Cocos2d makes it easy to convert your drab standard UIAlertViews into glorious customized color dialog boxes. Above is an actual UIAlertView / custom dialog box transition from my upcoming iPhone game. Here’s how I did it.

Implementing a Custom Dialog Box in Cocos2d

Cocos2d Scene Structure

Cocos2d uses a scene graph to hold everything being displayed at any given point in a game. Informally, this data structure is a hierarchy of objects, each of which might display something onscreen, contain other objects to be displayed onscreen, or both. In Cocos2d, these objects are descendants of the CCNode class.

Related objects can be placed into a single container object, which allows them all to be manipulated as a unit when necessary. The root, or lowest-level, container at any given time is an instance of the CCScene class. A CCScene contains CCLayer objects, which themselves contain other objects such as menu labels or sprites to be displayed. Layers, as the name implies, are displayed superimposed upon each other onscreen, which makes a CCLayer perfect for our custom dialog box.

Custom CCLayer

Here’s the code for DialogLayer.h and DialogLayer.m. These implement a basic but perfectly servicable custom iPhone dialog box. It can be used as-is or expanded in any number of ways for particular situations. Take a look through the finished product, and then we’ll go over it below.

DialogLayer.h

//
//  DialogLayer.h
//  concentrate
//
//  Created by Paul Legato on 12/4/10.
//  Copyright 2010 Paul Legato. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "cocos2d.h"

@interface DialogLayer : CCLayer {
  NSInvocation *callback;
}

-(id) initWithHeader:(NSString *)header andLine1:(NSString *)line1 andLine2:(NSString *)line2 andLine3:(NSString *)line3 target:(id)callbackObj selector:(SEL)selector;
-(void) okButtonPressed:(id) sender;

@end

DialogLayer.m

//
//  DialogLayer.m
//  concentrate
//
//  Created by Paul Legato on 12/4/10.
//  Copyright 2010 Paul Legato. All rights reserved.
//

#import "DialogLayer.h"
#import "GameSoundManager.h"

#define DIALOG_FONT @"bicho.fnt"

@implementation DialogLayer
-(id) initWithHeader:(NSString *)header andLine1:(NSString *)line1 andLine2:(NSString *)line2 andLine3:(NSString *)line3 target:(id)callbackObj selector:(SEL)selector
 {
   if((self=[super init])) {

     NSMethodSignature *sig = [[callbackObj class] instanceMethodSignatureForSelector:selector];
     callback = [NSInvocation invocationWithMethodSignature:sig];
     [callback setTarget:callbackObj];
     [callback setSelector:selector];
     [callback retain];

     CGSize screenSize = [CCDirector sharedDirector].winSize;

     CCSprite *background = [CCSprite node];
     [background initWithFile:@"Dialog.png"];
     [background setPosition:ccp(screenSize.width / 2, screenSize.height / 2)];
     [self addChild:background z:-1];

     CCLabelBMFont *headerShadow = [CCLabelBMFont labelWithString:header fntFile:DIALOG_FONT];
     headerShadow.color = ccGRAY;
     headerShadow.opacity = 127;
     [headerShadow setPosition:ccp(243, 262)];
     [self addChild:headerShadow];

     CCLabelBMFont *headerLabel = [CCLabelBMFont labelWithString:header fntFile:DIALOG_FONT];
     headerLabel.color = ccBLACK;
     [headerLabel setPosition:ccp(240, 265)];
     [self addChild:headerLabel];

     //////////////////

     CCLabelBMFont *line1Label = [CCLabelBMFont labelWithString:line1 fntFile:DIALOG_FONT];
     line1Label.color = ccBLACK;
     line1Label.scale = 0.84f;
     [line1Label setPosition:ccp(240, 200)];
     [self addChild:line1Label];

     CCLabelBMFont *line2Label = [CCLabelBMFont labelWithString:line2 fntFile:DIALOG_FONT];
     line2Label.color = ccBLACK;
     line2Label.scale = 0.84f;
     [line2Label setPosition:ccp(240, 160)];
     [self addChild:line2Label];

     CCLabelBMFont *line3Label = [CCLabelBMFont labelWithString:line3 fntFile:DIALOG_FONT];
     line3Label.color = ccBLACK;
     line3Label.scale = 0.84f;
     [line3Label setPosition:ccp(240, 120)];
     [self addChild:line3Label];

     CCMenuItemImage *okButton = [CCMenuItemImage itemFromNormalImage:@"OKButton.png" selectedImage:@"OKButtonSelected.png" target:self selector:@selector(okButtonPressed:)];
     [okButton setPosition:ccp(0, 60)];

     CCMenu *menu = [CCMenu menuWithItems: okButton, nil];
     menu.position = ccp(240,0);
     [self addChild:menu];
   }
   return self;
 }

-(void) okButtonPressed:(id) sender
 {
   [[GameSoundManager sharedManager].soundEngine playEffect:@"OK.caf"
                                    pitch:1.0f
                                    pan:0.0f
                                    gain:1.0f];

   [self removeFromParentAndCleanup:YES];
   [callback invoke];
 }

-(void) dealloc
 {
   [callback release];
   [super dealloc];
 }
@end

Custom dialog box code analysis

Overview

The dialog box is implemented as a subclass of CCLayer. An initWithHeader method is provided to initialize the layer with a given message and a specified callback method that will be invoked when the dialog’s “OK” button is pressed. It also plays a confirmation sound and switches the “OK” button’s image to a “pressed button” when the user taps it.

DialogLayer.m requires a few external supports, which must be available as project resources. Dialog.png is the dialog box’s background image. bicho.fnt is an AngelCode format bitmapped font, used for the labels, and can be substituted with any font you like. OKButton.png and OKButtonSelected.png are the images to be used for the button in unpressed and pressed forms. Finally, OK.caf is the audio file to be played when the user taps the button.

Analysis

Storing the callback

     NSMethodSignature *sig = [[callbackObj class] instanceMethodSignatureForSelector:selector];
     callback = [NSInvocation invocationWithMethodSignature:sig];
     [callback setTarget:callbackObj];
     [callback setSelector:selector];
     [callback retain];

This code is Objective C’s (massively redundant) way of storing the callback to be activated when the dialog’s button is pressed. It’s based on the callback code in Cocos2d itself. (We could also have used Objective C’s new closure data type, called “blocks”, but that syntax grates my Lispy sensibilities even more than the overly verbose Javaish mess here, besides not working out of the box on older versions of iOS.)

