diff --git a/simple.ed b/simple.ed
index 9eadde432..6e4d63acb 100644
--- a/simple.ed
+++ b/simple.ed
@@ -1,5 +1,5 @@
-fn main(x: i32, z: i32) -> i32 {
-    let y = 0;
+fn main(x: i64, z: i64) -> i64 {
+    let y: i64 = 0;
     if x == 5 {
         if x == z {
             y = 2 * x;
diff --git a/src/ast/mod.rs b/src/ast/mod.rs
index 9d39049c1..a9e94d996 100644
--- a/src/ast/mod.rs
+++ b/src/ast/mod.rs
@@ -86,7 +86,12 @@ impl Function {
 
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum Statement {
-    Variable {
+    Let {
+        name: String,
+        value: Box<Expression>,
+        type_name: Option<String>,
+    },
+    Mutate {
         name: String,
         value: Box<Expression>,
     },
diff --git a/src/codegen.rs b/src/codegen.rs
index d2d38417c..8f31da497 100644
--- a/src/codegen.rs
+++ b/src/codegen.rs
@@ -36,10 +36,14 @@ pub struct CodeGen<'ctx> {
     context: &'ctx Context,
     pub module: Module<'ctx>,
     builder: Builder<'ctx>,
+    fn_types: VariableTypes<'ctx>,
     _program: ProgramData,
     ast: ast::Program,
 }
 
+type Variables<'ctx> = HashMap<String, (BasicValueEnum<'ctx>, usize)>;
+type VariableTypes<'ctx> = HashMap<String, BasicTypeEnum<'ctx>>;
+
 impl<'ctx> CodeGen<'ctx> {
     pub fn new(
         context: &'ctx Context,
@@ -55,29 +59,38 @@ impl<'ctx> CodeGen<'ctx> {
             builder: context.create_builder(),
             _program,
             ast,
+            fn_types: HashMap::new(),
         };
 
         Ok(codegen)
     }
 
-    pub fn compile_ast(&self) -> Result<()> {
+    pub fn compile_ast(&mut self) -> Result<()> {
         let mut functions = vec![];
+        let mut types: VariableTypes<'ctx> = HashMap::new();
 
         // todo fix the grammar so top level statements are only functions and static vars.
 
         // create the llvm functions first.
+
         for statement in &self.ast.statements {
             match &statement {
-                Statement::Variable { .. } => unreachable!(),
+                Statement::Let { .. } => unreachable!(),
+                Statement::Mutate { .. } => unreachable!(),
                 Statement::Return(_) => unreachable!(),
                 Statement::If { .. } => unreachable!(),
                 Statement::Function(function) => {
                     functions.push(function);
-                    self.compile_function_signature(function)?;
+                    let ret_type = self.compile_function_signature(function)?;
+                    if let Some(ret_type) = ret_type {
+                        types.insert(function.name.clone(), ret_type);
+                    }
                 }
             }
         }
 
+        self.fn_types = types;
+
         // implement them.
         for function in functions {
             self.compile_function(function)?;
@@ -104,7 +117,10 @@ impl<'ctx> CodeGen<'ctx> {
     }
 
     /// creates the llvm function without the body, so other function bodies can call it.
-    fn compile_function_signature(&self, function: &Function) -> Result<()> {
+    fn compile_function_signature(
+        &self,
+        function: &Function,
+    ) -> Result<Option<BasicTypeEnum<'ctx>>> {
         let args_types: Vec<BasicTypeEnum<'ctx>> = function
             .params
             .iter()
@@ -116,13 +132,16 @@ impl<'ctx> CodeGen<'ctx> {
             args_types.into_iter().map(|t| t.into()).collect_vec();
 
         let fn_type = match &function.return_type {
-            Some(id) => self.get_llvm_type(id)?.fn_type(&args_types, false),
+            Some(id) => {
+                let return_type = self.get_llvm_type(id)?;
+                return_type.fn_type(&args_types, false)
+            }
             None => self.context.void_type().fn_type(&args_types, false),
         };
 
         self.module.add_function(&function.name, fn_type, None);
 
