diff --git a/nac3core/src/codegen/expr.rs b/nac3core/src/codegen/expr.rs
index 8e629f9b0..26ec835c4 100644
--- a/nac3core/src/codegen/expr.rs
+++ b/nac3core/src/codegen/expr.rs
@@ -1,12 +1,41 @@
 use std::{convert::TryInto, iter::once};
 
-use crate::top_level::{CodeGenContext, TopLevelDef};
-use crate::typecheck::typedef::{Type, TypeEnum};
-use inkwell::{types::BasicType, values::BasicValueEnum};
+use crate::{
+    top_level::DefinitionId,
+    typecheck::typedef::{Type, TypeEnum},
+};
+use crate::{
+    top_level::{CodeGenContext, TopLevelDef},
+    typecheck::typedef::FunSignature,
+};
+use inkwell::{
+    types::{BasicType, BasicTypeEnum},
+    values::BasicValueEnum,
+    AddressSpace,
+};
 use itertools::{chain, izip, zip, Itertools};
 use rustpython_parser::ast::{self, Boolop, Constant, Expr, ExprKind, Operator};
 
 impl<'ctx> CodeGenContext<'ctx> {
+    fn get_subst_key(&mut self, obj: Option<Type>, fun: &FunSignature) -> String {
+        let mut vars = obj
+            .map(|ty| {
+                if let TypeEnum::TObj { params, .. } = &*self.unifier.get_ty(ty) {
+                    params.clone()
+                } else {
+                    unreachable!()
+                }
+            })
+            .unwrap_or_default();
+        vars.extend(fun.vars.iter());
+        let sorted = vars.keys().sorted();
+        sorted
+            .map(|id| {
+                self.unifier.stringify(vars[id], &mut |id| id.to_string(), &mut |id| id.to_string())
+            })
+            .join(", ")
+    }
+
     fn get_attr_index(&mut self, ty: Type, attr: &str) -> usize {
         let obj_id = match &*self.unifier.get_ty(ty) {
             TypeEnum::TObj { obj_id, .. } => *obj_id,
@@ -22,17 +51,88 @@ impl<'ctx> CodeGenContext<'ctx> {
         index
     }
 
