How Tim Burks and Nu Stole the Show at C4[1]

Edit: Fixed some factual inaccuracies about the language itself.

Tim Burks, noted contributor to RubyCocoa and creator of RubyObjC, gave a talk at C4[1] about his experiences with creating a Ruby <-> ObjectiveC bridge, and the problems he overcame in doing so. It was an interesting presentation, and we were all suitably appreciative when he showed his custom visual chip-design software written in Ruby with a Cocoa interface.

And then he dropped a bombshell.

For the past year, Tim’s been working on a new dialect of Lisp – written in Objective-C – called Nu. Here are its features (more precisely, here are the ones that I remember; I was so awestruck that many went over my head):

• Interpreted, running on top of Objective-C.
• Scheme-y syntax. Everything is an s-expression (data is code, code is data). Variable assignment was done without let-clauses (which are a pain in the ass) – all one has to do was (set varname value).
• Variable sigils to indicate variable scope.
• True object-orientation – everything is an object.
• True closures with the do-statement – which, incidentally, is how Ruby should have done it.
• Macros. HOLY CRAP, MACROS! When Tim showed us an example of using define-macro for syntactical abstraction, Wolf Rentzsch and I started spontaneously applauding. His example even contained an example of absolutely beautiful exception handling that should be familiar to anyone with any ObjC or Ruby experience.
• Symbol generation (__) to make macros hygenic and prevent variable name conflicts.
• Nu data objects are Cocoa classes – the strings are NSStrings, the arrays NSArrays, etc.
• Ability to create new Obj-C classes from inside Nu.
• Interfaces with Cocoa libraries – you can access Core Data stores from within Nu in a much easier fashion than pure ObjC, thanks to Tim’s very clever idea of using a $session global to store the NSManagedObjectModel, NSManagedObjectContext, and NSPersistentStoreCoordinator.
• Ruby-style string interpolation with #{}.
• Regular expressions.
• Positively drool-inducing metaprogramming, including a simulation of Ruby’s method_missing functionality.
• A web-based templating system similar to ERb in 80 lines of Nu code – compare that with the 422 lines of code in erb.rb.

Tim showed us a MarsEdit-like blog editor written entirely in Nu, using Core Data as its backend – and then showed us the built-in Nu web server inside that program, complete with beautiful CSS/HTML/Ajax.

As F-Script is to Smalltalk, so Nu is to Lisp. Tim said that he hopes someday to open-source Nu; if he does, he will introduce what is quite possibly the most exciting development in the Lisp-related community in a long time. I don’t think I speak for just myself when I say I cannot wait to get my hands on it.

Map, Filter and Reduce in Cocoa

After working in Scheme, Python or Ruby, all of which (more or less) support function objects and the map(), filter() and reduce() functions, languages that don’t seem to be somewhat cumbersome. Cocoa manages to get these paradigms almost correctly implemented.

map()

One would think that Objective-C’s ability to pass functions around as objects in the form of selectors would make writing a map() method easy. Observe, however, the crucial differences of the NSArray equivalent to map(). (For those unfamilar with it, map(), when given an array and a method/function taking one argument, returns the result of mapping the function onto each item of the array.)

From the NSArray documentation:

makeObjectsPerformSelector:

Sends the aSelector message to each object in the array, starting with the first object and continuing through the array to the last object.

– (void)makeObjectsPerformSelector:(SEL)aSelector
Discussion

The aSelector method must not take any arguments. It shouldn’t have the side effect of modifying the receiving array.

This is different on no fewer than two levels. Firstly, even in the NSMutableArray subclass, this method is not allowed to have side effects. Frankly, I can think of few situations in which I would need to map an idempotent function onto an array; the point of map() is to be able to apply a function quickly to every element of an array and get back the changes! Secondly, an unaware or hurried programmer would think that this function was implemented so that one could write code like this:

- (void)printAnObject:(id)obj
{
NSLog([obj description]);
}

and then do this:
[anArray makeObjectsPerformSelector: @selector(printAnObject:)];

This is not the case – the above code would just make each element call printAnObject:, not call printAnObject with each element. I’m sure that to some it seems obvious, but I, for one, found this to be an insidiously tricky wart.

However, there is a (limited) workaround.

NSArray’s valueForKey: method (somewhat counter-intuitively) returns the result of invoking valueForKey on each of the array’s elements. As such, one can map KVC functions onto arrays. For example:

NSArray *arr = [NSArray arrayWithObjects: @"Get", @"CenterStage", @"0.6.2", @"because", @"it", @"rocks", nil];
[arr objectForKey: @"length"]; // returns [3, 10, 6, 7, 2, 5]

This can be used in many helpful ways; sadly, it only works on KVC-compliant properties/methods.

