diff --git a/src/eden/std/exceptions.cljc b/src/eden/std/exceptions.cljc
index daa3adf..f04e210 100644
--- a/src/eden/std/exceptions.cljc
+++ b/src/eden/std/exceptions.cljc
@@ -1,16 +1,22 @@
 (ns eden.std.exceptions)
 
 
+(defn generate-window
+  "TODO: write this function, with bigger window."
+  [{:keys [tokens index] :as astm}]
+  (let []))
+
+
 (defn generate-parser-data
   [{:keys [tokens index] :as astm}]
-  {:index index
+  {:position index
    :window
    (str 
-    (if (> index 1) "... " "")
-    (nth tokens (dec index)) " "
-    "<" (nth tokens index) "> "
-    (nth tokens (inc index))
-    (if (< index (- (count tokens) 3)) " ..." ""))})
+    (if (> index 1) "... " "|BEGINNING| ")
+    (get tokens (dec index) "") " "
+    "'" (get tokens index) "' "
+    (get tokens (inc index) "")
+    (if (< index (- (count tokens) 3)) " ..." " |END|"))})
 
 
 (defn parser-error
@@ -51,12 +57,10 @@
   (cond
     (parser-error? ex)
     (do
-      (println "Parser Error Caught")
       (throw ex))
 
     (runtime-error? ex)
     (do
-      (println "Runtime Error Caught")
       (throw ex))
 
     :else (throw ex)))
diff --git a/src/eden/std/impl/expression.cljc b/src/eden/std/impl/expression.cljc
index 7341dfc..a5b731b 100644
--- a/src/eden/std/impl/expression.cljc
+++ b/src/eden/std/impl/expression.cljc
@@ -135,9 +135,9 @@
 
   Expression
   (evaluate-expression [_]
-    (<=
-     (evaluate-expression left)
-     (evaluate-expression right)))
+    (let [lvalue (evaluate-expression left)
+          rvalue (evaluate-expression right)]
+      (<= lvalue rvalue)))
 
   display/Display
   (display-node [_]
@@ -155,9 +155,17 @@
 
   Expression
   (evaluate-expression [_]
-    (+
-     (evaluate-expression left)
-     (evaluate-expression right)))
+    (let [lvalue (evaluate-expression left)
+          rvalue (evaluate-expression right)]
+      (when-not (number? lvalue)
+        (runtime-error "Given lvalue for '+' operation is not a number"
+                       {:display (display-node left) :value lvalue}))
+
+      (when-not (number? rvalue)
+        (runtime-error "Given rvalue for '+' operation is not a number"
+                       {:display (display-node right) :value rvalue}))
+
+      (+ lvalue rvalue)))
 
   display/Display
   (display-node [_]
@@ -170,9 +178,17 @@
 
   Expression
   (evaluate-expression [_]
-    (-
-     (evaluate-expression left)
-     (evaluate-expression right)))
+    (let [lvalue (evaluate-expression left)
+          rvalue (evaluate-expression right)]
+      (when-not (number? lvalue)
+        (runtime-error "Given lvalue for '-' operation is not a number"
+                       {:display (display-node left) :value lvalue}))
+
+      (when-not (number? rvalue)
+        (runtime-error "Given rvalue for '-' operation is not a number"
+                       {:display (display-node right) :value rvalue}))
+
+    (- lvalue rvalue)))
 
   display/Display
   (display-node [_]
@@ -190,9 +206,17 @@
 
   Expression
   (evaluate-expression [_]
-    (*
-     (evaluate-expression left)
-     (evaluate-expression right)))
+    (let [lvalue (evaluate-expression left)
+          rvalue (evaluate-expression right)]
+      (when-not (number? lvalue)
+        (runtime-error "Given lvalue for '*' operation is not a number"
+                       {:display (display-node left) :value lvalue}))
+
+      (when-not (number? rvalue)
+        (runtime-error "Given rvalue for '*' operation is not a number"
+                       {:display (display-node right) :value rvalue}))
+
+    (* lvalue rvalue)))
 
   display/Display
   (display-node [_]
@@ -205,9 +229,17 @@
 
   Expression
   (evaluate-expression [_]
-    (/
-     (evaluate-expression left)
-     (evaluate-expression right)))
+    (let [lvalue (evaluate-expression left)
+          rvalue (evaluate-expression right)]
+      (when-not (number? lvalue)
+        (runtime-error "Given lvalue for '/' operation is not a number"
+                       {:display (display-node left) :value lvalue}))
+
+      (when-not (number? rvalue)
+        (runtime-error "Given rvalue for '/' operation is not a number"
+                       {:display (display-node right) :value rvalue}))
+
+    (/ lvalue rvalue)))
 
   display/Display
   (display-node [_]
@@ -238,7 +270,11 @@
 
   Expression
   (evaluate-expression [_]
-    (- (evaluate-expression value)))
+    (let [evalue (evaluate-expression value)]
+      (when-not (number? evalue)
+        (runtime-error "Given single value for '-' operation is not a number"
+                       {:display (display-node value) :value evalue}))
+      (- evalue)))
 
   display/Display
   (display-node [_]