The initWithHeader method is given a target object and a method selector. It looks up the class of the target object and then looks up the selector on that class at line 1, resulting in a NSMethodSignature object which encapsulates the class-method combination. Line 2 transforms that class-method signature into an “invocation,” which represents the act of actually calling that method on some as yet unspecified object. Line 3 associates the invocation with the particular target object we were given, and line 4 associates the selector (which is utterly odd, since the desired method is encoded in the signature object already.) Finally, we retain the callback so that we can save it for later (to be activated when the user actually presses the “OK” button) and it won’t be deallocated at the end of the present scope.

Setting up the background and labels

The remainder of the initWithHeader method, lines 25 to 68 in the main listing above, is pretty straightforward. A CCSprite object is created to store the background image and added to self, that is, to the new custom layer being created. Its position is set to be the center of the screen, and its level on the Z axis is set to -1 so that it will appear below anything else added in the layer.

CCLabelBMFonts are then created for the given text and added to the new layer. A shadow effect is created for the header by writing the same thing in grey at a slight offset underneath the main text. All lines except the header are then scaled down to 0.84 of their natural size, so as to enhance the prominence of the header.

A CCMenuItemImage is finally created to be the “OK” button. It’s added to a CCMenu, which handles the logic of button presses for us, and the CCMenu is then added to the new layer. It will take a little experimentation to get the exact placement of the OK button correct (lines 64 and 67) relative to your background image.

Note that the OK button itself takes callback arguments of exactly the same type that our initWithHeader method does (line 63). We might be tempted to forego the construction of our own callback object and just pass the given target and selector on to the OK button, simplifying the design of our DialogLayer a bit, but then we wouldn’t have the chance to play a sound when the OK buton is pressed. More seriously, we would have no way to destroy the dialog layer and remove it from the screen. So, we give the OK button a target of self and tell it to call the okButtonPressed method when OK is pressed.

The okButtonPressed callback

The CCMenuItemImage will invoke this method when the user presses “OK”. This means that it’s time to remove the DialogLayer from the scene (easy enough, line 80). Here, we also play a little click-type sound to make the game seem more interactive; also easy with CocosDenshion, lines 75-78. If you don’t want to play a sound, you can remove those lines and the GameSoundManager.h include file.

dealloc

Nothing fancy here; we just release our retained callback object and then defer to super.

Using the custom DialogLayer

Adding a custom dialog layer to your game scene is very easy:

   CCLayer *dialogLayer = [[[DialogLayer alloc]
                            initWithHeader:@"Level 1"
                            andLine1:@"Tilt The Phone"
                            andLine2:@"To Balance"
                            andLine3:@"The Ball!"
                            target:self
                            selector:@selector(beginLevelOne)] autorelease];
   [self addChild:dialogLayer z:200];

Future Expansions

DialogLayer is a simple modal dialog box that can only accept one action from the user. It can easily be enhanced in many ways, depending on your needs. The most obvious enhancement is to add multiple buttons to allow the user to make a choice rather than simply dismissing the box. It could be modified to accept an arbitrary amount of text and divide it up automatically into an aesthetically pleasing configuration.

One thing that DialogLayer does not do is pause the game. I decided that that is not necessarily desirable in all cases; many game scenarios need to present information to the user without stopping the action. It’s easy enough to pause manually when activating the dialog.

Little enhancements like this are critical as ‘polish’ for your game.

iPhone Pros

• iPhones are all (almost) exactly the same. 4th generation iPhones do support an optional higher screen resolution than previous models, but they’re 100% binary backwards compatible. This is a trivial difference when put up against the dozens of different Android devices available with radically different hardware, and the hoops that you as a developer have to jump through to make sure your app runs correctly on all of them.
• Large, high-end user base that demonstrably spends money on apps. iPhone users have already shown their willingness to buy large numbers of apps; Android users, not so much. This seems to be slowly changing over time as the Android app store matures, but for now, iPhone is still the cash cow.
• Beautiful and well-designed UI and system architecture. iPhones have been designed as an integrated and coherent experience. Apple’s renowned mastery of intuitive human-computer interaction is evident at every step while using the device. There is an overall (and well documented) unifying strategic vision of how different subsystems should fit together seamlessly and present a unified interface to the user. Their development libraries have been designed to support this theme at every level. Android, by contrast, reads much more like KDE or Gnome; it was bolted together out of disparate pieces written by different groups working in isolation with minimal coordination. Perceived technical coolness from the perspective of a software engineer who is already intimately familiar with the system’s internals took precedent over usability. Android’s user interface is generally much uglier and much less intuitive.
• iPhones support low latency audio. Android uses ALSA internally, which does support low-latency audio, but the low-latency API is not exposed to developers, and Google does not seem to be interested in fixing this. Android latency is unacceptably high for realtime sound synthesis unless you hack it and use the ALSA API, which is officially forbidden and in any case could change significantly and without warning between Android operating system updates and between phones. iPhone, on the other hand, has an officially supported low latency audio API, and there are many successful realtime sound synthesizer apps in the app store.
• XCode. XCode is a really well done IDE. I normally don’t like IDEs. I normally use Emacs for everything. But, in all fairness, I decided to give XCode and Eclipse with the Android plugins a shot. Eclipse is nice, but a little rough around the edges. I ran into a few bizarre IDE error messages while developing my Android test projects that required extensive Googling and hours of experimentation to unravel. XCode, as one would expect from a flagship Apple product, is immaculately polished and completely integrated from top to bottom.

iPhone cons

• Objective C. While Objective C was really cool and innovative in 1988, today it seems archaic. Nobody uses it except Apple. It’s laden with tons of cruft. It still feels like it’s a bolt-on afterthought preprocessor for a C compiler. For example:
  • Selectors: you can’t refer to method names directly; you have to use the special @selector() compiler directive to look up their index number. This probably made great sense when CPUs were very slow and doing a lookup in an ASCII table was a significant performance hit over doing an integer offset jump, but on modern computers, this is just annoying. CPU time is cheaper than engineer time.

  • Manual reference-counted memory management is an obsolete and extremely fragile pain in the ass. Lisp had GC ~40 years ago. The vast set of problems introduced by manual memory management, such as buffer overflows and memory leaks, is just not worth the small gain in efficiency that you get from not periodically running a garbage collector. Again, CPU time is cheaper than engineer time. Android does GC on a phone just fine, so I don’t buy the argument that a phone can’t support that kind of overhead. Garbage collection was recently introduced for the Mac OS X desktop system, but is as of yet not available on the iPhone.

    (I’m aware that C and C++ are widely used and both still make you do manual memory management, too. Far from regarding this as a vehicle for a macho display of programming prowess, I find manual memory management to be more of an annoyance, something that the computer should be handling for me automatically, much like how figuring out the exact memory address of a function was automated long ago and is no longer something we as application-level programmers ever need to worry about much.)

