Two new mixes

I’ve been pretty dormant in my music for the past few years, but I have been working on two two mixes in my sparse spare time: Tes Lyric, a weird blend of electronica, classical and rock, and Stage Superior, a progressive house mix. They’re up on my music page now; enjoy!

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Immutability and Blocks, Lambdas and Closures [UPDATE x2]

I recently ran into some “interesting” behaviour when using lambda in Python. Perhaps it’s only interesting because I learnt lambdas from a functional language background, where I expect that they work a particular way, and the rest of the world that learnt lambdas through Python, Ruby or JavaScript disagree. (Shouts out to you Objective-C developers who are discovering the wonders of blocks, too.) Nevertheless, I thought this would be blog-worthy. Here’s some Python code that shows the behaviour that I found on Stack Overflow:

Since I succumb to reading source code in blog posts by interpreting them as “blah”1, a high-level overview of what that code does is:

  1. iterate over a list of strings,
  2. create a new list of functions that prints out the strings, and then
  3. call those functions, which prints the strings.

Simple, eh? Prints “do”, then “re”, then “mi”, eh? Wrong. It prints out “mi”, then “mi”, then “mi”. Ka-what?

(I’d like to stress that this isn’t a theoretical example. I hit this problem in production code, and boy, it was lots of fun to debug. I hit the solution right away thanks to the wonders of Google and Stack Overflow, but it took me a long time to figure out that something was going wrong at that particular point in the code, and not somewhere else in my logic.)

The second answer to the Stack Overflow question is the clearest exposition of the problem, with a rather clever solution too. I won’t repeat it here since you all know how to follow links. However, while that answer explains the problem, there’s a deeper issue. The inconceivable Manuel Chakravarty provides a far more insightful answer when I emailed him to express my surprise at Python’s lambda semantics:

This is a very awkward semantics for lambdas. It is also probably almost impossible to have a reasonable semantics for lambdas in a language, such as Python.

The behaviour that the person on SO, and I guess you, found surprising is that the contents of the free variables of the lambdas body could change between the point in time where the closure for the lambda was created and when that closure was finally executed. The obvious solution is to put a copy of the value of the variable (instead of a pointer to the original variable) into the closure.

But how about a lambda where a free variable refers to a 100MB object graph? Do you want that to be deep copied by default? If not, you can get the same problem again.

So, the real issue here is the interaction between mutable storage and closures. Sometimes you want the free variables to be copied (so you get their value at closure creation time) and sometimes you don’t want them copied (so you get their value at closure execution time or simply because the value is big and you don’t want to copy it).

And, indeed, since I love being categorised as a massive Apple fanboy, I found the same surprising behaviour with Apple’s blocks semantics in C, too:

You can see the Gist page for this sample code to see how to work around the problem in Objective-C (basically: copy the block), and also to see what it’d look like in Haskell (with the correct behaviour).

In Python, the Stack Overflow solution that I linked to has an extremely concise way of giving the programmer the option to either copy the value or simply maintain a reference to it, and the syntax is clear enough—once you understand what on Earth what the problem is, that is. I don’t understand Ruby or JavaScript well enough to comment on how they’d capture the immediate value for lambdas or whether they considered this design problem. C++0x will, unsurprisingly, give programmers full control over lambda behaviour that will no doubt confuse the hell out of people. (See the C++0x language draft, section 5.1.2 on page 91.)

In his usual incredibly didactic manner, Manuel then went on to explain something else insightful:

I believe there is a deeper issue here. Copying features of FP languages is the hip thing in language design these days. That’s fine, but many of the high-powered FP language features derive their convenience from being unspecific, or at least unconventional, about the execution time of a piece of code. Lambdas delay code execution, laziness provides demand-dependent code execution plus memoisation, continuations capture closures including their environment (ie, the stack), etc. Another instance of that problem was highlighted by Joe Duffy in his STM retrospective.

I would say, mutability and flexible control flow are fundamentally at odds in language design.

Indeed, I’ve been doing some language exploration again lately as the lack of static typing in Python is really beginning to bug me, and almost all the modern languages that attempt to pull functional programming concepts into object-oriented land seem like a complete Frankenstein, partially due to mutability. Language designers, please, this is 2011: multicore computing is the norm now, whether we like it or not. If you’re going to make an imperative language—and that includes all your OO languages—I’ll paraphrase Tim Sweeney: in a concurrent world, mutable is the wrong default! I’d love a C++ or Objective-C where all variables are const by default.