Anyway, moving on…

filter()

With OS X 10.4, Apple introduced the NSArray filteredArrayUsingPredicate: method, which allows one to filter an array based on criteria established by an NSPredicate. Observe:

NSArray *arr = [NSArray arrayWithObjects: @"This", @"is", @"the", @"first", @"CenterStage", @"release", @"I", @"helped", nil];
arr = [arr filteredArrayWithPredicate: [NSPredicate predicateWithFormat: @"SELF.length > 5"]]; // arr is now [@"CenterStage", "@"release", @"helped"]

Verbose, but useful.

reduce()

Frankly, Apple don’t give us any way to do this in pure Cocoa. The best way I’ve found (if you really need this, which is less often than one needs map() and filter()) is to use the F-Script framework and apply the \ operator to an array. Unfortunately, this takes a bit of overhead.

In conclusion, Cocoa and Objective-C almost bring us the joys of functional programming. We can only hope that Leopard and Obj-C 2.0 improve on these in some way.

List comprehensions, anyone?

Why I Dig Objective-C

I apologize if this entry is incoherent or trivial. But I need to build up the habit of blogging.

I recently picked up a copy of Refactoring. I hadn’t heard of that book before I read Steve Yegge’s thoughts on it – indeed, my entire concept of refactoring had previously been limited to Eclipse’s built-in tools. To describe this book as an eye-opener would be an understatement; not only has it taught me many coding strategies (upon reading the entry for Introduce Null Object, I practically screamed “Why didn’t I think of that?”), but it’s also changed many of my ideas about the way I code.

Fowler’s clear, direct writing is one of the strengths of Refactoring; during the book, he introduces the concept of the code smell – a term which I had not heard before, and which perfectly describes a situation with which I have been familiar ever since I started coding – and points out that a sure-fire sign of smelly code is the presence of many comments; if you need to explain a method in excruciating detail, then you’re probably made it too complex.

One of the reasons why I adore Eclipse for Java development is because it shows JavaDoc attributes in its Intellisense methods – I don’t need to navigate through the Java API when I can hit Ctrl+Space and see the arguments the method takes. Without Eclipse, I have to either rely on memory (and in my opinion, life’s too short to memorize the proper sequence of arguments that a proper BufferedReader instance takes) or waste time navigating the API docs.

Objective-C, on the other hand, doesn’t have this – because method signatures can be broken up into multiple pieces. Take a look at these two method signatures – taken straight from the documentation:

- (NSRange)rangeOfString:(NSString *)subString options:(unsigned)mask range:(NSRange)aRange

versus:

public NSRange rangeOfString(String s, int i, NSRange nsrange)

When looking at the first method signature, I know that the second object will be an integer that controls the masking – simply by virtue of the fact that, like its parent Smalltalk, its methods recieve messages via keyword messaging. I can also make the assumption – and it is only an assumption – that - (NSRange)rangeOfString:(NSString *)subString options:(unsigned)mask and - (NSRange)rangeOfString:(NSString *)subString are valid methods. With the Java API, on the other hand, I have no clue what the second argument does; if I had to know, I’d need to be using Eclipse or XCode, not Emacs or Textmate. Textmate’s Cocoa completions don’t show any HeaderDoc information – but that doesn’t matter: the keywords tell me what the arguments are used for.

It seems to me that Objective-C’s syntax lends itself to clarity by its very nature; Java, on the other hand, depends on other programmers being clear and helpful in their method signatures. And as a short perusal through The Daily WTF reveals, programmers can be very, very unhelpful at times.

Objective-C 2.0

As many of you know, Apple is releasing the new incarnation of Objective-C (creatively dubbed Objective-C 2.0) along with XCode 3.0 and Leopard. Though much of it is hidden under Apple’s elaborate nondisclosure agreements, people like Andy Matushack and Scott Stevenson as well as other sites have uncovered more information from the ObjC mailing lists and repositories than the meager scraps of info that Apple released on their website.

Since I can’t give any new information about this, I’ll just list my opinions, then brace for the reaming I’ll receive in the comments.

Disclaimer: Apple may change all of these things. This post is naught but conjecture heaped upon conjecture.

I got you a present!

[Update: Looks like Dave Batton beat me to the punch. *cries*]

Merry Atheist Children Get Presents Day Christmas, everyone!

In celebration of this day, I am giving you a gift – yes, a gift – from me to you. Yes, you. It is my first piece of public code – well, actually, that’s a lie. I posted this some months ago – I consider it my cleverest Java hack EVER. I’m quite proud of it. No! Digression! Must…focus. Wait…my second was this.