  • The use of + and - to mark methods as class or instance methods. Principle of least surprise; why not just use, say, the English word class or static or something to that effect to designate class methods, and leave instance methods unmarked? Perhaps using 1 character symbols rather than entire English words as the token improved compile times on large, complex projects in the ’80s, but the time savings is infinitesimal on a modern computer, so I’ll vote for obviousness and clarity rather than crypticity.
  • The use of [] (square brackets) for message passing, and the requirement for named (but non-reorderable) arguments.

    Perhaps this was done out of a well-intentioned effort to make the programmer notice that these are messages and not function calls or C++ methods, and to make the code self-documenting. I find the introduction of new syntax rather unwarranted and distracting. I’d much prefer the familiar object.message(argument1, argument2) over the verbose and unwieldly [object message:argument1 parameterName:argument2]. You can’t even re-order the named arguments, so including the argument name is just noise in the code. I guess I’ve been spoiled by Clojure and Lisp, where there’s almost no syntax.

    If they really felt strongly about the need to pedantically differentiate messages from function calls or methods at the syntax level, they should at least have retained the general function/method syntactic template and chosen a different separator character, so as to enable programmers to re-use most of their neural syntax parsing circuitry rather than force us to introduce an entirely new syntactic template that performs only 1 task. For example, a more intuitive syntax that still emphasizes the fact that this is not a function or method call could be something like object#message(argument1, argument2) or object<-message(argument1, argument2).

• Closed platform. The source code to the vast Cocoa library is proprietary. You can develop with things Apple tells you you may develop with, and nothing else. They only recently relaxed the restriction on using any third party tools at all. The iPhone's CPU is widely reported to have built-in hardware level Java bytecode support, but that's off-limits.
• Vendor lock-in. This is a huge and obvious minus for the iPhone. Your ability to generate revenue is 100% dependent upon Apple's whims, upon remaining in Apple's good graces. Uncountable horror stories circulate online about developers who invested months and many thousands of dollars into a project only to have it rejected by Apple, often after a considerable delay and with little or no explanation.

Android pros

• Java (almost). Android apps are written in Java. (Well, technically not in Java due to licensing issues, but in a language that is 99.9% the same as Java.) If you already know Java, as I do, this is a big plus. You can easily re-use your code elsewhere, on any platform, with minor or no modifications. Objective C code is rather limited in what you can do with it: you can re-use it in your Apple desktop apps. (Yes, there is a free GNU Objective C compiler, but almost all real-world Objective C code is locked into Apple's proprietary Cocoa runtime system.)
• A JVM, almost. Android doesn't run a true JVM, unfortunately, as part of a licensing dispute between Google and Sun, but it's close. Android's Dalvik is a cleanroom reimplementation of part of Java, designed to avoid triggering what Google regards as onerous licensing fees for the use of a true JVM on a mobile handset. In any case, it's close enough that the door is open to potentially running Clojure and other JVM languages on it someday.
• No upfront developer fees. Develop for free. You can download the SDK and go nuts, upload your code to a real handset to test, send your apps to the app store for sale, whatever. There is no $99 developer tax.
• Larger ultimate user base. Android phones are now outselling iPhones, and this trend looks set to continue. The price you pay is coding for 200 slightly different handsets.
• Open source platform. Most of the Android system is freely available, precluding vendor lock-in, at least in theory.

Android cons

• Android emulator lacks hardware accelerated graphics. This is a big minus. I made a very simple bouncing-ball demo with Rokon that got about 6 or 7 fps in the emulator. Full games often take multiple seconds per frame, completely unusable. If you want to write games (or anything else where graphical performance is key), you absolutely need to have a hardware handset from the start. If you don't already own an Android phone anyway, this means you have to invest hundreds of dollars upfront, just to see if the platform is for you.
• Lack of interest from Google. Google has many, many pots cooking at once. While they do devote significant resources to Android's maintenance and expansion, it's nowhere near as important a product for them as the iPhone is for Apple, and you can tell. The iPhone is a core strategic product for Apple, whereas Android is a relatively minor satellite project for Google. Correspondingly, Apple's general support of the iPhone and its developers seems far better.
• Relatively few libraries. For the moment, there seem to be more third party libraries available for the iPhone, and they seem to be more mature and robust than what's out there for Android. This will change in time, as more and more app developers begin to use Android.

It was a close call. I did some exploratory coding on both platforms — there's no substitute for direct experience. Though I much prefer Java over Objective C as a language, the universe of extant Android libraries is much smaller and less mature. I may have been able to live with that in order to be able to code in Java and avoid Apple's vendor lock-in. In the end, the tipping point was practical: the Android emulator's lack of hardware accelerated graphics left me with an unusable framerate, so I'd have to spend hundreds of dollars on a handset to do even trivial demo development, whereas I already own an iPhone.

I'll be checking back in on Android periodically, as I expect its long-term position will improve substantially in time, but for now, I'm going with the iPhone.

The GUI is written entirely in Clojure with the Swing toolkit. Calculation state is stored in a series of atoms. Watches are used to update the output table automatically when an atom changes, an idea from Kotka. I used the excellent MiG Layout for general layout functionality, and generic Swing widgets (JTextFields and a JTable) for the input and output.

Clojure code

The Black-Scholes GUI is launched with themain function. The Swing GUI is actually created in the initialize-gui function, but since Swing widgets are not threadsafe, we must use the do-swing* helper function from Clojure Contrib to spin off the real GUI creation code into the Swing event thread.

(defn initialize-gui
  []
  (let [frame (JFrame. "Black-Scholes Option Modeler")]

    (doto frame
      (.setDefaultCloseOperation JFrame/DISPOSE_ON_CLOSE)
      (-> .getContentPane
          (.add (miglayout (JPanel.)

                           (input-panel
                            (fn [_]
                              (do-swing
                               (doto frame
                                 (.setVisible false)
                                 (.dispose)))))

                           (result-table)

                           )))

      (.setDefaultCloseOperation JFrame/DISPOSE_ON_CLOSE)
      (.pack)
      (.setVisible true))
     ))

(defn main
  []
  (do-swing* :now initialize-gui))

This is pretty straightforward. We make a (locally scoped) JFrame to be the main container at line 3. Within the context of the let, we add a JPane using MiG Layout to its content area (starting at line 8). The JPanel itself contains two subwidgets, which are those returned from the input-panel and result-table functions.

Note that we are passing an anonymous function to input-panel at line 11. The input panel contains the “Quit” button, and that button needs to know what to do when it’s pressed. We want it to close the GUI; that is, it should destroy the outer JFrame. To do that, it needs a reference to it. However, as a local variable in initialize-gui, the JFrame is not in scope elsewhere; we have to pass it in. The given function closes over the JFrame reference (line 13) and transports it to input-panel, where that function will be called as an event listener when the “Quit” button is pressed.