+    fn get_llvm_type(&mut self, ty: Type) -> BasicTypeEnum<'ctx> {
+        use TypeEnum::*;
+        // we assume the type cache should already contain primitive types,
+        // and they should be passed by value instead of passing as pointer.
+        self.type_cache.get(&ty).cloned().unwrap_or_else(|| match &*self.unifier.get_ty(ty) {
+            TObj { obj_id, fields, .. } => {
+                // a struct with fields in the order of declaration
+                let defs = self.top_level.definitions.read();
+                let definition = defs.get(obj_id.0).unwrap();
+                let ty = if let TopLevelDef::Class { fields: fields_list, .. } = &*definition.read()
+                {
+                    let fields = fields.borrow();
+                    let fields =
+                        fields_list.iter().map(|f| self.get_llvm_type(fields[&f.0])).collect_vec();
+                    self.ctx
+                        .struct_type(&fields, false)
+                        .ptr_type(AddressSpace::Generic)
+                        .into()
+                } else {
+                    unreachable!()
+                };
+                ty
+            }
+            TTuple { ty } => {
+                // a struct with fields in the order present in the tuple
+                let fields = ty.iter().map(|ty| self.get_llvm_type(*ty)).collect_vec();
+                self.ctx.struct_type(&fields, false).ptr_type(AddressSpace::Generic).into()
+            }
+            TList { ty } => {
+                // a struct with an integer and a pointer to an array
+                let element_type = self.get_llvm_type(*ty);
+                let fields = [
+                    self.ctx.i32_type().into(),
+                    element_type.ptr_type(AddressSpace::Generic).into(),
+                ];
+                self.ctx.struct_type(&fields, false).ptr_type(AddressSpace::Generic).into()
+            }
+            _ => unreachable!(),
+        })
+    }
+
+    fn gen_call(
+        &mut self,
+        obj: Option<(Type, BasicValueEnum<'ctx>)>,
+        fun: (&FunSignature, DefinitionId),
+        params: &[BasicValueEnum<'ctx>],
+        ret: Type,
+    ) -> Option<BasicValueEnum<'ctx>> {
+        let key = self.get_subst_key(obj.map(|(a, _)| a), fun.0);
+        let defs = self.top_level.definitions.read();
+        let definition = defs.get(fun.1.0).unwrap();
+        let val = if let TopLevelDef::Function { instance_to_symbol, .. } = &*definition.read() {
+            // TODO: codegen for function that are not yet generated
+            let symbol = instance_to_symbol.get(&key).unwrap();
+            let fun_val = self.module.get_function(symbol).unwrap_or_else(|| {
+                let params = fun.0.args.iter().map(|arg| self.get_llvm_type(arg.ty)).collect_vec();
+                let fun_ty = if self.unifier.unioned(ret, self.primitives.none) {
+                    self.ctx.void_type().fn_type(&params, false)
+                } else {
+                    self.get_llvm_type(ret).fn_type(&params, false)
+                };
+                self.module.add_function(symbol, fun_ty, None)
+            });
+            // TODO: deal with default parameters and reordering based on keys
+            self.builder.build_call(fun_val, params, "call").try_as_basic_value().left()
+        } else {
+            unreachable!()
+        };
+        val
+    }
+
     fn gen_const(&mut self, value: &Constant, ty: Type) -> BasicValueEnum<'ctx> {
         match value {
             Constant::Bool(v) => {
-                assert!(self.unifier.unioned(ty, self.top_level.primitives.bool));
+                assert!(self.unifier.unioned(ty, self.primitives.bool));
                 let ty = self.ctx.bool_type();
                 ty.const_int(if *v { 1 } else { 0 }, false).into()
             }
             Constant::Int(v) => {
-                let ty = if self.unifier.unioned(ty, self.top_level.primitives.int32) {
+                let ty = if self.unifier.unioned(ty, self.primitives.int32) {
                     self.ctx.i32_type()
-                } else if self.unifier.unioned(ty, self.top_level.primitives.int64) {
+                } else if self.unifier.unioned(ty, self.primitives.int64) {
                     self.ctx.i64_type()
                 } else {
                     unreachable!();
@@ -40,7 +140,7 @@ impl<'ctx> CodeGenContext<'ctx> {
                 ty.const_int(v.try_into().unwrap(), false).into()
             }
             Constant::Float(v) => {
-                assert!(self.unifier.unioned(ty, self.top_level.primitives.float));
+                assert!(self.unifier.unioned(ty, self.primitives.float));
                 let ty = self.ctx.f64_type();
                 ty.const_float(*v).into()
             }
@@ -134,7 +234,6 @@ impl<'ctx> CodeGenContext<'ctx> {
 
