0 as str)
 
 func err.set[code: i64, msg: str] : void
     mem.write64(_builtin_err_code(), code)
@@ -47,13 +47,13 @@ func mem._align[x: i64] : i64
 
 func mem._split_block[blk: mem.Block, needed: i64] : void
     if blk->size >= needed + MEM_BLOCK_SIZE + 8
-        let new_blk: mem.Block = blk + MEM_BLOCK_SIZE + needed
+        let new_blk: mem.Block = (blk as ptr + MEM_BLOCK_SIZE + needed) as mem.Block
         new_blk->size = blk->size - needed - MEM_BLOCK_SIZE
         new_blk->free = true
         new_blk->next = blk->next
         new_blk->prev = blk
 
-        if blk->next
+        if blk->next as ptr
             let next: mem.Block = blk->next
             next->prev = new_blk
         else
@@ -70,25 +70,27 @@ func mem._request_space?[size: i64] : mem.Block
     // PROT_READ | PROT_WRITE = 3
     // MAP_PRIVATE | MAP_ANONYMOUS = 34
     // fd = -1, offset = 0
-    let blk: mem.Block = _builtin_syscall(SYS_mmap, 0, alloc_size, 3, 34, -1, 0)
-    if blk == -1
+    let blk_ptr: ptr = _builtin_syscall(SYS_mmap, 0, alloc_size, 3, 34, -1, 0) as ptr
+    if blk_ptr == -1
         err.set(ERR_ALLOC_FAILED, "mem._request_space: mmap failed")
-        return 0
+        return 0 as mem.Block
+    
+    let blk: mem.Block = blk_ptr as mem.Block
 
     blk->size = alloc_size - MEM_BLOCK_SIZE
     blk->free = false
-    blk->next = 0
-    blk->prev = 0
+    blk->next = 0 as mem.Block
+    blk->prev = 0 as mem.Block
 
-    let tail: mem.Block = mem.read64(_builtin_heap_tail())
-    if tail
+    let tail: mem.Block = mem.read64(_builtin_heap_tail()) as mem.Block
+    if tail as ptr
         tail->next = blk
         blk->prev = tail
 
     mem.write64(_builtin_heap_tail(), blk)
     return blk
 
-func mem.alloc?[size: i64] : ptr
+func mem.alloc?[size: i64] : any
     err.clear()
     if size <= 0
         err.set(ERR_ALLOC_FAILED, "mem.alloc? called with non-positive size")
@@ -96,12 +98,12 @@ func mem.alloc?[size: i64] : ptr
 
     size = mem._align(size)
 
-    let cur: mem.Block = mem.read64(_builtin_heap_head())
-    while cur
+    let cur: mem.Block = mem.read64(_builtin_heap_head()) as mem.Block
+    while cur as ptr
         if cur->free && cur->size >= size
             mem._split_block(cur, size)
             cur->free = false
-            return cur + MEM_BLOCK_SIZE
+            return cur as ptr + MEM_BLOCK_SIZE
         cur = cur->next
     
     let blk: mem.Block = mem._request_space?(size)
@@ -113,51 +115,51 @@ func mem.alloc?[size: i64] : ptr
     
     mem._split_block(blk, size)
     
-    return blk + MEM_BLOCK_SIZE
+    return blk as ptr + MEM_BLOCK_SIZE
 
-func mem.alloc[size: i64] : ptr
+func mem.alloc[size: i64] : any
     return must(mem.alloc?(size))
 
-func mem.free[x: ptr] : void
-    if !x
+func mem.free[x: any] : void
+    if !(x as ptr)
         return 0
 
-    let blk: mem.Block = x - MEM_BLOCK_SIZE
+    let blk: mem.Block = (x as ptr - MEM_BLOCK_SIZE) as mem.Block
     blk->free = true
 
     let next: mem.Block = blk->next
-    if next && next->free
-        let expected_next: mem.Block = blk + MEM_BLOCK_SIZE + blk->size
-        if expected_next == next
+    if next as ptr && next->free
+        let expected_next: mem.Block = (blk as ptr + MEM_BLOCK_SIZE + blk->size) as mem.Block
+        if expected_next as ptr == next as ptr
             blk->size = blk->size + MEM_BLOCK_SIZE + next->size
             blk->next = next->next
-            if next->next
+            if next->next as ptr
                 let b: mem.Block = next->next
                 b->prev = blk
-            if mem.read64(_builtin_heap_tail()) == next
+            if mem.read64(_builtin_heap_tail()) == next as ptr
                 mem.write64(_builtin_heap_tail(), blk)
 
