The correspondence of Nu and Objective-C

Monday, 24 Mar 2008

Here’s a small example that implements the same method in Nu and in Objective-C. In this case, we’ll look at the method that allows us to index arrays with integers, which lets us do things like this: 
[Oxygen:~/Desktop/Repositories/Nu] tim% nush
Nu Shell.
% (set a (array 'x 'y 'z))
<NSCFArray:361620>
% (a 1)
y
First, here is the Nu version: 
(class NSArray   
     ;; When an unknown message is received by an array,
     ;; if it is an integer, treat it as a call to objectAtIndex:.
     (- (id) handleUnknownMessage:(id) method withContext:(id) context is
        (let ((m ((method car) evalWithContext:context)))
             (if (m isKindOfClass:NSNumber)
                 (then (if (and (< m (self count)) (>= m 0))
                           (then (self objectAtIndex:m))
                           (else nil)))
                 (else (super handleUnknownMessage:method
                              withContext:context))))))
Now here is the same method in Objective-C: 
@implementation NSArray(Nu)

- (id) handleUnknownMessage:(NuCell *) method
       withContext:(NSMutableDictionary *) context
{
    id m = [[method car] evalWithContext:context];
    if ([m isKindOfClass:[NSNumber class]]) {
        int mm = [m intValue];
        if ((mm < [self count]) && (mm >= 0)) {
            return [self objectAtIndex:mm];
        }
        else {
            return nil;
        }
    }
    else {
        return [super handleUnknownMessage:method 
                      withContext:context];
    }
}

@end

What’s different? The Objective-C version is compiled and fast, the Nu version is interpreted and easy to write and test. I wrote the Nu version first, then wrote some tests, and much later (today) converted it to Objective-C in a few minutes. Since the Objective-C version could be tested with the same tests that verified the Nu one, I just had to rerun my existing tests – and now this change is good to go.

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