-        Ok(())
+        Ok(fn_type.get_return_type())
     }
 
     fn compile_function(&self, function: &Function) -> Result<()> {
@@ -131,18 +150,16 @@ impl<'ctx> CodeGen<'ctx> {
 
         self.builder.position_at_end(entry_block);
 
-        let mut variables: HashMap<String, (BasicValueEnum<'ctx>, usize)> = HashMap::new();
+        let mut variables: Variables = HashMap::new();
+        let mut types: VariableTypes = HashMap::new();
 
         for (i, param) in function.params.iter().enumerate() {
             let id = param.ident.clone();
-            variables.insert(
-                id.clone(),
-                (
-                    func.get_nth_param(i.try_into().unwrap())
-                        .expect("parameter"),
-                    0,
-                ),
-            );
+            let param = func
+                .get_nth_param(i.try_into().unwrap())
+                .expect("parameter");
+            variables.insert(id.clone(), (param, 0));
+            types.insert(id.clone(), param.get_type());
         }
 
         let mut has_return = false;
@@ -151,7 +168,7 @@ impl<'ctx> CodeGen<'ctx> {
             if let Statement::Return(_) = statement {
                 has_return = true
             }
-            self.compile_statement(statement, &mut variables)?;
+            self.compile_statement(statement, &mut variables, &mut types)?;
         }
 
         if !has_return {
@@ -161,30 +178,70 @@ impl<'ctx> CodeGen<'ctx> {
         Ok(())
     }
 
+    fn find_expr_type(
+        &self,
+        expr: &Expression,
+        types: &VariableTypes<'ctx>,
+    ) -> Option<BasicTypeEnum<'ctx>> {
+        match expr {
+            Expression::Literal(x) => match x {
+                LiteralValue::String => todo!(),
+                LiteralValue::Integer {
+                    bits,
+                    signed,
+                    value,
+                } => bits.map(|bits| self.context.custom_width_int_type(bits).into()),
+            },
+            Expression::Variable(x) => types.get(x).cloned(),
+            Expression::Call { function, args } => types.get(function).cloned(),
+            Expression::BinaryOp(lhs, op, rhs) => self
+                .find_expr_type(lhs, types)
+                .or_else(|| self.find_expr_type(rhs, types)),
+        }
+    }
+
     fn compile_statement(
         &self,
         statement: &Statement,
         // value, assignments
-        variables: &mut HashMap<String, (BasicValueEnum<'ctx>, usize)>,
+        variables: &mut Variables<'ctx>,
+        types: &mut VariableTypes<'ctx>,
     ) -> Result<()> {
         match statement {
             // Variable assignment
-            Statement::Variable { name, value } => {
+            Statement::Let {
+                name,
+                value,
+                type_name,
+            } => {
+                let type_hint = if let Some(type_name) = type_name {
+                    self.get_llvm_type(type_name)?
+                } else {
+                    self.find_expr_type(value, types)
+                        .expect("type should be found")
+                };
+                types.insert(name.clone(), type_hint);
+
                 let result = self
-                    .compile_expression(value, variables)?
+                    .compile_expression(value, variables, types, Some(type_hint))?
                     .expect("should have result");
 
-                let accesses = if let Some(x) = variables.get(name) {
-                    x.1 + 1
-                } else {
-                    0
-                };
-                variables.insert(name.clone(), (result, accesses));
+                variables.insert(name.clone(), (result, 0));
+            }
+            Statement::Mutate { name, value } => {
+                let type_hint = *types.get(name).expect("should exist");
+                let result = self
+                    .compile_expression(value, variables, types, Some(type_hint))?
+                    .expect("should have result");
+
+                let (old_val, acc) = variables.get(name).expect("variable should exist");
+                variables.insert(name.clone(), (result, acc + 1));
             }
             Statement::Return(ret) => {
                 if let Some(ret) = ret {
+                    let type_hint = self.find_expr_type(ret, types);
                     let result = self
-                        .compile_expression(ret, variables)?
+                        .compile_expression(ret, variables, types, type_hint)?
                         .expect("should have result");
                     self.builder.build_return(Some(&result));
                 } else {
@@ -196,8 +253,9 @@ impl<'ctx> CodeGen<'ctx> {
                 body,
                 else_body,
             } => {
+                let type_hint_cond = self.find_expr_type(condition, types);
                 let condition = self
-                    .compile_expression(condition, variables)?
+                    .compile_expression(condition, variables, types, type_hint_cond)?
                     .expect("should produce a value");
 