We could also have simply passed the bare JFrame rather than a function, and moved the frame-closing code into input-panel, but that would imply that the input panel must always be in a JFrame, which will not necessarily the case. Passing in an external anonymous function to be activated by the “Quit” button allows the input panel to be used generically in any sort of container, with the container deciding what to do when “Quit” is pressed.

The Input Panel

(defn- input-panel [quit-action]
    (let [
          ;; Fields
          spot (doto (JFormattedTextField. (NumberFormat/getNumberInstance))
                 (.setColumns 6)
                 (.setValue @*spot*))

          strike (doto (JFormattedTextField. (NumberFormat/getNumberInstance))
                   (.setColumns 6)
                   (.setValue @*strike*))

          days-till-expiry (doto (JFormattedTextField. (NumberFormat/getIntegerInstance))
                             (.setColumns 6)
                             (.setValue @*days-till-expiry*))

          riskfree (doto (JFormattedTextField. (NumberFormat/getNumberInstance))
                     (.setColumns 6)
                     (.setValue @*riskfree*))

          volatility (doto (JFormattedTextField. (NumberFormat/getNumberInstance))
                       (.setColumns 6)
                       (.setValue @*volatility*))

          ;; Buttons
          calculate (JButton. "Calculate")
          quit (JButton. "Quit")
          ]

      (add-action-listener
       calculate
       (fn [_]
         (update-all-calculations
          (Double/parseDouble (.getText spot))
          (/ (Integer/parseInt (.getText days-till-expiry)) *trading-days-per-year*)
          (Double/parseDouble (.getText strike))
          (Double/parseDouble (.getText riskfree))
          (Double/parseDouble (.getText volatility)))))

      (add-action-listener
       quit
       quit-action)

      (miglayout (JPanel.)
                 :layout  [:wrap 2 ]

                 (JLabel. "Spot") [:align "right"]
                 spot

                 (JLabel. "Strike") [:align "right"]
                 strike

                 (JLabel. "Risk-free rate") [:align "right"]
                 riskfree

                 (JLabel. "Volatility") [:align "right"]
                 volatility

                 (JLabel. "Days till expiry") [:align "right"]
                 days-till-expiry

                 calculate
                 quit)))

The input widgets are local variables in the input panel function, along with the two buttons. The widgets are initialized from the state atoms (which are then unused again; see discussion below.) The “Quit” button is given the event callback passed in as an argument (so it can manipulate the enclosing JFrame as per above.) The “Calculate” button is given an event callback that calls the update-all-calculations function with the values in the input widgets.

The lot are then stuffed into a JPanel with a MiG Layout and returned.

The Output Table

(defn- update-cell
  "Updates the given AbstractTableModel cell and fires a change event for that cell."
  [model data row col]
  (.setValueAt model data row col)
  (.fireTableCellUpdated model row col))

(defn- result-table []
  (let [
        column-names ["" "Call" "Put"]
        row-names ["Price" "Delta" "Theta" "Rho" "Gamma" "Vega"]
        table-model (proxy [AbstractTableModel] []
                      (getColumnCount [] (count column-names))
                      (getRowCount [] (count row-names))
                      (isCellEditable [] false)
                      (getColumnName [col] (nth column-names col))
                      (getValueAt [row col]
                                  (condp = col
                                      0 (nth row-names row) ;; Return the row name for column zero
                                      (condp = [row col]
                                          [0 1] @*call-price*
                                          [0 2] @*put-price*

                                          [1 1] @*call-delta*
                                          [1 2] @*put-delta*

                                          [2 1] @*call-theta*
                                          [2 2] @*put-theta*

                                          [3 1] @*call-rho*
                                          [3 2] @*put-rho*

                                          [4 1] @*gamma*
                                          [4 2] "=="

                                          [5 1] @*vega*
                                          [5 2] "=="

                                          nil))))

        table (doto (JTable. table-model)
                (.setGridColor java.awt.Color/DARK_GRAY))
        ]

    (add-watch *call-price* ::update-call-price (fn [_ _ _ newprice] (update-cell table-model newprice 0 1)))
    (add-watch *put-price* ::update-put-price (fn [_ _ _ newprice] (update-cell table-model newprice 0 2)))

    (add-watch *call-delta* ::update-call-delta (fn [_ _ _ newval] (update-cell table-model newval 1 1)))
    (add-watch *put-delta* ::update-put-delta (fn [_ _ _ newval] (update-cell table-model newval 1 2)))

    (add-watch *call-theta* ::update-call-theta (fn [_ _ _ newval] (update-cell table-model newval 2 1)))
    (add-watch *put-theta* ::update-put-theta (fn [_ _ _ newval] (update-cell table-model newval 2 2)))

    (add-watch *call-rho* ::update-call-rho (fn [_ _ _ newval] (update-cell table-model newval 3 1)))
    (add-watch *put-rho* ::update-put-rho (fn [_ _ _ newval] (update-cell table-model newval 3 2)))

    (add-watch *gamma* ::update-gamma (fn [_ _ _ newval] (update-cell table-model newval 4 1)))
    (add-watch *vega* ::update-vega (fn [_ _ _ newval] (update-cell table-model newval 5 1)))

    ;; This shrinks the table's preferred viewport down to its actual size.
    ;; (The default is to make a huge viewport, even though the table is small.)
    (.setPreferredScrollableViewportSize table (.getPreferredSize table))

    (JScrollPane. table)
    ))

The output table itself is a standard JTable. Its model is initialized from the output state atoms. The watches at lines 44-57 are the most interesting part; they allow arbitrary code to be called when the output state atoms change. This allows the Black-Scholes calculation to happen independently of the output widget. When one of those atoms is changed, it self-updates automatically via the watches.

The helper function at lines 1-5 updates the appropriate model cell and fires an event to tell the JTable to repaint it.

The scrollable viewport business at line 61 is to work around a questionable Java design decision where a JTable, by default, tells its container to make its viewport enormous, even though it have very little data. (That’s not an error, that’s subjunctive.)