     pub fn gen_expr(&mut self, expr: &Expr<Option<Type>>) -> BasicValueEnum<'ctx> {
         let zero = self.ctx.i32_type().const_int(0, false);
-        let primitives = &self.top_level.primitives;
         match &expr.node {
             ExprKind::Constant { value, .. } => {
                 let ty = expr.custom.unwrap();
@@ -146,25 +245,36 @@ impl<'ctx> CodeGenContext<'ctx> {
             }
             ExprKind::List { elts, .. } => {
                 // this shall be optimized later for constant primitive lists...
+                // we should use memcpy for that instead of generating thousands of stores
                 let elements = elts.iter().map(|x| self.gen_expr(x)).collect_vec();
                 let ty = if elements.is_empty() {
                     self.ctx.i32_type().into()
                 } else {
                     elements[0].get_type()
                 };
-                // this length includes the leading length element
+                let arr_ptr = self.builder.build_array_alloca(
+                    ty,
+                    self.ctx.i32_type().const_int(elements.len() as u64, false),
+                    "tmparr",
+                );
                 let arr_ty = self.ctx.struct_type(
-                    &[self.ctx.i32_type().into(), ty.array_type(elements.len() as u32).into()],
+                    &[
+                        self.ctx.i32_type().into(),
+                        ty.ptr_type(AddressSpace::Generic).into(),
+                    ],
                     false,
                 );
-                let arr_ptr = self.builder.build_alloca(arr_ty, "tmparr");
+                let arr_str_ptr = self.builder.build_alloca(arr_ty, "tmparrstr");
                 unsafe {
-                    let len_ptr = arr_ptr
-                        .const_in_bounds_gep(&[zero, self.ctx.i32_type().const_int(0u64, false)]);
                     self.builder.build_store(
-                        len_ptr,
+                        arr_str_ptr.const_in_bounds_gep(&[zero, zero]),
                         self.ctx.i32_type().const_int(elements.len() as u64, false),
                     );
+                    self.builder.build_store(
+                        arr_str_ptr
+                            .const_in_bounds_gep(&[zero, self.ctx.i32_type().const_int(1, false)]),
+                        arr_ptr,
+                    );
                     let arr_offset = self.ctx.i32_type().const_int(1, false);
                     for (i, v) in elements.iter().enumerate() {
                         let ptr = self.builder.build_in_bounds_gep(
@@ -175,7 +285,7 @@ impl<'ctx> CodeGenContext<'ctx> {
                         self.builder.build_store(ptr, *v);
                     }
                 }
-                arr_ptr.into()
+                arr_str_ptr.into()
             }
             ExprKind::Tuple { elts, .. } => {
                 let element_val = elts.iter().map(|x| self.gen_expr(x)).collect_vec();
@@ -266,9 +376,9 @@ impl<'ctx> CodeGenContext<'ctx> {
                 // when doing code generation for function instances
                 if ty1 != ty2 {
                     unimplemented!()
-                } else if [primitives.int32, primitives.int64].contains(&ty1) {
+                } else if [self.primitives.int32, self.primitives.int64].contains(&ty1) {
                     self.gen_int_ops(op, left, right)
-                } else if primitives.float == ty1 {
+                } else if self.primitives.float == ty1 {
                     self.gen_float_ops(op, left, right)
                 } else {
                     unimplemented!()
@@ -277,7 +387,7 @@ impl<'ctx> CodeGenContext<'ctx> {
             ExprKind::UnaryOp { op, operand } => {
                 let ty = self.unifier.get_representative(operand.custom.unwrap());
                 let val = self.gen_expr(operand);
-                if ty == primitives.bool {
+                if ty == self.primitives.bool {
                     let val =
                         if let BasicValueEnum::IntValue(val) = val { val } else { unreachable!() };
                     match op {
@@ -286,7 +396,7 @@ impl<'ctx> CodeGenContext<'ctx> {
                         }
                         _ => val.into(),
                     }
-                } else if [primitives.int32, primitives.int64].contains(&ty) {
+                } else if [self.primitives.int32, self.primitives.int64].contains(&ty) {
                     let val =
                         if let BasicValueEnum::IntValue(val) = val { val } else { unreachable!() };
                     match op {
@@ -303,7 +413,7 @@ impl<'ctx> CodeGenContext<'ctx> {
                             .into(),
                         _ => val.into(),
                     }
-                } else if ty == primitives.float {
+                } else if ty == self.primitives.float {
                     let val = if let BasicValueEnum::FloatValue(val) = val {
                         val
                     } else {
@@ -334,7 +444,7 @@ impl<'ctx> CodeGenContext<'ctx> {
                 )
                 .fold(None, |prev, (lhs, rhs, op)| {
                     let ty = lhs.custom.unwrap();
-                    let current = if [primitives.int32, primitives.int64, primitives.bool]
+                    let current = if [self.primitives.int32, self.primitives.int64, self.primitives.bool]
                         .contains(&ty)
                     {
                         let (lhs, rhs) =
@@ -355,7 +465,7 @@ impl<'ctx> CodeGenContext<'ctx> {
                             _ => unreachable!(),
                         };
                         self.builder.build_int_compare(op, lhs, rhs, "cmp")
-                    } else if ty == primitives.float {
+                    } else if ty == self.primitives.float {
                         let (lhs, rhs) = if let (
                             BasicValueEnum::FloatValue(lhs),
                             BasicValueEnum::FloatValue(rhs),
diff --git a/nac3core/src/top_level.rs b/nac3core/src/top_level.rs
index ef2651cd6..e205af288 100644
--- a/nac3core/src/top_level.rs
+++ b/nac3core/src/top_level.rs
@@ -3,7 +3,7 @@ use std::{collections::HashMap, sync::Arc};
 use super::typecheck::type_inferencer::PrimitiveStore;
 use super::typecheck::typedef::{SharedUnifier, Type, Unifier};
 use crate::symbol_resolver::SymbolResolver;
-use inkwell::{builder::Builder, context::Context, module::Module, values::PointerValue};
+use inkwell::{builder::Builder, context::Context, module::Module, types::BasicTypeEnum, values::PointerValue};
 use parking_lot::RwLock;
 use rustpython_parser::ast::Stmt;
 