     let prev: mem.Block = blk->prev
-    if prev && prev->free
-        let expected_blk: mem.Block = prev + MEM_BLOCK_SIZE + prev->size
-        if expected_blk == blk
+    if prev as ptr && prev->free
+        let expected_blk: mem.Block = (prev as ptr + MEM_BLOCK_SIZE + prev->size) as mem.Block
+        if expected_blk as ptr == blk as ptr
             prev->size = prev->size + MEM_BLOCK_SIZE + blk->size
             prev->next = blk->next
-            if blk->next
+            if blk->next as ptr
                 let b: mem.Block = blk->next
                 b->prev = prev
-            if mem.read64(_builtin_heap_tail()) == blk
+            if mem.read64(_builtin_heap_tail()) == blk as ptr
                 mem.write64(_builtin_heap_tail(), prev)
             blk = prev
 
     let block_total: i64 = blk->size + MEM_BLOCK_SIZE
-    if (blk & 4095) == 0 && (block_total & 4095) == 0
-        if blk->prev
+    if (blk as i64 & 4095) == 0 && (block_total & 4095) == 0
+        if blk->prev as ptr
             let b: mem.Block = blk->prev
             b->next = blk->next
         else
             mem.write64(_builtin_heap_head(), blk->next)
-        if blk->next
+        if blk->next as ptr
             let b: mem.Block = blk->next
             b->prev = blk->prev
         else
@@ -171,38 +173,38 @@ func mem.realloc?[x: ptr, new_size: i64] : ptr
     
     if new_size < 0
         err.set(ERR_ALLOC_FAILED, "mem.realloc? called with negative size")
-        return 0
+        return 0 as ptr
 
     if new_size == 0
         mem.free(x)
-        return 0
+        return 0 as ptr
 
     new_size = mem._align(new_size)
 
-    let blk: mem.Block = x - MEM_BLOCK_SIZE
+    let blk: mem.Block = (x - MEM_BLOCK_SIZE) as mem.Block
     if blk->size >= new_size
         mem._split_block(blk, new_size)
         return x
 
     let next: mem.Block = blk->next
-    let expected_next: mem.Block = blk + MEM_BLOCK_SIZE + blk->size
+    let expected_next: mem.Block = (blk as ptr + MEM_BLOCK_SIZE + blk->size) as mem.Block
     
-    if next && next->free && expected_next == next
+    if next as ptr && next->free && expected_next as ptr == next as ptr
         let combined: i64 = blk->size + MEM_BLOCK_SIZE + next->size
         if combined >= new_size
             blk->size = combined
             blk->next = next->next
-            if next->next
+            if next->next as ptr
                 let b: mem.Block = next->next
                 b->prev = blk
-            if mem.read64(_builtin_heap_tail()) == next
+            if mem.read64(_builtin_heap_tail()) == next as ptr
                 mem.write64(_builtin_heap_tail(), blk)
             mem._split_block(blk, new_size)
             return x
 
     let new_ptr: ptr = mem.alloc?(new_size)
     if err.check()
-        return 0
+        return 0 as ptr
 
     mem.copy(x, new_ptr, math.min(blk->size, new_size))
     mem.free(x)
@@ -214,7 +216,7 @@ func mem.zero_and_free[x: ptr, size: i64] : void
 
 func mem.zero[x: ptr, size: i64] : void
     for i in 0..size
-        x[i] = 0
+        x[i] = 0 as u8
 
 func mem.copy[src: ptr, dst: ptr, n: i64] : void
     if dst > src
@@ -230,16 +232,16 @@ func mem.read8[x: ptr] : u8
     return x[0]
 
 func mem.read16[x: ptr] : i64
-    return x[0] | (x[1] << 8)
+    return x[0] as i64 | (x[1] << 8)
 
 func mem.read16be[x: ptr] : i64
-    return (x[0] << 8) | x[1]
+    return (x[0] << 8) as i64 | x[1]
 
 func mem.read32[x: ptr] : i64
-    return x[0] | (x[1] << 8) | (x[2] << 16) | (x[3] << 24)
+    return x[0] as i64 | (x[1] << 8) | (x[2] << 16) | (x[3] << 24)
 
 func mem.read32be[x: ptr] : i64
-    return (x[0] << 24) | (x[1] << 16) | (x[2] << 8) | x[3]
+    return (x[0] as i64 << 24) | (x[1] << 16) | (x[2] << 8) | x[3]
 
 func mem.read64[x: ptr] : i64
     return _builtin_read64(x)
@@ -258,11 +260,11 @@ func mem.write16be[x: ptr, d: i64] : void
     x[0] = (d >> 8) & 0xff
     x[1] = d & 0xff
 
-func mem.write64[x: ptr, d: i64] : void
+func mem.write64[x: ptr, d: any] : void
     _builtin_set64(x, d)
 