                 let func = self
@@ -224,7 +282,7 @@ impl<'ctx> CodeGen<'ctx> {
                 let mut variables_if = variables.clone();
                 self.builder.position_at_end(if_block);
                 for s in body {
-                    self.compile_statement(s, &mut variables_if)?;
+                    self.compile_statement(s, &mut variables_if, types)?;
                 }
                 self.builder.build_unconditional_branch(merge_block);
                 if_block = self.builder.get_insert_block().unwrap(); // update for phi
@@ -234,7 +292,7 @@ impl<'ctx> CodeGen<'ctx> {
                     self.builder.position_at_end(else_block);
 
                     for s in else_body {
-                        self.compile_statement(s, &mut variables_else)?;
+                        self.compile_statement(s, &mut variables_else, types)?;
                     }
                     self.builder.build_unconditional_branch(merge_block);
                     else_block = self.builder.get_insert_block().unwrap(); // update for phi
@@ -288,14 +346,18 @@ impl<'ctx> CodeGen<'ctx> {
     pub fn compile_expression(
         &self,
         expr: &Expression,
-        variables: &mut HashMap<String, (BasicValueEnum<'ctx>, usize)>,
+        variables: &mut Variables<'ctx>,
+        types: &mut VariableTypes<'ctx>,
+        type_hint: Option<BasicTypeEnum<'ctx>>,
     ) -> Result<Option<BasicValueEnum<'ctx>>> {
         Ok(match expr {
-            Expression::Variable(term) => Some(self.compile_variable(term, variables)?),
-            Expression::Literal(term) => Some(self.compile_literal(term)?),
-            Expression::Call { function, args } => self.compile_call(function, args, variables)?,
+            Expression::Variable(term) => Some(self.compile_variable(term, variables, types)?),
+            Expression::Literal(term) => Some(self.compile_literal(term, type_hint)?),
+            Expression::Call { function, args } => {
+                self.compile_call(function, args, variables, types)?
+            }
             Expression::BinaryOp(lhs, op, rhs) => {
-                Some(self.compile_binary_op(lhs, op, rhs, variables)?)
+                Some(self.compile_binary_op(lhs, op, rhs, variables, types, type_hint)?)
             }
         })
     }
@@ -304,15 +366,17 @@ impl<'ctx> CodeGen<'ctx> {
         &self,
         func_name: &str,
         args: &[Box<Expression>],
-        variables: &mut HashMap<String, (BasicValueEnum<'ctx>, usize)>,
+        variables: &mut Variables<'ctx>,
+        types: &mut VariableTypes<'ctx>,
     ) -> Result<Option<BasicValueEnum<'ctx>>> {
         let function = self.module.get_function(func_name).expect("should exist");
 
         let mut value_args: Vec<BasicMetadataValueEnum> = Vec::with_capacity(args.len());
 
         for arg in args {
+            let type_enum = self.find_expr_type(arg, types);
             let res = self
-                .compile_expression(arg, variables)?
+                .compile_expression(arg, variables, types, type_enum)?
                 .expect("should have result");
             value_args.push(res.into());
         }
@@ -333,14 +397,16 @@ impl<'ctx> CodeGen<'ctx> {
         lhs: &Expression,
         op: &OpCode,
         rhs: &Expression,
-        variables: &mut HashMap<String, (BasicValueEnum<'ctx>, usize)>,
+        variables: &mut Variables<'ctx>,
+        types: &mut VariableTypes<'ctx>,
+        type_hint: Option<BasicTypeEnum<'ctx>>,
     ) -> Result<BasicValueEnum<'ctx>> {
         let lhs = self
-            .compile_expression(lhs, variables)?
+            .compile_expression(lhs, variables, types, type_hint)?
             .expect("should have result")
             .into_int_value();
         let rhs = self
-            .compile_expression(rhs, variables)?
+            .compile_expression(rhs, variables, types, type_hint)?
             .expect("should have result")
             .into_int_value();
 
