diff --git a/nac3core/src/expression.rs b/nac3core/src/expression.rs
index c91e8c10a4..29b9565b53 100644
--- a/nac3core/src/expression.rs
+++ b/nac3core/src/expression.rs
@@ -1,13 +1,13 @@
 use crate::inference::resolve_call;
+use crate::operators::*;
 use crate::primitives::*;
 use crate::typedef::{GlobalContext, Type, Type::*};
-use rustpython_parser::ast::{Expression, ExpressionType};
+use rustpython_parser::ast::{Comparison, Expression, ExpressionType, Operator, UnaryOperator};
 use std::collections::HashMap;
-use std::convert::TryFrom;
 use std::rc::Rc;
 
 type SymTable<'a> = HashMap<&'a str, Rc<Type>>;
-type ParserResult = Result<Rc<Type>, String>;
+type ParserResult = Result<Option<Rc<Type>>, String>;
 
 pub fn parse_expr(ctx: &GlobalContext, sym_table: &SymTable, expr: &Expression) -> ParserResult {
     Err("not supported".into())
@@ -22,7 +22,7 @@ fn parse_constant(
     match value {
         Number::Integer { .. } => {
             // not check the range now
-            Ok(PrimitiveType(INT32_TYPE).into())
+            Ok(Some(PrimitiveType(INT32_TYPE).into()))
             // if i32::try_from(&value).is_ok() {
             //     Ok(PrimitiveType(INT32_TYPE).into())
             // } else if i64::try_from(&value).is_ok() {
@@ -31,40 +31,47 @@ fn parse_constant(
             //     Err("integer out of range".into())
             // }
         }
-        Number::Float { .. } => Ok(PrimitiveType(FLOAT_TYPE).into()),
+        Number::Float { .. } => Ok(Some(PrimitiveType(FLOAT_TYPE).into())),
         _ => Err("not supported".into()),
     }
 }
 
 fn parse_identifier(_: &GlobalContext, sym_table: &SymTable, name: &str) -> ParserResult {
     match sym_table.get(name) {
-        Some(v) => Ok(v.clone()),
+        Some(v) => Ok(Some(v.clone())),
         None => Err("unbounded variable".into()),
     }
 }
 
 fn parse_list(ctx: &GlobalContext, sym_table: &SymTable, elements: &[Expression]) -> ParserResult {
     if elements.len() == 0 {
-        return Ok(ParametricType(LIST_TYPE, vec![BotType.into()]).into());
+        return Ok(Some(ParametricType(LIST_TYPE, vec![BotType.into()]).into()));
     }
 
     let mut types = elements.iter().map(|v| parse_expr(&ctx, sym_table, v));
 
     let head = types.next().unwrap()?;
+    if head.is_none() {
+        return Err("list elements must have some type".into());
+    }
     for v in types {
         if v? != head {
             return Err("inhomogeneous list is not allowed".into());
         }
     }
-    Ok(ParametricType(LIST_TYPE, vec![head]).into())
+    Ok(Some(ParametricType(LIST_TYPE, vec![head.unwrap()]).into()))
 }
 
 fn parse_tuple(ctx: &GlobalContext, sym_table: &SymTable, elements: &[Expression]) -> ParserResult {
-    let types: Result<Vec<_>, String> = elements
+    let types: Result<Option<Vec<_>>, String> = elements
         .iter()
         .map(|v| parse_expr(&ctx, sym_table, v))
         .collect();
-    Ok(ParametricType(TUPLE_TYPE, types?).into())
+    if let Some(t) = types? {
+        Ok(Some(ParametricType(TUPLE_TYPE, t).into()))
+    } else {
+        Err("tuple elements must have some type".into())
+    }
 }
 
 fn parse_attribute(
@@ -73,7 +80,7 @@ fn parse_attribute(
     value: &Expression,
     name: String,
 ) -> ParserResult {
-    let value = parse_expr(ctx, sym_table, value)?;
+    let value = parse_expr(ctx, sym_table, value)?.ok_or("no value".to_string())?;
     if let TypeVariable(id) = value.as_ref() {
         let v = ctx.get_variable(*id);
         if v.bound.len() == 0 {
@@ -91,12 +98,12 @@ fn parse_attribute(
                 return Err("unknown field (type mismatch between variants)".into());
             }
         }
-        return Ok(ty.unwrap().clone());
+        return Ok(Some(ty.unwrap().clone()));
     }
 
     match value.get_base(ctx) {
         Some(b) => match b.fields.get(name.as_str()) {
-            Some(t) => Ok(t.clone()),
+            Some(t) => Ok(Some(t.clone())),
             None => Err("no such field".into()),
         },
         None => Err("this object has no fields".into()),
@@ -109,15 +116,28 @@ fn parse_bool_ops(
     values: &[Expression],
 ) -> ParserResult {
     assert_eq!(values.len(), 2);
-    let left = parse_expr(ctx, sym_table, &values[0])?;
-    let right = parse_expr(ctx, sym_table, &values[1])?;
+    let left = parse_expr(ctx, sym_table, &values[0])?.ok_or("no value".to_string())?;
+    let right = parse_expr(ctx, sym_table, &values[1])?.ok_or("no value".to_string())?;
 
     let b = PrimitiveType(BOOL_TYPE);
     if left.as_ref() == &b && right.as_ref() == &b {
-        Ok(b.into())
+        Ok(Some(b.into()))
     } else {
         Err("bool operands must be bool".into())
     }
 }
 
+fn parse_bin_ops(
+    ctx: &GlobalContext,
+    sym_table: &SymTable,
+    op: &Operator,
+    left: &Expression,
+    right: &Expression,
+) -> ParserResult {
+    let left = parse_expr(ctx, sym_table, left)?.ok_or("no value".to_string())?;
+    let right = parse_expr(ctx, sym_table, right)?.ok_or("no value".to_string())?;
+    let fun = binop_name(op);
+    let mut assumptions = HashMap::new();
+    resolve_call(ctx, Some(left), fun, &[right], &mut assumptions)
+}
 
diff --git a/nac3core/src/operators.rs b/nac3core/src/operators.rs
index 31990d6976..a33f4aa882 100644
--- a/nac3core/src/operators.rs
+++ b/nac3core/src/operators.rs
@@ -1,6 +1,6 @@
 use rustpython_parser::ast::{Comparison, Operator, UnaryOperator};
 
-pub fn binop_name(op: Operator) -> &'static str {
+pub fn binop_name(op: &Operator) -> &'static str {
     match op {
         Operator::Add => "add",
         Operator::Sub => "sub",
@@ -18,7 +18,7 @@ pub fn binop_name(op: Operator) -> &'static str {
     }
 }
 
-pub fn unaryop_name(op: UnaryOperator) -> &'static str {
+pub fn unaryop_name(op: &UnaryOperator) -> &'static str {
     match op {
         UnaryOperator::Pos => "pos",
         UnaryOperator::Neg => "neg",
@@ -27,7 +27,7 @@ pub fn unaryop_name(op: UnaryOperator) -> &'static str {
     }
 }
 
-pub fn comparison_name(op: Comparison) -> Option<&'static str> {
+pub fn comparison_name(op: &Comparison) -> Option<&'static str> {
     match op {
         Comparison::Less => Some("lt"),
         Comparison::LessOrEqual => Some("le"),