Design considerations

GUI and business logic decoupling

Above all, the GUI has to be decoupled from the model itself. Internal changes to one should never affect the other, and the two should be independently testable. In such a simple application, this was easy enough. A more complex app that requires several information round trips would be trickier. We now have a good foundation for such an app. No business calculations are performed in the GUI code namespace. The sole point of connection is in the update-all-calculations function:

(defn- update-all-calculations [spot timeleft strike riskfree sigma]
  (swap! *call-price* (fn [_] (bs/call spot timeleft strike riskfree sigma)))
  (swap! *put-price* (fn [_] (bs/put spot timeleft strike riskfree sigma)))

  (swap! *call-delta* (fn [_] (bs/call-delta spot timeleft strike riskfree sigma)))
  (swap! *put-delta* (fn [_] (bs/put-delta spot timeleft strike riskfree sigma)))

  (swap! *call-theta* (fn [_] (bs/call-theta spot timeleft strike riskfree sigma)))
  (swap! *put-theta* (fn [_] (bs/put-theta spot timeleft strike riskfree sigma)))

  (swap! *call-rho* (fn [_] (bs/call-rho spot timeleft strike riskfree sigma)))
  (swap! *put-rho* (fn [_] (bs/put-rho spot timeleft strike riskfree sigma)))

  (swap! *vega* (fn [_] (bs/vega spot timeleft strike riskfree sigma)))
  (swap! *gamma* (fn [_] (bs/gamma spot timeleft strike riskfree sigma))))

Atoms to hold state

As we can see above, set of atoms holds state for the GUI, in keeping with the general Clojure principle of isolating state into transactional memory. (Actually, I cheated a bit. Our full state consists of the input values and the results of our Black-Scholes calculations. Although I created atoms for the input state values as well, I calculate the results directly from the widgets. It is a trivial exercise to alter the “Calculate” button callback to use the atoms. This would be more useful in the case where changes to a widget fire an event that triggers recalculation, which is not done in this demo as per below. I would use the atoms in a real application.)

I/O is inherently stateful, as are GUIs. There was therefore the temptation to use the Swing widget objects themselves as state-holders, and in fact it requires extra code to shuttle data back and forth between the widgets and the atoms. In such a simple app as this, using the GUI widgets as state-holders does not make much practical difference, but in a more complex app, a well designed set of state atoms and watches can automate much of the tedious GUI logic.

Using the widgets directly as state holders implies a tight coupling between the input widgets, the business logic, and the output widgets. Changing any one of them would invariably have required changes to the linking logic.

For example, suppose we were using widgets-as-state, and we changed the “days left till expiry” input to a slider rather than a text box, or suppose we wanted to read it from a file or from an API. We would have to update the code that reads it, calculates the model, and puts the results into the output table. Touching that code implies a non-zero likelihood of breaking it. Why even introduce the potential for breaking something that is unrelated to the task at hand, in this case the business logic and output code?

Again, this seems trivial with such a very small application that is only a few lines long, but the design principle is what’s important. Imagine applying it to a much larger application with hundreds of inputs spread across 4 windows, 3 databases, and 6 web feeds to see why this is a good idea.

Auto-updating results when input changes

I wanted the results to auto-update anytime an input is changed, without resorting to a “Calculate” button. Having the button trigger the calculation and output update allows the state of the input and the output to become decoupled, which can be confusing for the user. Worse, it could be unnoticed by a user who is doing other things at the same time.

Due to typical Java overengineering, there is no straightforward way to simply (add-action-listener) on the JTextField and get notifications when it changes. Instead, each Swing widget implements full-blown model-view separation, and so you have to get the document underlying the JTextField. Yes, the view components of your MVC application themselves contain both models and views. Each and every field contains an entire implementation of the document object and associated cruft, all for itself. If you want to monitor changes in the widget, you need to build out a DocumentListener to watch for such events.

In a real application, I’d bite the bullet and implement it, but it’s not worth the extra time for the purposes of this demo, so we are left with a “Calculate” button and manual updating.

Clojure-Options on Github

The entire package so far is now available on my GitHub page. Patches are welcome. Atomizing the input widgets and adding the appropriate event callbacks and watches to eliminate the “Calculate” button is particularly welcome.

> (log/spy [1 2 3 4 5])
[1 2 3 4 5] ;;; prints " [1 2 3 4 5] => [1 2 3 4 5]" to the log
> (log/spy (take 3 [1 2 3 4 5]))
(1 2 3) ;;; prints "(take 3 [1 2 3 4 5]) => clojure.lang.LazySeq@8592" to the log

The spy macro logs a string realization of the sequence per se, rather than its constituents. In the case of a concrete, fully realized sequence such as the literal vector in the first example, all values are known in advance, and the vector’s string realization in the log can helpfully be its actual contents. In the case of the lazy sequence returned by take, however, we get a less than helpful class name and some sort of instance identifier code rather than the actual contents of the sequence. A lazy sequence saves resources by merely promising to provide the values when asked. If they’re never asked for, they’re never computed. Since they haven’t been computed, they can’t be printed. A lazy sequence can also potentially be infinitely long, another good reason why they are not realized by default when converting the sequence to a string.

In our case, we don’t care about the resource usage, and we’re sure the sequence is finite. We want to force all values to be computed for manual inspection. (There are several ways of going about this.) The following modification of the spy macro converts the lazy sequence into a regular seq:

(defmacro lazy-spy
  "Evaluates expr and outputs the form and its result to the debug log.
   If the result is a lazy sequence, it is realized concretely in the log.
   Returns the result of expr."
  [expr]
  `(let [a# ~expr] (log/log :debug (str '~expr " => "
                                    (if (= clojure.lang.LazySeq (class a#))
                                      (seq a#)
                                       a#)))
        a#))

This gives us:

> (lazy-spy  (take 3 [1 2 3 4 5]))
(1 2 3) ;;; prints "(take 3 [1 2 3 4 5]) => (1 2 3)" to the log

Clojure posts you might also like:

• Log4J and Clojure Configuration
• Setting Clojure’s Log Level
• SQL WHERE clauses in Clojure from S-Expressions
• XML DTD Validation in Clojure: Turning It Off, Parsing Malformed XML

1. If using Leiningen, add

   :dependencies [
                    [log4j "1.2.15" :exclusions [javax.mail/mail
                                                 javax.jms/jms
                                                 com.sun.jdmk/jmxtools
                                                 com.sun.jmx/jmxri]]
   ]
   

   to your project.clj file and rerun lein deps to auto-install the Log4J JAR.

   If not using Leiningen, acquire a Log4J JAR and put it on your classpath.

2. Create a file called log4j.properties on your classpath and put the following into it:

   # Based on the example properties given at http://logging.apache.org/log4j/1.2/manual.html
   # Set root logger level to DEBUG and its only appender to A1.
   log4j.rootLogger=DEBUG, A1
   
   # A1 is set to be a ConsoleAppender.
   log4j.appender.A1=org.apache.log4j.ConsoleAppender
   
   # A1 uses PatternLayout.
   log4j.appender.A1.layout=org.apache.log4j.PatternLayout
   log4j.appender.A1.layout.ConversionPattern= %-5p %c - %m%n
   

That’s the short version. Read on for the gory details.