 // see emit_prologue for the io.printf wrapper
-func io._printf_impl[s: str, args: ptr] : void
+func io._printf_impl[s: ptr, args: ptr] : void
     while s[0]
         if s[0] == '%'
             s = s + 1
@@ -274,10 +276,10 @@ func io._printf_impl[s: str, args: ptr] : void
                 io.print_i64_hex(mem.read64(args))
                 args = args + 8
             else if s[0] == 's'
-                io.print(mem.read64(args))
+                io.print(mem.read64(args) as str)
                 args = args + 8
             else if s[0] == 'c'
-                io.print_char(mem.read64(args))
+                io.print_char(mem.read64(args) as u8)
                 args = args + 8
             else if s[0] == '%'
                 io.print_char('%')
@@ -287,11 +289,11 @@ func io._printf_impl[s: str, args: ptr] : void
             io.print_char(s[0])
         s = s + 1
 
-func io.print_sized[x: str, size: i64] : void
+func io.print_sized[x: ptr, size: i64] : void
     _builtin_syscall(SYS_write, 1, x, size)
 
 func io.print[x: str] : void
-    io.print_sized(x, str.len(x))
+    io.print_sized(x as ptr, str.len(x))
 
 func io.println[x: str] : void
     io.print(x)
@@ -322,7 +324,7 @@ func io.println_i64[x: i64] : void
     mem.free(s)
 
 func io.read_char[] : u8
-    let c: u8 = 0
+    let c: u8 = 0 as u8
     _builtin_syscall(SYS_read, 0, ^c, 1)
     return c
 
@@ -333,7 +335,7 @@ func io.read_line[]: str
     if n < 0
         n = 0
     buffer[n] = 0
-    buffer = must(mem.realloc?(buffer, n + 1))
+    buffer = must(mem.realloc?(buffer as ptr, n + 1))
     return buffer
 
 struct io.Buffer
@@ -347,7 +349,7 @@ func io.Buffer.alloc?[size: i64] : io.Buffer
     buffer->data = mem.alloc?(size)
     if err.check()
         mem.free(buffer)
-        return 0
+        return 0 as io.Buffer
     return buffer
 
 func io.Buffer.free[buf: io.Buffer] : void
@@ -362,15 +364,15 @@ func io.read_text_file?[path: str] : str
     let fd: i64 = _builtin_syscall(SYS_openat, -100, path, 0, 0)
     if fd < 0
         err.set(ERR_OPEN_FAILED, "io.read_text_file?: failed to open file")
-        return 0
+        return 0 as str
 
     let size: i64 = _builtin_syscall(SYS_lseek, fd, 0, 2)
     _builtin_syscall(SYS_lseek, fd, 0, 0)
 
-    let buffer: str = mem.alloc?(size + 1)
+    let buffer: str = mem.alloc?(size + 1) as str
     if err.check()
         _builtin_syscall(SYS_close, fd)
-        return 0
+        return 0 as str
 
     let n: i64 = _builtin_syscall(SYS_read, fd, buffer, size)
     _builtin_syscall(SYS_close, fd)
@@ -378,7 +380,7 @@ func io.read_text_file?[path: str] : str
     if n != size
         mem.free(buffer)
         err.set(ERR_READ_FAILED, "io.read_text_file?: failed to read file")
-        return 0
+        return 0 as str
 
     buffer[n] = 0
     return buffer
@@ -388,7 +390,7 @@ func io.read_binary_file?[path: str] : io.Buffer
     let fd: i64 = _builtin_syscall(SYS_openat, -100, path, 0, 0)
     if fd < 0
         err.set(ERR_OPEN_FAILED, "io.read_binary_file?: failed to open file")
-        return 0
+        return 0 as io.Buffer
     
     let buf: io.Buffer = new io.Buffer
     buf->size = _builtin_syscall(SYS_lseek, fd, 0, 2)
@@ -398,7 +400,7 @@ func io.read_binary_file?[path: str] : io.Buffer
     if err.check()
         io.Buffer.free(buf)
         _builtin_syscall(SYS_close, fd)
-        return 0
+        return 0 as io.Buffer
 
     let n: i64 = _builtin_syscall(SYS_read, fd, buf->data, buf->size)
     _builtin_syscall(SYS_close, fd)
@@ -406,12 +408,12 @@ func io.read_binary_file?[path: str] : io.Buffer
     if n != buf->size
         io.Buffer.free(buf)
         err.set(ERR_READ_FAILED, "io.read_binary_file?: failed to read file")
-        return 0
+        return 0 as io.Buffer
 
     return buf
 
 func io.write_file?[path: str, content: str] : void
-    io.write_binary_file?(path, content, str.len(content))
+    io.write_binary_file?(path, content as ptr, str.len(content))
 
