diff --git a/calculator.cabal b/calculator.cabal
--- a/calculator.cabal
+++ b/calculator.cabal
@@ -1,5 +1,5 @@
 name:                calculator
-version:             0.1.5.2
+version:             0.1.5.4
 synopsis:            A calculator repl.
 description:         A calculator repl that processes mathematical expressions.
                      Does basic arithmetic, and provides pre-defined basic mathematical functions.
@@ -21,6 +21,7 @@
                      , Calculator.Evaluator.Cmd
                      , Calculator.Evaluator.Expr
                      , Calculator.Evaluator.Statement
+                     , Calculator.Evaluator.Func
                      , Calculator.Parser.Base
                      , Calculator.Parser.Cmd
                      , Calculator.Parser.Expr
diff --git a/src/Calculator/Evaluator/Cmd.hs b/src/Calculator/Evaluator/Cmd.hs
--- a/src/Calculator/Evaluator/Cmd.hs
+++ b/src/Calculator/Evaluator/Cmd.hs
@@ -2,18 +2,23 @@
 
 --------------------------------------------------------------------------------
 
-import Calculator.Prim.Expr (Bindings, defBinds, fromConst, addVar, addFun)
-import Calculator.Prim.Cmd (Cmd(..))
-import Calculator.Evaluator.Func (mkFun)
-import Calculator.Evaluator.Expr (evalExpr)
-import Calculator.Help (help)
+import           Calculator.Evaluator.Expr (evalExpr)
+import           Calculator.Evaluator.Func (mkFun)
+import           Calculator.Help           (help)
+import           Calculator.Prim.Cmd       (Cmd (..))
+import           Calculator.Prim.Expr      (Bindings, Expr (Message, Constant),
+                                            addFun, addVar, defBinds)
 
 --------------------------------------------------------------------------------
 
 evalCmd :: Bindings -> Cmd -> Either [String] Bindings
 evalCmd _ (Help)         = Left help
 evalCmd _ (Reset)        = Right defBinds
-evalCmd b (Assign s e)   = Right $ addVar (s, fromConst $ evalExpr b e) b
+evalCmd b (Assign s e)   =
+    case evalExpr b e of
+      Message xxs -> Left xxs
+      Constant c  -> Right $ addVar (s, c) b
+      _           -> Left [" ~~ Erroneous Result ~~ "]
 evalCmd b f@(Func i _ _) = Right $ addFun (i, mkFun b f) b
 
 --------------------------------------------------------------------------------
diff --git a/src/Calculator/Evaluator/Expr.hs b/src/Calculator/Evaluator/Expr.hs
--- a/src/Calculator/Evaluator/Expr.hs
+++ b/src/Calculator/Evaluator/Expr.hs
@@ -4,11 +4,7 @@
 
 import           Calculator.Prim.Base (Number)
 import           Calculator.Prim.Expr (Bindings, Expr (..), Operator (..),
-                                       fromConst, isConst, joinMessage)
-
---------------------------------------------------------------------------------
-
-import           Data.Function        (on)
+                                       joinMessage)
 
 --------------------------------------------------------------------------------
 
@@ -41,15 +37,12 @@
 
 evalPart :: Bindings -> Expr -> (Operator, Expr) -> Expr
 evalPart b e1 ((BinaryOp op), e2) =
-  let val1 = evalExpr b e1
-      val2 = evalExpr b e2
-  in if ((&&) `on` isConst) val1 val2
-     then Constant $ (op `on` fromConst) val1 val2
-     else case (val1, val2) of
-           (e@(Message _), f@(Message _)) -> joinMessage e f
-           (_, e@(Message _))             -> e
-           (e@(Message _), _)             -> e
-           _ -> Message ["Could not find matching pairs for evalPart"]
+  case (evalExpr b e1, evalExpr b e2) of
+    (Constant n1, Constant n2)     -> Constant $ op n1 n2
+    (e@(Message _), f@(Message _)) -> joinMessage e f
+    (_, e@(Message _))             -> e
+    (e@(Message _), _)             -> e
+    _ -> Message ["Could not find matching pairs for evalPart"]
 evalPart _ _ _ = Message ["Could not find suitable pattern for evalPart"]
 
 --------------------------------------------------------------------------------
diff --git a/src/Calculator/Evaluator/Func.hs b/src/Calculator/Evaluator/Func.hs
new file mode 100644
--- /dev/null
+++ b/src/Calculator/Evaluator/Func.hs
@@ -0,0 +1,18 @@
+module Calculator.Evaluator.Func where
+
+--------------------------------------------------------------------------------
+
+import Calculator.Prim.Base (Number)
+import Calculator.Prim.Cmd (Cmd(Func))
+import Calculator.Prim.Expr (addVar, Bindings, Expr(Constant))
+import Calculator.Evaluator.Expr (evalExpr)
+
+--------------------------------------------------------------------------------
+
+mkFun :: Bindings -> Cmd -> (Number -> Number)
+mkFun b (Func _ x e) = \v -> case evalExpr (addVar (x, v) b) e of
+                               Constant z -> z
+                               _          -> error "Invalid Result"
+mkFun _ _            = error "Making function from invalid cmd"
+
+--------------------------------------------------------------------------------
diff --git a/src/Calculator/Parser/Expr.hs b/src/Calculator/Parser/Expr.hs
--- a/src/Calculator/Parser/Expr.hs
+++ b/src/Calculator/Parser/Expr.hs
@@ -8,7 +8,7 @@
 
 --------------------------------------------------------------------------------
 
-import           Control.Applicative           ((<$>))
+import           Control.Applicative           ((<$>), (<*))
 import           Text.ParserCombinators.Parsec
 
 --------------------------------------------------------------------------------
@@ -81,8 +81,7 @@
 
 parseCall :: Parser Expr
 parseCall = do
-  ident <- optionMaybe parseId
-  _     <- char '('
+  ident <- try (optionMaybe parseId <* char '(')
   expr  <- parseExpr
   _     <- char ')'
   return $ case ident of
diff --git a/src/Calculator/Prim/Cmd.hs b/src/Calculator/Prim/Cmd.hs
--- a/src/Calculator/Prim/Cmd.hs
+++ b/src/Calculator/Prim/Cmd.hs
@@ -2,12 +2,7 @@
 
 --------------------------------------------------------------------------------
 
-import           Calculator.Prim.Base (Number)
 import           Calculator.Prim.Expr (Expr)
-
---------------------------------------------------------------------------------
-
-type Bindings = [(String, Number)]
 
 --------------------------------------------------------------------------------
 
diff --git a/src/Calculator/Prim/Expr.hs b/src/Calculator/Prim/Expr.hs
--- a/src/Calculator/Prim/Expr.hs
+++ b/src/Calculator/Prim/Expr.hs
@@ -74,16 +74,6 @@
 
 --------------------------------------------------------------------------------
 
-fromConst :: Expr -> Number
-fromConst (Constant v)   = v
-fromConst (Function _ _) = error "fromConst Function"
-fromConst (UnOp _ _)     = error "fromConst UnOp"
-fromConst (BinOp _)      = error "fromConst BinOp"
-fromConst (Variable _)   = error "fromConst Variable"
-fromConst (Message _)    = error "fromConst Message"
-
---------------------------------------------------------------------------------
-
 isConst :: Expr -> Bool
 isConst (Constant _) = True
 isConst _            = False