Logging philosophy

My logging requirements are simple at the moment. I want to write messages to the console, each tagged with a log level (“INFO”, “WARN”, “ERROR”, etc.) I want to be able to set a log level, and only see log messages of that severity or higher. That’s it. I take this to be the canonical base case of logging that covers 80% of needs. All other logging features (e.g. logging to a file, differentiating by class or namespace) are nice, but much less common, and ought to be handled via configurable deviations from this default case.

clojure.contrib.logging does not presently make it easy to do any of this common base case out of the box. The Lispy façade quickly falls apart; you’re dumped into the deep end of the Java overengineering mire without a life vest when you simply want to change the log level temporarily.

What’s wrong with clojure.contrib.logging’s defaults?

clojure.contrib.logging with no special configuration done prefaces each and every logged line on my system with:

[null] May 16, 2010 1:08:48 PM clojure.contrib.logging$eval__2077$impl_write_BANG___2088 invoke

I have no idea why. This makes it very hard to read the log when logging any nontrivial amount of information.

clojure.contrib.logging’s documentation contains a rather cryptic offhanded remark, with no further explanation: Note: your log configuration should display the name that was passed to the logging implementation, and not perform stack-inspection, otherwise you’ll see something like “fn__72$impl_write_BANG__39__auto____81″ in your logs.

My log configuration? Stack inspection? Why do I need to have a log configuration to just dump strings to the console? Why is the default case to print a verbose and useless header for each line? Isn’t this Javaesque mess — obscure 50 line XML configuration files just to get basic logging functionality working — exactly what we’re trying to avoid?

Log4J with clojure.contrib.logging configuration

Ah, well, there’s nothing for it, I suppose. Better just get a serious logging library and figure out how to configure it. The docs say that clojure.contrib.logging delegates to the first supported logging implementation that it finds. I am apparently using the builtin JDK logger by default, and Apache Commons’ logger is just another wrapper, so I added Log4J to Leiningen’s project.clj and it installed the jar for me.

Now, I get:

     [null] log4j:WARN No appenders could be found for logger (my-project-name).
     [null] log4j:WARN Please initialize the log4j system properly.

Followed by dead silence.

Hm.

I can only imagine that Log4j was never intended to be used without a custom configuration. Odd. Why people design software that comes without reasonable defaults pre-set to cover the common base case is beyond me. I fully understand that strange setups will require customized configuration, but “dumping strings to the console” does not seem like a bizarre edge case for a logging package. A default case of “send all log messages to the black hole unless configured to do otherwise” makes little sense.

So, how hard can it be to configure Log4J for this base case functionality? The “short introduction to log4j” is 19 pages long. Sigh. (The complete manual is proudly described as “over 200 pages.” I sometimes wonder if they overengineer Java libraries on purpose just to drive book sales.)

I do not want to know what a Nested Diagnostic Context is, or the precise semantic distinction between “Loggers,” “Appenders,” and “Layouts.” I don’t care about “Appender Additivity” and “Named Hierarchies,” and I certainly don’t want to write an XML configuration file. I Just want to log tagged strings to the console, and have them print without a bizarre autogenerated header line. That’s it. I fully appreciate that those things will be useful for some people — maybe even for me, when my project grows to become more complex. I have nothing against their existence; but for the base case, one should not need to know of it.

On page 9 of the “short” manual, after a boatload of irrelevant information, we have an answer: there is a class called BasicConfigurator with a static method that does reasonable initialization for logging to the console. We can just do (org.apache.log4j.BasicConfigurator/configure) and get that.

However, we are still getting log messages like:

     [null] 3702 [8633270@qtp-3149669-2] INFO my.namespace - log message

We have to learn something about Layouts now. The thread names generated by Clojure are apparently not very useful, so we’re going to turn them off. And while interesting when profiling, the number of milliseconds since program start is just going to be visual clutter 98% of the time, so I’m going to turn that off, too, until I actually need it.

The result is the log4j.properties file listed above. Just copy and paste it into your classpath. If you want more or different information in your logs, take a look at the PatternLayout API documentation and alter the ConversionPattern line according to your preferences.

The result:

     [null] INFO  my.namespace - log message

I’m still not sure where the [null] comes from, but overall, it’s very readable.

Clojure posts you might also like:

• Lazy-spy: Clojure logging/spy for lazy sequences
• Setting Clojure’s Log Level
• SQL WHERE clauses in Clojure from S-Expressions
• XML DTD Validation in Clojure: Turning It Off, Parsing Malformed XML

error: java.io.IOException: Server returned HTTP response code: 503 for URL: http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd

It seems that the W3C blocked access to the DTDs two years ago, but Java still tries to load them by default anyway. The following allows clojure.xml to run without checking external DTDs:

(defn startparse-sax-non-validating [s ch]
  (.. (doto (. javax.xml.parsers.SAXParserFactory (newInstance))
       (.setValidating false)
       (.setFeature "http://apache.org/xml/features/nonvalidating/load-dtd-grammar" false)
       (.setFeature "http://apache.org/xml/features/nonvalidating/load-external-dtd" false)
       (.setFeature "http://xml.org/sax/features/validation" false)
       (.setFeature "http://xml.org/sax/features/external-general-entities" false)
       (.setFeature "http://xml.org/sax/features/external-parameter-entities" false))

       (newSAXParser) (parse s ch))))

Then you can simply (xml/parse "sourcefile.xml" startparse-sax-non-validating). This is not the ideal solution — ideally, we want to use a locally cached DTD — but it works well enough for one-off code. Read on for further information.

W3C’s malformed specification

The W3C webserver was being DDoSed by the massive number of XML parsers trying to download the DTDs constantly, so they just blocked everyone. This is a direct result of the poorly considered technical specification for DTD identification and URIs themselves, then exacerbated by the W3C’s institutional culture and philosophy.

Technical commentary

The root of the problem is, as they put it, that “these [DTD URIs] are not hyperlinks; these URIs are used for identification” (emphasis in original.) This is an abuse of the URL format. If it’s “for identification,” then it doesn’t need to specify a transport protocol! They are relying upon their particular interpretation of the old URI/URL debate, reducing the transport protocol element to a namespace identifier instead, and then assuming everyone will defer to their judgement and wisdom on the matter.

The W3C (and others who share their URL/URI interpretation) decided to conflate the semantic functions of locating and transporting a resource with the very distinct function of identifying a resource. “My name” is not the same thing as an instruction to “get in your car, drive to my street address, pick up someone with my name here, and then drive me back to your house,” even though there is indeed only one person with my name at this address. If you want to identify me, you would not give someone the latter instruction. Most likely, they didn’t think about this at all and just did it. Now everyone suffers for this poor design decision. They are being DDoSed and paying massive amounts of money to keep the servers going, and everyone’s XML parsers are broken because they’re now returing 503s for all the DTD URIs.

