diff --git a/schala-lang/language/src/tree_walk_eval/mod.rs b/schala-lang/language/src/tree_walk_eval/mod.rs
index 8b4e2b8..5fe6edd 100644
--- a/schala-lang/language/src/tree_walk_eval/mod.rs
+++ b/schala-lang/language/src/tree_walk_eval/mod.rs
@@ -9,7 +9,7 @@ use std::convert::From;
 
 mod test;
 
-type EvalResult<T> = Result<T, String>;
+type EvalResult<T> = Result<T, RuntimeError>;
 
 #[derive(Debug)]
 pub struct State<'a> {
@@ -49,6 +49,14 @@ impl From<String> for RuntimeError {
     }
 }
 
+impl From<&str> for RuntimeError {
+    fn from(msg: &str) -> Self {
+        Self {
+            msg: msg.to_string(),
+        }
+    }
+}
+
 impl RuntimeError {
     fn get_msg(&self) -> String {
         format!("Runtime error: {}", self.msg)
@@ -160,7 +168,7 @@ impl<'a> State<'a> {
                 },
                 Ok(_) => (),
                 Err(error) => {
-                    acc.push(Err(error.into()));
+                    acc.push(Err(error.msg));
                     return acc;
                 }
             }
@@ -210,21 +218,21 @@ impl<'a> State<'a> {
                           // This just checks that the function exists in "memory" by ID, we don't
                           // actually retrieve it until `apply_function()`
                           Some(RuntimeValue::Function(_)) => Primitive::Callable(Function::UserDefined(id.clone())),
-                          x => return Err(format!("Function not found for id: {} : {:?}", id, x)),
+                          x => return Err(format!("Function not found for id: {} : {:?}", id, x).into()),
                       }
                   },
                   Lookup::Param(n) => {
                       let mem = n.into();
                       match self.environments.lookup(&mem) {
                           Some(RuntimeValue::Primitive(prim)) => prim.clone(),
-                          e => return Err(format!("Param lookup error, got {:?}", e)),
+                          e => return Err(format!("Param lookup error, got {:?}", e).into()),
                       }
                   },
                   kind @ Lookup::LocalVar | kind @ Lookup::GlobalVar => {
                       let mem = id.into();
                       match self.environments.lookup(&mem) {
                           Some(RuntimeValue::Primitive(expr)) => expr.clone(),
-                          _ => return Err(format!("Nothing found for variable lookup {} of kind {:?}", id, kind)),
+                          _ => return Err(format!("Nothing found for variable lookup {} of kind {:?}", id, kind).into()),
                       }
                   },
               }
@@ -232,19 +240,18 @@ impl<'a> State<'a> {
           Expression::Assign { ref lval, box rval } => {
               let mem = lval.into();
               let mut env = self.environments.lookup(&mem);
-              return Err("Assign not implemented".to_string());
+              return Err("Assign not implemented".into());
           },
           Expression::Call { box f, args } => self.call_expression(f, args)?,
           Expression::Callable(func) => Primitive::Callable(func),
           Expression::ReductionError(e) => return Err(e.into()),
-          e => return Err(format!("Can't yet handle {:?}", e)),
       })
   }
 
   fn call_expression(&mut self, f: Expression, args: Vec<Expression>) -> EvalResult<Primitive> {
       let func = match self.expression(f)? {
           Primitive::Callable(func) => func,
-          other => return Err(format!("Trying to call non-function value: {:?}", other)),
+          other => return Err(format!("Trying to call non-function value: {:?}", other).into()),
       };
       match func {
           Function::Builtin(builtin) => self.apply_builtin(builtin, args),
@@ -255,12 +262,12 @@ impl<'a> State<'a> {
                       let body = body.clone(); //TODO ideally this clone would not happen
                       self.apply_function(body, args)
                   },
-                  e => Err(format!("Error looking up function with id {}: {:?}", def_id, e))
+                  e => Err(format!("Error looking up function with id {}: {:?}", def_id, e).into())
               }
           },
           Function::Lambda { arity, body } => {
               if arity as usize != args.len() {
-                  return Err(format!("Lambda expression requries {} arguments, only {} provided", arity, args.len()));
+                  return Err(format!("Lambda expression requries {} arguments, only {} provided", arity, args.len()).into());
               }
               let body = body.clone(); //TODO again ideally, no cloning here
               self.apply_function(body, args)
@@ -279,7 +286,7 @@ impl<'a> State<'a> {
 
       Ok(match (builtin, evaled_args.as_slice()) {
           (FieldAccess, /*&[Node::PrimObject { .. }]*/ _) => {
-              return Err("Field access unimplemented".to_string());
+              return Err("Field access unimplemented".into());
           }
           (binop, &[ref lhs, ref rhs]) => match (binop, lhs, rhs) {
               (Add, Lit(Nat(l)), Lit(Nat(r))) => Nat(l + r).into(),
@@ -288,7 +295,7 @@ impl<'a> State<'a> {
               (Multiply, Lit(Nat(l)), Lit(Nat(r))) => Nat(l * r).into(),
               (Divide, Lit(Nat(l)), Lit(Nat(r))) => Float((*l as f64)/ (*r as f64)).into(),
               (Quotient, Lit(Nat(l)), Lit(Nat(r))) => if *r == 0 {
-                  return Err("Divide-by-zero error".to_string());
+                  return Err("Divide-by-zero error".into());
               } else {
                   Nat(l / r).into()
               },
@@ -320,7 +327,7 @@ impl<'a> State<'a> {
               (GreaterThanOrEqual, Lit(Int(l)), Lit(Int(r))) => Bool(l >= r).into(),
               (GreaterThanOrEqual, Lit(Float(l)), Lit(Float(r))) => Bool(l >= r).into(),
 
-              (binop, lhs, rhs) => return Err(format!("Invalid binop expression {:?} {:?} {:?}", lhs, binop, rhs)),
+              (binop, lhs, rhs) => return Err(format!("Invalid binop expression {:?} {:?} {:?}", lhs, binop, rhs).into()),
           },
           (prefix, &[ref arg]) => match (prefix, arg) {
               (BooleanNot, Lit(Bool(true))) => Bool(false),
@@ -329,7 +336,7 @@ impl<'a> State<'a> {
               (Negate, Lit(Int(n))) => Int(-(*n as i64)),
               (Increment, Lit(Int(n))) => Int(*n),
               (Increment, Lit(Nat(n))) => Nat(*n),
-              _ => return Err("No valid prefix op".to_string())
+              _ => return Err("No valid prefix op".into())
           }.into(),
           /* builtin functions */
           (IOPrint, &[ref anything]) => {
@@ -345,7 +352,7 @@ impl<'a> State<'a> {
               std::io::stdin().read_line(&mut buf).expect("Error readling line in 'getline'");
               StringLit(Rc::new(buf.trim().to_string())).into()
           },
-          (x, args) => return Err(format!("bad or unimplemented builtin {:?} | {:?}", x, args)),
+          (x, args) => return Err(format!("bad or unimplemented builtin {:?} | {:?}", x, args).into()),
       })
   }