Anyway, it’s my third piece of public code, and I hope you like it. In short, it’s called PVCGradientCell, and it’s a subclass of NSTextFieldCell that uses the CTGradient class to mimic the Source List found in iTunes 6. It’s notable for several reasons:

1. Despite the name of the file (PVCGradientTable) , it is actually not a subclass of NSTableView – all the code is contained in the NSTextFieldCell subclass. This is unlike Matt Gemmell’s iTableView, which requires subclassing of NSTableView. (By the way, props to Matt – his iTableView helped me fix a lot of bugs.)
2. It uses Chad Weider’s CTGradient code to render the gradient – the majority of implementations use either stretching images or bare CoreImage code.
3. It allows you to enable centering the text vertically, a lá Daniel Jalkut’s RSVerticallyCenteredTextCell.
4. It allows one to specify whether the text should be bold when clicked upon.
5. It freshens your breath.

Here’s a screenshot, with the text bolded and vertically centered:

You can grab it here, or check it out from my Subversion repository (courtesy of Assembla – great job, guys!) here:
http://tools.assembla.com/svn/importantshock

Use the username and password ‘anonymous‘ (without quotes, of course) to access the repository.

Bug reports, accolades, fame, and large sacks of cash are welcome at:

ironswallow at gmail dot etc

By the way, PVC was the initial acronym for the Cocoa application I’m working on.

Anyway, I hope that y’all have a rockin’ Christmas. Oh, and check out Scott Stevenson’s really neat THCalendarInfo present to you; it kinda owns my present in terms of complexity. (It reminds me of the Ruby linguistics module in terms of slickness and usefulness.)

Bye!

mogenerator, or how I nearly abandoned Core Data

Unfortunately, my Macbook Pro is out of commission due to a broken fan (I’ve sent it in to Apple; they’d better send it back soon! I’m dying here!), so I’m just going to blog about a mini-renaissance I had when working with Core Data last week.

When I first saw Core Data, I couldn’t believe my eyes. Apple’s developer tools now had a simple way to graphically model as many parts of an MVC application as possible – Interface Builder for the view, Cocoa Bindings (and custom logic) for the controller, and now Core Data for the modeling. Add in the fact that I had heard nothing but praise for it, and I was completely sold. I created the model for my application, prototyped a view, and flipped to the documentation on Core Data’s NSManagedObject.

It was, to say the least, an unpleasant surprise. Though I adored the fact that Core Data would take care of undo/redo, saving, archival formats, and saving data, I didn’t want to start using valueForKey:. setValue: forKey, primitiveValueForKey:, and setPrimitiveValue: forKey: instead of the mutator methods that I had grown to love for their combination of ease-of-use and added maintainability. Sure, I could have made a million subclasses of NSManagedObject, but the idea of doing that manually struck me as tedious – and if there’s anything I loathe, it’s tedium. And updating every object every time I made a change to the Core Data model struck me as a maintainability nightmare.

Though it may reflect poorly on me as a programmer, I considered abandoning Core Data. The magic which it brought to undo/redo/saving/archiving could not overcome the reluctance I had to view all my objects as NSManagedObjects – which, frankly, seems like quite a breach of the Model part of the MVC philosophy.

But hope lay in wait. At the bottom of some Google results about subclassing NSManagedObject, I found this page from Jonathan ‘Wolf’ Rentzsch – one of my idols, both for his coding and his vocal pro-Cocoa advocacy – about an unbelievably clever tool named mogenerator.

In short, mogenerator reads the data you have stored in an .xcdatamodel file, extracts the information about each Entity one has created, creates two subclasses of NSManagedObject for each Entity, changes the .xcdatamodel file automatically so that your Entities inherit from their proper classes, and adds code for all necessary accessor and mutator methods – in short, it removes everything I resented about Core Data.

Why two subclasses? Because if I decide to make a change to the data model, I don’t wont to worry about overwriting my own code with the automatically generated code that mogenerator creates for me. To solve this problem, Wolf has his program use one of the two files for automatically generated code, and allows one to use the second – which extends the first file’s class – for one’s own nefarious purposes. If you ever update the .xcdatamodel file, all you need to do is run mogenerator again, and only the file with the automatically generated code will be overwritten; you can be sure that the custom application logic you’ve written will stay intact.

This is a stunningly useful program, and I don’t know why people aren’t proclaiming it’s merits hither and yon. mogenerator allows one to forgo all compromise with Core Data – you get all the advantages of an NSManagedObject without sacrificing the familiar paradigm of creating specific .c/.h files for each object’s code. My thanks go out to Rentzsch for such an amazing tool.