 func io.write_binary_file?[path: str, content: ptr, size: i64] : void
     err.clear()
@@ -435,7 +437,7 @@ func str.len[s: str] : i64
 func str.make_copy[s: str] : str
     let size: i64 = str.len(s) + 1
     let dup: str = mem.alloc(size)
-    mem.copy(s, dup, size)
+    mem.copy(s as ptr, dup as ptr, size)
     return dup
 
 func str.equal[a: str, b: str] : bool
@@ -471,8 +473,8 @@ func str.concat[a: str, b: str] : str
     let a_len: i64 = str.len(a)
     let b_len: i64 = str.len(b)
     let out: str = mem.alloc(a_len + b_len + 1)
-    mem.copy(a, out, a_len)
-    mem.copy(b, out + a_len, b_len)
+    mem.copy(a as ptr, out as ptr, a_len)
+    mem.copy(b as ptr, out as ptr + a_len, b_len)
     out[a_len + b_len] = 0
     return out
 
@@ -504,7 +506,7 @@ func str.substr[s: str, start: i64, length: i64] : str
         panic("str.substr out of bounds")
 
     let out: str = mem.alloc(length + 1)
-    mem.copy(s + start, out, length)
+    mem.copy(s as ptr + start, out as ptr, length)
     out[length] = 0
     return out
 
@@ -591,7 +593,7 @@ func str.from_i64[n: i64] : str
     if neg
         buf[end] = '-'
         end = end - 1
-    let s: str = str.make_copy(buf + end + 1)
+    let s: str = str.make_copy((buf as ptr + end + 1) as str)
     mem.free(buf)
     return s
 
@@ -662,7 +664,7 @@ func str.hex_encode[s: str, s_len: i64] : str
     out[j] = 0
     return out
 
-func str._hex_digit_to_int[d: u8] : i64
+func str._hex_digit_to_int[d: u8] : u8
     if d >= 'a' && d <= 'f'
         return d - 'a' + 10
     if d >= 'A' && d <= 'F'
@@ -769,10 +771,10 @@ struct Array
 func array.new[] : Array
     return new Array
 
-func array.new_with_size_zeroed[size: i64] : Array
+func array.new_preallocated_and_zeroed[size: i64] : Array
     let xs: Array = new Array
     if size > 0
-        xs->data = must(mem.realloc?(xs->data, size * 8))
+        xs->data = must(mem.realloc?(xs->data, size * 8)) as ptr
         mem.zero(xs->data, size * 8)
         xs->size = size
         xs->capacity = size
@@ -815,13 +817,13 @@ func array.slice[xs: Array, start: i64, length: i64] : Array
     if start < 0 || length < 0 || start + length > xs->size
         panic("array.slice out of bounds")
 
-    let new_array: Array = array.new_with_size_zeroed(length)
+    let new_array: Array = array.new_preallocated_and_zeroed(length)
     for i in 0..length
         array.set(new_array, i, array.nth(xs, start + i))
     return new_array
 
 func array.concat[a: Array, b: Array] : Array
-    let new_array: Array = array.new_with_size_zeroed(a->size + b->size)
+    let new_array: Array = array.new_preallocated_and_zeroed(a->size + b->size)
     for i in 0..a->size
         array.set(new_array, i, array.nth(a, i))
     for i in 0..b->size
@@ -841,7 +843,7 @@ func alg._partition[arr: Array, low: i64, high: i64] : i64
     let pivot: i64 = array.nth(arr, high)
     let i: i64 = low - 1
     for j in (low)..high
-        if array.nth(arr, j) <= pivot
+        if array.nth(arr, j) as i64 <= pivot
             i = i + 1
             let temp: i64 = array.nth(arr ,i)
             array.set(arr, i, array.nth(arr, j))
@@ -858,20 +860,20 @@ func alg.count[arr: Array, item: any] : i64
             count = count + 1
     return count
 
-func alg.map[arr: Array, fn: ptr] : Array
-    let out: Array = array.new_with_size_zeroed(arr->size)
+func alg.map[arr: Array, fn: fnptr] : Array
+    let out: Array = array.new_preallocated_and_zeroed(arr->size)
     for i in 0..arr->size
         array.set(out, i, fn(array.nth(arr, i)))
     return out
 
-func alg.filter[arr: Array, fn: ptr] : Array
+func alg.filter[arr: Array, fn: fnptr] : Array
     let out: Array = []
     for i in 0..arr->size
         if fn(array.nth(arr, i))
             array.push(out, array.nth(arr, i))
     return out
 
