Fundamental Redesign of Evaluation

MathPad/MPEvaluationContext.m

@@ -32,8 +32,10 @@ static MPEvaluationContext *sharedContext;
     if (self) {
         _stack = [[NSMutableArray alloc] init];
         [self push];
-        [self defineVariable:@"e" withValue:[[NSDecimalNumber alloc] initWithDouble:M_E]];
-        [self defineVariable:@"π" withValue:[[NSDecimalNumber alloc] initWithDouble:M_PI]];
+        [self defineVariable:@"e"
+                       value:[[NSDecimalNumber alloc] initWithDouble:M_E]];
+        [self defineVariable:@"π"
+                       value:[[NSDecimalNumber alloc] initWithDouble:M_PI]];
     }
     return self;
 }
@@ -48,10 +50,23 @@ static MPEvaluationContext *sharedContext;
     [self.stack removeLastObject];
 }
 
-- (void)defineVariable:(NSString *)variable withValue:(NSDecimalNumber *)value
+- (BOOL)defineVariable:(NSString *)variable
+                 value:(NSDecimalNumber *)value
 {
+    if ([self isVariableDefined:variable]) {
+        return NO;
+    }
     NSMutableDictionary *currentBindings = self.stack.lastObject;
     currentBindings[variable] = value;
+    return YES;
+}
+
+- (BOOL)redefineVariable:(NSString *)variable
+                   value:(NSDecimalNumber *)value
+{
+    [self undefineVariable:variable];
+    return [self defineVariable:variable
+                          value:value];
 }
 
 - (void)undefineVariable:(NSString *)variable
@@ -60,6 +75,11 @@ static MPEvaluationContext *sharedContext;
     [currentBindings removeObjectForKey:variable];
 }
 
+- (BOOL)isVariableDefined:(NSString *)variable
+{
+    return [self valueForVariable:variable] != nil;
+}
+
 - (NSDecimalNumber *)valueForVariable:(NSString *)variable
 {
     NSUInteger currentIndex = self.stack.count;
@@ -72,9 +92,4 @@ static MPEvaluationContext *sharedContext;
     return value;
 }
 
-- (BOOL)isVariableDefined:(NSString *)variable
-{
-    return [self valueForVariable:variable] != nil;
-}
-
 @end