The W3C’s proposed solution is that everyone else should a) defer to their views on the URL/URI matter, and 2) rewrite their XML parsers to be massively more complex and build local caching DTD catalogs for every single one-off parse. In practice, most people will just turn DTD validation off, potentially breaking a lot of other things.

Institutional commentary on the W3C itself

The tone of naive incredulity in the W3C’s plea is most amazing of all. They seem genuinely baffled as to why anyone would ever choose to ignore their wisdom on the URL/URI matter, and even more confused as to how anyone could ignore any detail of a baroque and poorly communicated specification like those for XML and DTD cataloging. “We spent a long time writing that, and we have [the self-appointed] institutional authority to write specifications and make determinations like what a URI should be, so surely people would never just ignore us!”

For them, the only conceivable explanation for every detail of the specification not being implemented is sheer ignorance. They even gently chide the readers about their failure to use URIs properly and to build DTD caches into everything, much as a kindergarten teacher would a well-meaning but rather dumb child who continually ignores instructions about how to use his crayons. They think that they only have to patiently remind people enough times that there is indeed a right way to do it, that which is sanctioned by the appropriate institutional authorities (them, of course) — and finally everything will be fine once people get that message.

This is the sort of ingenuous attitude that can only develop in a pure research institute and among people who mostly have very high power distance index scores. The high PDI explains their naive arrogation of global authority and utterly genuine assumption that everyone ought to just do as they say, simply because they’re the (self-)designated authority. The research institute setting means that they’re shut off from the economic real world. Since money is never a big issue, the worship of artificially created fancy titles and institutional authority can become rampant, and it’s perfectly fine to spend extended amounts of time parsing a 2 page XML document “the right way” rather than 15 minutes hacking it and moving on to more pressing problems so your company doesn’t go bankrupt in the interim.

Moreover, they are still intransigently defending the original poor design choice to use URIs with embedded transport protocols as unique identifiers, because they (as an institution) cannot admit the possibility that this was a poor choice. That would create a great deal of cognitive dissonance vis-à-vis their conception of the nature of an institution and its role in dictating what is correct and what is not in the world; their view that institutional authority must be right, because, well, otherwise it wouldn’t be an institution. After all, everyone there does have more fancy titles than most people who are not there.

These attitudes are pervasive throughout most W3C work, and are the main reason that real-world implementations ignore much of their work.

Parsing malformed HTML

So, in any case, I have another problem besides Java trying to load the unavailable DTD. The input HTML is malformed. Not just a little; it’s utterly broken. It’s both externally-generated and unreplaceable, so fixing it at the source is not an option. I’m stuck with it; it’s the only way to get the data I need. (That’s real-world programming as opposed to academia right there.)

Disabling XML validation

Disabling all XML validation is the obvious solution to both problems, but there is no readily apparent way to do that clojure.xml. Calling setValidating(false) on the factory is insufficient. Thanks to Stack Overflow, I found that you must also use
setFeature() with several obscure “SAX2 Standard Feature Flags“, as above.

HTML Tidy to the rescue

This still leaves the problem of the malformed HTML. There does not seem to be any easy way to make Sax ignore errors. ((.setFeature "http://apache.org/xml/features/continue-after-fatal-error" true) has no apparent effect; it still doesn’t continue after a fatal parse error.)

In the end, I just ran the malformed input HTML through HTML Tidy before attempting to parse it. HTML Tidy coerces the malformed input into valid XML, and Sax is finally happy.

(I now get “Exception in thread "Swank REPL Thread" java.lang.RuntimeException: java.lang.IllegalMonitorStateException” from SLIME when I try to actually display the parsed XML in Emacs, but that’s another matter…)

The Psyleron system, unfortunately, costs hundreds of dollars and only runs on Windows, so I wrote some quick Clojure to do experimentation with randomness, using /dev/random as the source. (It appears that /dev/random on Mac OS X does not provide the same quality guarantees as it does on Linux, but it’s fine for a prototype and the implementation allows easy substitution of another randomness source later.)

Clojure random walk generator

(ns randomwalker.core
  (:use [incanter core charts]))

(defn random-walk
  "Returns a random walk of length steps. random-fn should return a random integer."
  [steps list random-fn]
  (if (= 1 steps)
    list
    (recur (dec steps)
           (conj list
                   (+
                    (if (odd? (random-fn)) 1 -1)
                    (or (last list) 0)))
           random-fn)))

(defn dev-random-walk
  "Returns a random walk of length steps based on random numbers read from /dev/random."
  [steps]
  (with-open [random (java.io.FileInputStream. "/dev/random")]
    (random-walk steps [0] (fn [] (.read random)))))

(defn view-walk
  "Visualize a random walk."
  [steps]
  (view (incanter.charts/xy-plot (range 0 (dec steps)) (dev-random-walk steps))))

(defn walk-ratio
  "Generates the given number of random walks and returns the ratio of (walks ending > 0) / (walks ending < 0)."
  [runs steps-per-run greater less]
  (if (zero? runs)
    (double (/ greater less))
    (let [final (last (dev-random-walk steps-per-run))
          ;foo (println final)
          new-greater (if (> final 0) (inc greater) greater)
          new-less (if (< final 0) (inc less) less)]
      (recur (dec runs) steps-per-run new-greater new-less))))

This code generates a simple random walk. The user can easily visualize the random walk with Incanter, and it looks suspiciously like a stock chart.

Screenshot of the Clojure Random Walker in action

Psyleron type functionality first vaguely appears in walk-ratio. This generates the requested number of random walks and returns the ratio of those that ended above zero to those that ended below. I imagine this will become the basis for some Psyleron Reflector style games and experiments.

This is probably not the most efficient way to implement this; any suggestions are welcome. Any ideas for a better source of randomness? Any insight into how the Psyleron games work? Their descriptions are a bit short on the details.