-func alg.reduce[arr: Array, fn: ptr, acc: any] : any
+func alg.reduce[arr: Array, fn: fnptr, acc: any] : any
     for i in 0..arr->size
         acc = fn(acc, array.nth(arr, i))
     return acc
@@ -900,13 +902,13 @@ func os.urandom?[n: i64]: ptr
     err.clear()
     let buffer: ptr = mem.alloc(n)
     if err.check()
-        return 0
+        return 0 as ptr
 
     let fd: i64 = _builtin_syscall(SYS_openat, -100, "/dev/urandom", 0, 0)
     if fd < 0
         mem.free(buffer)
         err.set(ERR_READ_FAILED, "os.urandom?: failed to open /dev/urandom")
-        return 0
+        return 0 as ptr
 
     let bytes_read: i64 = _builtin_syscall(SYS_read, fd, buffer, n)
     _builtin_syscall(SYS_close, fd)
@@ -914,7 +916,7 @@ func os.urandom?[n: i64]: ptr
     if n != bytes_read
         mem.free(buffer)
         err.set(ERR_READ_FAILED, "os.urandom?: failed to read /dev/urandom")
-        return 0
+        return 0 as ptr
     
     return buffer
 
@@ -966,7 +968,7 @@ func os.list_directory?[path: str] : Array
     let fd: i64 = _builtin_syscall(SYS_openat, -100, path, 0, 0)
     if fd < 0
         err.set(ERR_OPEN_FAILED, "os.list_directory?: failed to open dir")
-        return 0
+        return 0 as Array
 
     let files: Array = []
     let buf: ptr = mem.alloc(1024)
@@ -981,14 +983,14 @@ func os.list_directory?[path: str] : Array
 
             _builtin_syscall(SYS_close, fd)
             err.set(ERR_SYSCALL_FAILED, "os.list_directory?: getdents64() failed")
-            return 0
+            return 0 as Array
         else if n == 0
             break
 
         let pos = 0
         while pos < n
             let len: i64 = mem.read16(buf + pos + 16)
-            let name: str = buf + pos + 19
+            let name: ptr = buf + pos + 19
             if name[0]
                 let skip: bool = false
                 // skip if name is exactly '.' or '..'
@@ -999,7 +1001,7 @@ func os.list_directory?[path: str] : Array
                         if name[2] == 0
                             skip = true
                 if !skip
-                    array.push(files, str.make_copy(name))
+                    array.push(files, str.make_copy(name as str))
             pos = pos + len
 
     mem.free(buf)
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index 3056a7a..445c93d 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -55,6 +55,7 @@ pub enum TokenType {
     KeywordExport,
     KeywordStruct,
     KeywordNew,
+    KeywordAs,
 