One take-away point from all this is to try to keep your language semantics simple. I love Dan Ingall’s quote from Design Principles Behind Smalltalk: “if a system is to serve the creative spirit, it must be entirely comprehensible to a single individual”. I love Objective-C partially because its message-passing semantics are straightforward, and its runtime has a amazingly compact API and implementation considering how powerful it is. I’ve been using Python for a while now, and I still don’t really know the truly nitty-gritty details about subtle object behaviours (e.g. class variables, multiple inheritance). And I mostly agree with Guido’s assertion that Python should not have included lambda nor reduce, given what Python’s goals are. After discovering this quirk about them, I’m still using the lambda in production code because the code savings does justify the complexity, but you bet your ass there’s a big comment there saying “warning, pretentous code trickery be here!”

1. See point 13 of Knuth et al.’s Mathematical Writing report.

UPDATE: There’s a lot more subtlety at play here than I first realised, and a couple of statements I’ve made above are incorrect. Please see the comments if you want to really figure out what’s going on: I’d summarise the issues, but the interaction between various language semantics are extremely subtle and I fear I’d simply get it wrong again. Thank you to all the commenters for both correcting me and adding a lot of value to this post. (I like this Internet thing! Other people do my work for me!)

Update #2

I’ve been overwhelmed by the comments, in both the workload sense and in the pleasantly-surprised-that-this-provoked-some-discussion sense. Boy, did I get skooled in a couple of areas. I’ve had a couple of requests to try to summarise the issues here, so I’ll do my best to do so.

Retrospective: Python

It’s clear that my misunderstanding of Python’s scoping/namespace rules is the underlying cause of the problem: in Python, variables declared in for/while/if statements will be declared in the compound block’s existing scope, and not create a new scope. So in my example above, using a lambda inside the for loop creates a closure that references the variable m, but m’s value has changed by the end of the for loop to “mi”, which is why it prints “mi, mi, mi”. I’d prefer to link to the official Python documentation about this here rather than writing my own words that may be incorrect, but I can’t actually find anything in the official documentation that authoritatively defines this. I can find a lot of blog posts warning about it—just Google for “Python for while if scoping” to see a few—and I’ve perused the entire chapter on Python’s compound statements, but I just can’t find it. Please let me know in the comments if you do find a link, in which case I’ll retract half this paragraph and stand corrected, and also a little shorter.

I stand by my assertion that Python’s for/while/if scoping is slightly surprising, and for some particular scenarios—like this—it can cause some problems that are very difficult to debug. You may call me a dumbass for bringing assumptions about one language to another, and I will accept my dumbassery award. I will happily admit that this semantics has advantages, such as being able to access the last value assigned in a for loop, or not requiring definitions of variables before executing an if statement that assigns to those variables and using it later in the same scope. All language design decisions have advantages and disadvantages, and I respect Python’s choice here. However, I’ve been using Python for a few years, consider myself to be at least a somewhat competent programmer, and only just found out about this behaviour. I’m surprised 90% of my code actually works as intended given these semantics. In my defence, this behaviour was not mentioned at all in the excellent Python tutorials, and, as mentioned above, I can’t a reference for it in the official Python documentation. I’d expect that this behaviour is enough of a difference vs other languages to at least be mentioned. You may disagree with me and regard this as a minor issue that only shows up when you do crazy foo like use lambda inside a for loop, in which case I’ll shrug my shoulders and go drink another beer.

I’d be interested to see if anyone can come up an equivalent for the “Closures and lexical closures” example at http://c2.com/cgi/wiki?ScopeAndClosures, given another Python scoping rule that assignment to a variable automatically makes it a local variable. (Thus, the necessity for Python’s global keyword.) I’m guessing that you can create the createAdder closure example there with Python’s lambdas, but my brain is pretty bugged out today so I can’t find an equivalent for it right now. You can simply write a callable class to do that and instantiate an object, of course, which I do think is about 1000x clearer. There’s no point using closures when the culture understands objects a ton better, and the resulting code is more maintainable.

Python summary: understand how scoping in for/while/if blocks work, otherwise you’ll run into problems that can cost you hours, and get skooled publicly on the Internet for all your comrades to laugh at. Even with all the language design decisions that I consider weird, I still respect and like Python, and I feel that Guido’s answer to the stuff I was attempting would be “don’t do that”. Writing a callable class in Python is far less tricky than using closures, because a billion times more people understand their semantics. It’s always a design question of whether the extra trickiness is more maintainable or not.

Retrospective: Blocks in C

My C code with blocks failed for a completely different reason unrelated to the Python version, and this was a total beginner’s error with blocks, for which I’m slightly embarrassed. The block was being stack-allocated, so upon exit of the for loop that assigns the function list, the pointers to the blocks are effectively invalid. I was a little unlucky that the program didn’t crash. The correct solution is to perform a Block_copy, in which case things work as expected.