(defmacro sql-expand
  "Transforms nested s-expressions into SQL, for use in WHERE or HAVING clauses.

e.g.: (sql-expand (and (> foo 3)
                       (< bar 4)))

     -> '(foo > 3 AND bar < 4)'

 Nested clauses:

      (sql-expand (or (and (> foo 3) (< bar 4))
                      (> baz 6)))

      -> '((foo > 3 AND bar < 4) OR baz > 6)'

 Embedded arbitrary SQL:
         (sql-expand (and (> foo 3)
                          (< bar \"(SELECT max(foo) + 10 FROM bar)\")))
      -> '(foo > 3 AND bar < (SELECT max(foo) + 10 FROM bar))'

"
  [form]
  (let [head (first form)]
    (if (includes? '(and or) head)
      `(str "(" (sql-expand ~(second form)) " "
            ~(.toUpperCase (str head)) " "
            (sql-expand ~(last form)) ")")
      (str (second form) " " head " " (last form)))))

Generating SQL from Lisp has been approached in a few different ways:

CL-SQL version

CL-SQL uses reader macros to do something similar, but with square brackets and new syntax:

;; From the CL-SQL docs
(sql [select [foo] [bar] :from [baz]] 'having [= [foo id] [bar id]]
     'and [foo val] '< 5)
=> "SELECT FOO,BAR FROM BAZ HAVING (FOO.ID = BAR.ID) AND FOO.VAL < 5"

I like my version better as it works with regular S-expressions. No new syntactic constructs are necessary, so you can re-use the parsers already set up in your brain for quick and easy mental code scanning and construction.

Some people mentioned that you have to jump through a lot of hoops to make sure that CL-SQL’s reader macros are activated and deactivated correctly around your code. Apparently it’s the only major Common Lisp package that actually uses reader macros extensively. (Clojure doesn’t support user-provided reader macros anyway, at least not without messy unsupported hackery.)

Cynojure’s functional version

Cynojure has a similar approach to mine:

;; From Cynojure's blog
 :where (sql-and (sql-= 'role +role-programmer+)
                                 (sql-< 'age 30)))

Cynojure uses special sql-* function names instead of transforming an s-expression with a macro. Since and and or are already in use by Clojure itself, he had to use other names. Symbols naming SQL fields (role and age, in this case) must also be quoted. This adds a lot of unnecessary noise to the signal and, as he notes, “interrupts the flow” of the code when reading.

By virtue of not automatically evaluating its arguments, a macro-based version can re-use already reserved symbols like and and or, and the user doesn’t have to bother with messily quoting symbols. This results in much cleaner and more readable code. The user-supplied data for the macro version contain 100% signal and no noise.

Chouser’s recursive function within a macro version

Chouser has a different macro-based approach involving a recursive anonymous function:

;; By Chouser, from http://groups.google.com/group/clojure/msg/62437f3b96835b58
(defmacro where [e]
  (let [f (fn f [e]
            (if-not (list? e)
              [(str e)]
              (let [[p & r] e]
                (if (= p `unquote)
                  r
                  (apply concat (interpose [(str " " p " ")]
                                           (map f r)))))))]
    (list* `str "where " (f e))))

His macro allows variable interpolation with quasiquotes, which is really cool. For example, you can do:

;; Chouser's examples
user=> (where (and (> i (- 3 1)) (< i ~(+ 3 1))))
"where i > 3 - 1 and i < 4"
user=> (let [my-name "'chouser'"] (where (and (> id 0) (= name ~my-name))))
"where id > 0 and name = 'chouser'"

This version also avoids the inelegant repetition of the SQL template at lines 25-28 in my version, at the price of slightly more complexity.

The only problem is that it doesn’t write grouping parentheses in the output SQL string, which discards potentially important grouping information embedded in the s-expression tree and leaves evaluation grouping order to the SQL server:

(where (or (<= baz 123)
           (and (> foo 3) (> bar 6))))
-> "baz <= 123 or foo > 3 and bar > 6"

My version

By being aware of the and and or list heads as special case operators which join other clauses together, my version can wrap their output in parentheses, which preserves the evaluation priority implicit within the input S-expression in the final SQL output. It seems that there should be a way to do the same thing both generically and recursively (that is, without repetition of similar code and without conditional recursion), but I can’t make that work without making the macro significantly longer and more complex (read: fragile), so for the moment, I’m going to live with simple and slightly repetitious.

One problem lies in knowing when to add parentheses. Although bluntly wrapping the result of every SQL operator and outputting SQL like “((foo < 3) AND (bar > 4))” will execute fine on the SQL server, the extra parentheses are superfluous. Ideally, we want to use parentheses only when strictly necessary to preserve the semantic intent of the S-expression tree, not blindly wrap every single expression.

Some sort of awareness of clause-combinatory SQL operators such as AND and OR as special cases is therefore necessary to know when to add parentheses in the output, but I don’t like having to repeat what’s essentially the same almost identical output template. In all cases, we have 3 elements, and we’re just swapping the first two and then taking a string output.

The other issue is that the AND/OR case needs to recurse, but the ordinary case doesn’t; the ordinary case can be translated directly to a string. The logic would have to be adjusted to either differentiate between these cases and recurse conditionally, or else recurse unconditionally and then have a way to deal with the base case of translating non-trinary lists. That is, with unconditional recursion, a foo symbol will eventually be fed into the macro by itself and must return just the string "foo", which complicates the management of the SQL string template significantly by forcing you to have multiple templating procedures and then decide between them. (Well, we’re doing that anyway now, but that would be even worse.)

I messed around with conditionally interpolateing the template result into a '("(" ")") list, and with conditionally stringifying directly or else building a trinary list based on the structure of the input, but the result was ugly, far longer and far more complicated and confusing than just repeating the template. Any suggestions?

I also haven’t figured out the best way to add unquote support without making a huge mess. That would be a very cool feature. Any ideas on that are also welcome.

Someday, this macro could be a small part of a user-transparent Clojure object persistence library like Common Lisp’s Elephant. That would truly be a killer feature for the Clojure language.

Suppose that you’re working at a REPL and you have namespaces a, b, and c, where c :uses both a and b (meaning that it imports their namespace contents into its own namespace.) You write a function foo in a, but then later you decide to move it to b. Bam, you get a namespace collision exception the next time you reload c.

The symbol foo in c’s namespace is already bound to your code in memory. Simply deleting the source code from a’s source file is not enough.

Of course, you could be brutish and simply kill your Clojure REPL process and restart it, but who wants to go through all that? We need a way to unbind the obsolete symbol in a. The solution is simple: just

(ns-unmap 'a 'foo)

and the symbol is removed from package a’s namespace. (This is called undef or unintern in other Lisp dialects.)

Edit: Since we’ve :used a from c, there will also now be a foo symbol in the c namespace, and we must also (ns-unmap 'c 'foo). Thanks Meikel!

JDBC (at least, the PostgreSQL driver) can’t use Joda Time directly (without explicit type mapping). One way to convert a Joda LocalDate into something JDBC can use:

(defn to-sql-date [date]
  "Convert any Joda-readable date object (including a string) to a java.sql.Date"
  (java.sql.Date. (.. (LocalDate. date) toDateMidnight toInstant getMillis)))

It’s not pretty, but it works. You can follow a similar procedure for any of the Joda classes.