Started to Implement Evaluation

Corrected Some Errors
Cleaned Code

MathPad/MPExpression.h

@@ -9,7 +9,7 @@
 @import Foundation;
 #import "NSString+MPExpressionElement.h"
 
-@class MPExpression, MPFunction, MPRangePath;
+@class MPExpression, MPFunction, MPRangePath, MPExpressionEvaluator;
 @protocol MPExpressionElement;
 
 /*!
@@ -269,8 +269,26 @@
 - (void)replaceSymbolsInRange:(NSRange)range
                  withElements:(NSArray *)elements;
 
-#warning Evaluating must possibly return error
-- (double)doubleValue; // Evaluates Expression
+
+/*!
+ @method     elements
+ @brief      Returns an array of all elements in the receiver.
+ 
+ @discussion The elements in the returned array are not copied before they are
+ returned.
+ 
+ @return     An array of all elements from the receiver.
+ */
+- (NSArray *)elements;
+
+
+#pragma mark Evaluating Expressions
+
+- (double)evaluateExpression:(NSError *__autoreleasing *)error;
+
+
+// TODO: Private?
+@property (readonly, nonatomic, strong) MPExpressionEvaluator *evaluator;
 
 #pragma mark Notifications
 // All notification methods should create a new rangePath with the receiver's index added to the beginning of the path and then ascend the message to it's parent
@@ -312,7 +330,7 @@
     @return     @c YES if @c anExpression is equal to the receiver, @c NO
                 otherwise.
  */
-- (BOOL)isEqualToExpression:(MPExpression *)anExpression;
+// - (BOOL)isEqualToExpression:(MPExpression *)anExpression;
 
 - (NSString *)description;
 - (NSUInteger)hash;
@@ -534,21 +552,6 @@
 - (void)deleteElementsInRange:(NSRange)range;
 
 
-#pragma mark Querying Expressions
-
-
-/*!
-    @method     elements
-    @brief      Returns an array of all elements in the receiver.
- 
-    @discussion The elements in the returned array are not copied before they are
-                returned.
- 
-    @return     An array of all elements from the receiver.
- */
-- (NSArray *)elements;
-
-
 /*!
     @method     mutableElements
     @brief      Returns a proxy mutable array object that responds to all methods
@@ -564,3 +567,4 @@
 // - (NSMutableArray *)mutableElements;
 
 @end
+