@@ -16,14 +16,17 @@ pub enum TopLevelDef {
         object_id: DefinitionId,
         // type variables bounded to the class.
         type_vars: Vec<Type>,
-        // class fields and method signature.
+        // class fields
         fields: Vec<(String, Type)>,
         // class methods, pointing to the corresponding function definition.
-        methods: Vec<(String, DefinitionId)>,
+        methods: Vec<(String, Type, DefinitionId)>,
         // ancestor classes, including itself.
         ancestors: Vec<DefinitionId>,
     },
     Function {
+        // prefix for symbol, should be unique globally, and not ending with numbers
+        name: String,
+        // function signature.
         signature: Type,
         /// Function instance to symbol mapping
         /// Key: string representation of type variable values, sorted by variable ID in ascending
@@ -48,7 +51,6 @@ pub struct CodeGenTask {
 }
 
 pub struct TopLevelContext {
-    pub primitives: PrimitiveStore,
     pub definitions: Arc<RwLock<Vec<RwLock<TopLevelDef>>>>,
     pub unifiers: Arc<RwLock<Vec<SharedUnifier>>>,
 }
@@ -61,4 +63,6 @@ pub struct CodeGenContext<'ctx> {
     pub unifier: Unifier,
     pub resolver: Box<dyn SymbolResolver>,
     pub var_assignment: HashMap<String, PointerValue<'ctx>>,
+    pub type_cache: HashMap<Type, BasicTypeEnum<'ctx>>,
+    pub primitives: PrimitiveStore,
 }
diff --git a/nac3core/src/typecheck/typedef/mod.rs b/nac3core/src/typecheck/typedef/mod.rs
index daf7fcfe7..c880b6c2d 100644
--- a/nac3core/src/typecheck/typedef/mod.rs
+++ b/nac3core/src/typecheck/typedef/mod.rs
@@ -30,6 +30,8 @@ pub struct Call {
 pub struct FuncArg {
     pub name: String,
     pub ty: Type,
+    // TODO: change this to an optional value
+    // for primitive types
     pub is_optional: bool,
 }
 
diff --git a/nac3core/src/typecheck/unification_table.rs b/nac3core/src/typecheck/unification_table.rs
index 7a95a2a20..7475afcef 100644
--- a/nac3core/src/typecheck/unification_table.rs
+++ b/nac3core/src/typecheck/unification_table.rs
@@ -1,6 +1,6 @@
 use std::rc::Rc;
 
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 pub struct UnificationKey(usize);
 
 pub struct UnificationTable<V> {