@@ -363,7 +429,11 @@ impl<'ctx> CodeGen<'ctx> {
         Ok(result.as_basic_value_enum())
     }
 
-    pub fn compile_literal(&self, term: &LiteralValue) -> Result<BasicValueEnum<'ctx>> {
+    pub fn compile_literal(
+        &self,
+        term: &LiteralValue,
+        type_hint: Option<BasicTypeEnum<'ctx>>,
+    ) -> Result<BasicValueEnum<'ctx>> {
         let value = match term {
             LiteralValue::String => todo!(),
             LiteralValue::Integer {
@@ -371,13 +441,19 @@ impl<'ctx> CodeGen<'ctx> {
                 signed: _,
                 value,
             } => {
-                // todo: type resolution for bit size?
-                let bits = bits.unwrap_or(32);
+                if let Some(type_hint) = type_hint {
+                    type_hint
+                        .into_int_type()
+                        .const_int(value.parse().unwrap(), false)
+                        .as_basic_value_enum()
+                } else {
+                    let bits = bits.unwrap_or(32);
 
-                self.context
-                    .custom_width_int_type(bits)
-                    .const_int(value.parse().unwrap(), false)
-                    .as_basic_value_enum()
+                    self.context
+                        .custom_width_int_type(bits)
+                        .const_int(value.parse().unwrap(), false)
+                        .as_basic_value_enum()
+                }
             }
         };
 
@@ -387,7 +463,8 @@ impl<'ctx> CodeGen<'ctx> {
     pub fn compile_variable(
         &self,
         variable: &str,
-        variables: &mut HashMap<String, (BasicValueEnum<'ctx>, usize)>,
+        variables: &mut Variables<'ctx>,
+        types: &mut VariableTypes<'ctx>,
     ) -> Result<BasicValueEnum<'ctx>> {
         let var = *variables.get(variable).expect("value");
         Ok(var.0)
diff --git a/src/grammar.lalrpop b/src/grammar.lalrpop
index 54cea829e..3edcc16bb 100644
--- a/src/grammar.lalrpop
+++ b/src/grammar.lalrpop
@@ -69,10 +69,14 @@ Statement: ast::Statement = {
     <f:Function> => ast::Statement::Function(f),
 };
 
+TypeInfo: String =  {
+    ":" <i:"identifier"> => i
+};
+
 // statements not including function definitions
 BasicStatement: ast::Statement = {
-    "let" <i:"identifier"> "=" <e:Expr> ";" => ast::Statement::Variable { name: i, value: e},
-    <i:"identifier"> "=" <e:Expr> ";" => ast::Statement::Variable { name: i, value: e},
+    "let" <i:"identifier"> <t:TypeInfo?> "=" <e:Expr> ";" => ast::Statement::Let { name: i, value: e, type_name: t},
+    <i:"identifier"> "=" <e:Expr> ";" => ast::Statement::Mutate { name: i, value: e},
     "if" <cond:Expr> "{" <s:Statements> "}" <e:ElseExpr?> => ast::Statement::If { condition: cond, body: s, else_body: e},
     "return" <e:Expr?> ";" => ast::Statement::Return(e),
 };
diff --git a/src/main.rs b/src/main.rs
index 148eedf52..c277715c2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -123,7 +123,7 @@ fn main() -> Result<()> {
 
             println!("{:#?}", ast);
             let context = Context::create();
-            let codegen = codegen::CodeGen::new(&context, &file_name, program, ast)?;
+            let mut codegen = codegen::CodeGen::new(&context, &file_name, program, ast)?;
             codegen.compile_ast()?;
             let generated_llvm_ir = codegen.generated_code();
 
@@ -144,7 +144,7 @@ fn main() -> Result<()> {
             let file_name = input.file_name().unwrap().to_string_lossy();
 
             let context = Context::create();
-            let codegen = codegen::CodeGen::new(&context, &file_name, program, ast)?;
+            let mut codegen = codegen::CodeGen::new(&context, &file_name, program, ast)?;
             codegen.compile_ast()?;
             let execution_engine = codegen
                 .module