Decisions are Bullshit
So many new things to say, so many thoughts, so many feelings…I’m not particularly interested in leaving California (70°F as I write this, among other reasons) but I have affairs to settle in Dallas as well as people I’d like to see. Again I’m struck with the feeling that I need to make better use of the time I spend in Dallas. Not only because life is short and I need to always be making the best possible use of the time that I have, but also because of a growing awareness that the sun is setting on my time with that city.Ruby: Implementing progn from Lisp
While hacking on some Ruby code today I started to miss progn from my Common Lisp programming days. If you’re not familiar with progn, it’s a special form which evaluates all of the contained expressions and returns the value of the last one.
Background
The progn construct is more important in the (more-or-less) functional1 programming world than in the imperative world because it allows us to insert multiple expressions where, syntactically, we could otherwise only insert one.
Let’s start with a trivial example:
(if (> x 0)
x
0)If x is greater than zero, return x, else return 0.
But what if we also wanted to output a message whenever x is not greater than 0? We can’t just add our printing line in before (or after) 0 because the compiler knows what’s what based on the positions of the subexpressions in the if expression: it has to be (if test-form then-form [else-form]) 2.
Enter progn:
(if (> x 0)
x
(progn
(print "Returning zero!")
0))Boom! It works. How very exciting. Ok, maybe not. In fact, I called progn a “special form” earlier—which basically means that it’s something which defies the basic evaluation rule of the language—but the truth is it’s probably the least special of the special forms. In fact, implicit progns are all over the place in Lisp: case forms, catch forms, when forms, let forms, but most importantly, function bodies.
Rails: Finding Associated Objects Safely
This one’s simple but I’ve run across it a dozen times today so I’ve decided to vent my frustration via education (I think this is healthy…right?). You have the following simple models: class Customer < ActiveRecord::Base has_many :orders end class Order < ActiveRecord::Base belongs_to :customer end Now, almost invariably, in your OrdersController you have a destroy method that needs to ensure that Customers can only delete their own Orders. Assuming @customer holds the current user’s Customer record, don’t waste your time doing this:iOS: Categories in a Static Library Not Working?
Grrr…if you have category methods in a static library and those methods don’t seem to exist at runtime (unrecognized selector sent to class 0x...), first make sure that your project target is passing the -ObjC flag to the linker (in Other linker flags). If that doesn’t fix the issue, change the -ObjC flag to -all_load. Details from Apple in this Technical Q&A. You might also try using nm to see if your category methods are being properly compiled into your .iOS: Maintaining Compatibility While Leveraging New Frameworks
If you starting using a framework that didn’t exist in a previous version of iOS, you’ll get dynamic linker errors when attempting to run your application on older devices. For example: dyld: Library not loaded: /System/Library/Frameworks/EventKit.framework/EventKit Referenced from: /Volumes/(...) Reason: image not found To fix this you need to modify your application so that the framework is weak-linked, meaning that the application will continue to run even if linking fails.Rails: Application-Specific Config File
If there’s one thing I hate in programming or web design, it’s having to repeat myself. So I always do my best to abstract out repeated bits of information and put them in a single, easily changeable location. A nice way to do this in Ruby on Rails is to create your own YAML config file. First create the config file in config/config.yml (or any other name), and then we’ll get it loaded.NSPasteboard and Dynamic UTIs
When dealing with pasted or dragged data in Cocoa, we receive the passed data on an NSPasteboard. Prior to v10.6 (Snow Leopard), this pasteboard could only contain a single item, but now it can contain multiple items, which are returned as an NSArray via the -pasteboardItems method.
The data passed in via the pasteboard can be in a variety of types (for instance, a single image could be passed both in JPEG format and TIFF format), and for compatibility reasons these types can be expressed in various ways—they can be MIME types, UTIs, OSTypes, or PboardTypes.
If we want to handle pastes and drags in the proper, modern, multi-item fashion, you’d think we could just iterate over the -pasteboardItems (each is an NSPasteboardItem), ask each for its -types, and handle the data in our preferred format. Unfortunately things aren’t always that simple.