     Indent,
     Dedent,
@@ -371,6 +372,7 @@ impl Tokenizer {
             "export" => TokenType::KeywordExport,
             "struct" => TokenType::KeywordStruct,
             "new" => TokenType::KeywordNew,
+            "as" => TokenType::KeywordAs,
             "true" => TokenType::True,
             "false" => TokenType::False,
             _ => TokenType::Identifier,
diff --git a/src/typechecker.rs b/src/typechecker.rs
new file mode 100644
index 0000000..07fb7a7
--- /dev/null
+++ b/src/typechecker.rs
@@ -0,0 +1,481 @@
+use std::collections::HashMap;
+
+use crate::{
+    analyzer::{Analyzer, Type},
+    parser::{Expr, Stmt},
+    tokenizer::{TokenType, ZernError, error},
+};
+
+macro_rules! expect_type {
+    ($expr_type:expr, $expected:expr, $loc:expr) => {{
+        let actual = $expr_type;
+        if $expected != "any" && actual != "any" && actual != $expected {
+            return error!(
+                $loc,
+                format!("expected type '{}', got '{}'", $expected, actual)
+            );
+        }
+    }};
+}
+
+macro_rules! expect_types {
+    ($expr_type:expr, [$( $expected:expr ),+], $loc:expr) => {
+        if $expr_type != "any" && $( $expr_type != $expected )&&+ {
+            return error!(
+                $loc,
+                format!(
+                    "expected one of [{}], got '{}'",
+                    [$( $expected ),+].join(", "),
+                    $expr_type
+                )
+            );
+        }
+    };
+}
+
+static BUILTIN_TYPES: [&str; 8] = ["void", "u8", "i64", "str", "bool", "ptr", "fnptr", "any"];
+
+pub struct Env {
+    scopes: Vec<HashMap<String, Type>>,
+}
+
+impl Env {
+    pub fn new() -> Env {
+        Env {
+            scopes: vec![HashMap::new()],
+        }
+    }
+
+    pub fn push_scope(&mut self) {
+        self.scopes.push(HashMap::new());
+    }
+
+    pub fn pop_scope(&mut self) {
+        assert!(!self.scopes.is_empty());
+        self.scopes.pop();
+    }
+
+    pub fn define_var(&mut self, name: String, var_type: String) {
+        self.scopes.last_mut().unwrap().insert(name, var_type);
+    }
+
+    fn get_var_type(&self, name: &str) -> Option<&Type> {
+        for scope in self.scopes.iter().rev() {
+            if let Some(var) = scope.get(name) {
+                return Some(var);
+            }
+        }
+        None
+    }
+}
+
+pub struct TypeChecker<'a> {
+    analyzer: &'a Analyzer,
+    current_function_return_type: String,
+}
+
+impl<'a> TypeChecker<'a> {
+    pub fn new(analyzer: &'a Analyzer) -> TypeChecker<'a> {
+        TypeChecker {
+            analyzer,
+            current_function_return_type: String::new(),
+        }
+    }
+
+    pub fn typecheck_stmt(&mut self, env: &mut Env, stmt: &Stmt) -> Result<(), ZernError> {
+        match stmt {
+            Stmt::Expression(expr) => {
+                self.typecheck_expr(env, expr)?;
+            }
+            Stmt::Let {
+                name,
+                var_type,
+                initializer,
+            } => {
+                let mut actual_type = self.typecheck_expr(env, initializer)?;
+                if let Some(var_type) = var_type {
+                    if !self.is_valid_type_name(&var_type.lexeme) {
+                        return error!(
+                            &name.loc,
+                            "unrecognized type: ".to_owned() + &var_type.lexeme
+                        );
+                    }
+                    expect_type!(actual_type.clone(), var_type.lexeme, var_type.loc);
+
+                    if actual_type == "any" {
+                        actual_type = var_type.lexeme.clone();
+                    }
+                }
+
+                env.define_var(name.lexeme.clone(), actual_type);
+            }
+            Stmt::Const { name: _, value: _ } => {
+                // handled in the analyzer
+            }
+            Stmt::Block(stmts) => {
+                env.push_scope();
+                for stmt in stmts {
+                    self.typecheck_stmt(env, stmt)?;
+                }
+                env.pop_scope();
+            }
+            Stmt::If {
+                keyword,
+                condition,
+                then_branch,
+                else_branch,
+            } => {
+                expect_types!(
+                    self.typecheck_expr(env, condition)?,
+                    ["i64", "u8", "ptr", "bool"],
+                    keyword.loc
+                );
+                self.typecheck_stmt(env, then_branch)?;
+                self.typecheck_stmt(env, else_branch)?;
+            }
+            Stmt::While {
+                keyword,
+                condition,
+                body,
+            } => {
+                expect_types!(
+                    self.typecheck_expr(env, condition)?,
+                    ["i64", "u8", "ptr", "bool"],
+                    keyword.loc
+                );
+                self.typecheck_stmt(env, body)?;
+            }
+            Stmt::For {
+                var,
+                start,
+                end,
+                body,
+            } => {
+                expect_type!(self.typecheck_expr(env, start)?, "i64", var.loc);
+                expect_type!(self.typecheck_expr(env, end)?, "i64", var.loc);
+
+                env.push_scope();
+                env.define_var(var.lexeme.clone(), "i64".into());
+                self.typecheck_stmt(env, body)?;
+                env.pop_scope();
+            }
+            Stmt::Function {
+                name: _,
+                params,
+                return_type,
+                body,
+                exported: _,
+            } => {
+                if !self.is_valid_type_name(&return_type.lexeme) {
+                    return error!(
+                        &return_type.loc,
+                        "unrecognized type: ".to_owned() + &return_type.lexeme
+                    );
+                }
+
+                self.current_function_return_type = return_type.lexeme.clone();
+
+                env.push_scope();
+
+                for param in params {
+                    if !self.is_valid_type_name(&param.var_type.lexeme) {
+                        return error!(
+                            &param.var_name.loc,
