Added the MPExpressionTree Classes

MathPad/MPPowerFunction.m

@@ -7,7 +7,7 @@
 //
 
 #import "MPPowerFunction.h"
-#import "MPExpressionEvaluator.h"
+#import "MPExpression.h"
 
 @implementation MPPowerFunction
 
@@ -18,19 +18,24 @@ MPFunctionAccessorImplementation(ExponentExpression, _exponentExpression)
     return @[@"exponentExpression"];
 }
 
-- (MPTerm *)parseWithError:(MPParseError *__autoreleasing *)error
+- (BOOL)validate:(MPParseError *__autoreleasing *)error
 {
-    MPExpressionEvaluator *exponent = [[MPExpressionEvaluator alloc] initWithExpression:self.exponentExpression];
-    MPTerm *exponentTerm = [exponent parseExpectingVariable:NO
-                                                      error:error];
-    if (exponentTerm == nil) {
-        return nil;
+    if (!self.baseValue) {
+        if (error) {
+            *error = MPParseError(NSMakeRange([self.parent convertIndex:[self.parent indexOfElement:self]
+                                                     fromReferenceFrame:MPElementReferenceFrame
+                                                       toReferenceFrame:MPSymbolReferenceFrame], 0), @"No Base for Power");
+        }
+        return NO;
     }
-    
-    return [[MPTerm alloc] initWithBlock:^NSDecimalNumber *{
-        double power = pow([self.baseTerm evaluate].doubleValue, [exponentTerm evaluate].doubleValue);
-        return [[NSDecimalNumber alloc] initWithDouble:power];
-    }];
+    return [self.baseValue validate:error] && [[self.exponentExpression parse] validate:error];
+}
+
+- (NSDecimalNumber *)evaluate
+{
+    NSDecimalNumber *base = [self.baseValue evaluate];
+    NSDecimalNumber *exponent = [[self.exponentExpression parse] evaluate];
+    return [[NSDecimalNumber alloc] initWithDouble:pow(base.doubleValue, exponent.doubleValue)];
 }
 
 - (NSString *)description