+                            "unrecognized type: ".to_owned() + &param.var_type.lexeme
+                        );
+                    }
+
+                    env.define_var(param.var_name.lexeme.clone(), param.var_type.lexeme.clone());
+                }
+
+                self.typecheck_stmt(env, body)?;
+
+                env.pop_scope();
+            }
+            Stmt::Return { expr, keyword } => {
+                let expected = if self.current_function_return_type == "void" {
+                    "i64".into()
+                } else {
+                    self.current_function_return_type.clone()
+                };
+                expect_type!(self.typecheck_expr(env, expr)?, expected, keyword.loc);
+            }
+            Stmt::Break => {}
+            Stmt::Continue => {}
+            Stmt::Extern(_) => {
+                // handled in the analyzer
+            }
+            Stmt::Struct { name: _, fields } => {
+                for field in fields {
+                    if !self.is_valid_type_name(&field.var_type.lexeme) {
+                        return error!(
+                            &field.var_type.loc,
+                            format!("unknown type: {}", &field.var_type.lexeme)
+                        );
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+
+    pub fn typecheck_expr(&self, env: &mut Env, expr: &Expr) -> Result<Type, ZernError> {
+        match expr {
+            Expr::Binary { left, op, right } => {
+                let left_type = self.typecheck_expr(env, left)?;
+
+                match op.token_type {
+                    TokenType::Plus | TokenType::Minus => {
+                        expect_types!(left_type, ["i64", "ptr", "u8"], op.loc);
+                        expect_types!(
+                            self.typecheck_expr(env, right)?,
+                            ["i64", "ptr", "u8"],
+                            op.loc
+                        );
+                        Ok(left_type)
+                    }
+                    TokenType::Star
+                    | TokenType::Slash
+                    | TokenType::Mod
+                    | TokenType::Xor
+                    | TokenType::BitAnd
+                    | TokenType::BitOr
+                    | TokenType::ShiftLeft
+                    | TokenType::ShiftRight => {
+                        expect_types!(left_type, ["i64", "u8"], op.loc);
+                        expect_types!(self.typecheck_expr(env, right)?, ["i64", "u8"], op.loc);
+                        Ok(left_type)
+                    }
+                    TokenType::DoubleEqual
+                    | TokenType::NotEqual
+                    | TokenType::Greater
+                    | TokenType::GreaterEqual
+                    | TokenType::Less
+                    | TokenType::LessEqual => {
+                        expect_types!(left_type, ["i64", "ptr", "u8"], op.loc);
+                        expect_types!(
+                            self.typecheck_expr(env, right)?,
+                            ["i64", "ptr", "u8"],
+                            op.loc
+                        );
+                        Ok("bool".into())
+                    }
+                    _ => unreachable!(),
+                }
+            }
+            Expr::Logical { left, op, right } => {
+                expect_types!(
+                    self.typecheck_expr(env, left)?,
+                    ["bool", "i64", "ptr"],
+                    op.loc
+                );
+                expect_types!(
+                    self.typecheck_expr(env, right)?,
+                    ["bool", "i64", "ptr"],
+                    op.loc
+                );
+                Ok("bool".into())
+            }
+            Expr::Grouping(expr) => self.typecheck_expr(env, expr),
+            Expr::Literal(token) => match token.token_type {
+                TokenType::Number => Ok("i64".into()),
+                TokenType::Char => Ok("u8".into()),
+                TokenType::String => Ok("str".into()),
+                TokenType::True => Ok("bool".into()),
+                TokenType::False => Ok("bool".into()),
+                _ => unreachable!(),
+            },
+            Expr::Unary { op, right } => {
+                let right_type = self.typecheck_expr(env, right)?;
+                match op.token_type {
+                    TokenType::Minus => {
+                        expect_type!(right_type, "i64", op.loc);
+                        Ok("i64".into())
+                    }
+                    TokenType::Bang => {
+                        expect_types!(right_type, ["bool", "i64", "ptr", "u8"], op.loc);
+                        Ok("bool".into())
+                    }
+                    _ => unreachable!(),
+                }
+            }
+            Expr::Variable(name) => {
+                if self.analyzer.constants.contains_key(&name.lexeme) {
+                    Ok("i64".into())
+                } else {
+                    match env.get_var_type(&name.lexeme) {
+                        Some(x) => Ok(x.clone()),
+                        None => error!(name.loc, format!("undefined variable: {}", &name.lexeme)),
+                    }
+                }
+            }
+            Expr::Assign { left, op, value } => {
+                let value_type = self.typecheck_expr(env, value)?;
+
+                match left.as_ref() {
+                    Expr::Variable(name) => {
+                        let existing_var_type = match env.get_var_type(&name.lexeme) {
+                            Some(x) => x,
+                            None => {
+                                return error!(
+                                    name.loc,
+                                    format!("undefined variable: {}", &name.lexeme)
+                                );
+                            }
+                        };
+                        expect_type!(value_type.clone(), *existing_var_type, name.loc);
+                    }
+                    Expr::Index {
+                        expr,
+                        bracket,
+                        index,
+                    } => {
+                        expect_types!(self.typecheck_expr(env, expr)?, ["ptr", "str"], bracket.loc);
+                        expect_types!(self.typecheck_expr(env, index)?, ["i64", "u8"], bracket.loc);
+                        expect_types!(value_type.clone(), ["u8", "i64"], bracket.loc);
+                    }
+                    Expr::MemberAccess { left, field } => {
+                        let left_type = self.typecheck_expr(env, left)?;
+
+                        let fields = match self.analyzer.structs.get(&left_type) {
+                            Some(f) => f,
+                            None => {
+                                return error!(
+                                    &field.loc,
+                                    format!("unknown struct type: {}", left_type)
+                                );
+                            }
+                        };
+
+                        let f = match fields.get(&field.lexeme) {
+                            Some(o) => o,
+                            None => {
+                                return error!(
+                                    &field.loc,
+                                    format!("unknown field: {}", &field.lexeme)
+                                );
+                            }
+                        };
+
+                        expect_type!(value_type.clone(), f.field_type, field.loc);
+                    }
+                    _ => return error!(&op.loc, "invalid assignment target"),
+                }
+                Ok(value_type)
+            }
+            Expr::Call {
+                callee,
+                paren,
+                args,
+            } => {
+                if let Expr::Variable(callee_name) = &**callee {
+                    if self.analyzer.functions.contains_key(&callee_name.lexeme) {
+                        let fn_type = &self.analyzer.functions[&callee_name.lexeme];
+                        // its a function (defined/builtin/extern)
+                        if let Some(params) = fn_type.params.clone() {
+                            for (i, arg) in args.iter().enumerate() {
+                                // arity is checked in the analyzer
+                                expect_type!(self.typecheck_expr(env, arg)?, params[i], paren.loc);
+                            }
+                        } else {
+                            // its a variadic function, cant check arg types
+                            for arg in args {
+                                self.typecheck_expr(env, arg)?;
+                            }
+                        }
+                        Ok(fn_type.return_type.clone())
+                    } else {
+                        // its a variable containing function address
+                        expect_type!(self.typecheck_expr(env, callee)?, "fnptr", paren.loc);
+
+                        for arg in args {
+                            self.typecheck_expr(env, arg)?;
+                        }
+                        Ok("any".into())
+                    }
+                } else {
+                    // its an expression that evalutes to function address
+                    expect_type!(self.typecheck_expr(env, callee)?, "fnptr", paren.loc);
+
+                    for arg in args {
+                        self.typecheck_expr(env, arg)?;
+                    }
+                    Ok("any".into())
+                }
+            }
+            Expr::ArrayLiteral(exprs) => {
+                for expr in exprs {
+                    self.typecheck_expr(env, expr)?;
+                }
+                Ok("Array".into())
+            }
+            Expr::Index {
+                expr,
+                bracket,
+                index,
+            } => {
+                expect_types!(self.typecheck_expr(env, expr)?, ["ptr", "str"], bracket.loc);
+                expect_types!(self.typecheck_expr(env, index)?, ["i64", "u8"], bracket.loc);
+                Ok("u8".into())
+            }
+            Expr::AddrOf { op, expr } => match expr.as_ref() {
+                Expr::Variable(name) => {
+                    if self.analyzer.functions.contains_key(&name.lexeme) {
+                        Ok("fnptr".into())
+                    } else {
+                        Ok("ptr".into())
+                    }
+                }
+                _ => {
+                    error!(&op.loc, "can only take address of variables and functions")
+                }
+            },
+            Expr::New(struct_name) => {
+                if !self.analyzer.structs.contains_key(&struct_name.lexeme) {
+                    return error!(
+                        &struct_name.loc,
+                        format!("unknown struct name: {}", &struct_name.lexeme)
+                    );
+                }
+                Ok(struct_name.lexeme.clone())
+            }
+            Expr::MemberAccess { left, field } => {
+                let left_type = self.typecheck_expr(env, left)?;
+
+                let fields = match self.analyzer.structs.get(&left_type) {
+                    Some(f) => f,
+                    None => {
+                        return error!(&field.loc, format!("unknown struct type: {}", left_type));
+                    }
+                };
+
+                let field = match fields.get(&field.lexeme) {
+                    Some(o) => o,
+                    None => return error!(&field.loc, format!("unknown field: {}", &field.lexeme)),
+                };
+
+                Ok(field.field_type.clone())
+            }
+            Expr::Cast { expr, type_name } => {
+                self.typecheck_expr(env, expr)?;
+                if !self.is_valid_type_name(&type_name.lexeme) {
+                    return error!(
+                        &type_name.loc,
+                        format!("unknown type: {}", &type_name.lexeme)
+                    );
+                }
+                Ok(type_name.lexeme.clone())
+            }
+        }
+    }
+
+    fn is_valid_type_name(&self, name: &str) -> bool {
+        if BUILTIN_TYPES.contains(&name) {
+            return true;
+        }
+        if self.analyzer.structs.contains_key(name) {
+            return true;
+        }
+        false
+    }
+}