Retrospective: Closures

Not all closures are the same; or, rather, closures are closures, but their semantics can differ from language to language due to many different language design decisions—such as how one chooses to define the lexical environment. Wikipedia’s article on closures has an excellent section on differences in closure semantics.

Retrospective: Mutability

I stand by all my assertions about mutability. This is where the Haskell tribe will nod their collective heads, and all the anti-Haskell tribes think I’m an idiot. Look, I use a lot of languages, and I love and hate many things about each of them, Haskell included. I fought against Haskell for years and hated it until I finally realised that one of its massive benefits is that things bloody well work an unbelievable amount of the time once your code compiles. Don’t underestimate how much of a revelation this is, because that’s the point where the language’s beauty, elegance and crazy type system fade into the background and, for the first time, you see one gigantic pragmatic advantage of Haskell.

One of the things that Haskell does to achieve this is the severe restriction on making things immutable. Apart from the lovely checkbox reason that you can write concurrent-safe algorithms with far less fear, I truly believe that this makes for generally more maintainable code. You can read code and think once about what value a variable holds, rather than keep it in the back of your mind all the time. The human mind is better at keeping track of multiple names, rather than a single name with different states.

The interaction of state and control flow is perhaps the most complex thing to reason about in programming—think concurrency, re-entrancy, disruptive control flow such as longjmp, exceptions, co-routines—and mutability complicates that by an order of magnitude. The subtle difference in behaviour between all the languages discussed in the comments is exemplar that “well-understood” concepts such as lexical scoping, for loops and closures can produce a result that many people still don’t expect; at least for this simple example, these issues would have been avoided altogether if mutability was disallowed. Of course mutability has its place. I’m just advocating that we should restrict it where possible, and at least a smattering of other languages—and hopefully everyone who has to deal with thread-safe code—agrees with me.

Closing

I’d truly like to thank everyone who added their voice and spent the time to comment on this post. It’s been highly enlightening, humbling, and has renewed my interest in discussing programming languages again after a long time away from it. And hey, I’m blogging again. (Though perhaps after this post, you may not think that two of those things are good things.) It’s always nice when you learn something new, which I wouldn’t have if not for the excellent peer review. Science: it works, bitches!

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The Projectionist

There comes in the career of every motion picture that final occasion when all the artistry, all the earnest constructive endeavor of all the man-power and genius of the industry, and all the capital investment, too, must pour through the narrow gate of the projector on its way to the fulfilment of its purpose, the final delivery to the public.

The delivery is a constant miracle of men and mechanism in the projection rooms of the world’s fifty thousand theatres. That narrow ribbon, thirty-five millimetres, flowing at twenty-four frames a second through the scintillating blaze of the spot at the picture aperture and coursing by at an exactingly-precise 90 feet a minute past the light slit of the sound system, demands a quality of skill and faithful, unfailing attention upon which the whole great industry depends.

The projector lens is the neck in the bottle through which all must pass. The projectionist presiding over that mechanism is responsible for the ultimate performance upon which we must all depend.

The projector must not fail, and more importantly still, the man must not fail or permit it to waiver in its performance. It is to the tremendous credit of the skill of the modern projectionist that perfect presentation of the motion picture upon the screen is today a commonplace, a perfection that is taken as a matter of course.

Adolph Zukor, Chairman of Paramount Pictures, 1935. It still applies as much today as it did back then.

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It Gets Better

Pixar’s latest short film. I’m so proud and honoured to be working with such an amazing group of people.

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Learning Photography with the Panasonic GF1

Thanks to several evil friends of mine, I started to take an interest in photography at the end of last year. I’ve always wanted to have a “real” camera instead of a point and shoot, so at the start of 2010, I bit the bullet and bought myself a Panasonic Lumix DMC-GF1, usually just called The GF1 amongst the camera geeks.

I tossed up between the GF1 and the then-just-released Canon EOS 550D (a.k.a. the Rebel T2i in the USA) for a long time. I figured that getting a compact camera would make me tote it around a lot more, and after ten months of using it, I think I was right. I recently went to a wedding in Sydney, and I literally kept the camera in my suit pocket instead of having to lug it around strapped to my body or neck. I definitely wouldn’t be able to do that with a Canon or Nikon DSLR. The camera’s so small with the kit 20mm f/1.7 lens that I stopped using the UV filter with it, because I didn’t like the 2-3mm that the filter added to the camera depth. Here’s a size comparison of the Nikon D3000 vs the GF1.



(Image stolen from dpreview.com’s review.)

I won’t write up a comprehensive review of the GF1 here; other sites have done that, in much more depth than I can be bothered to go into. If you’re after a good review, the three articles that swayed me to the GF1 in the end were DPreview’s review, and Craig Mod’s GF1 photo field test article and video tests. What follows is my own impressions and experiences of using the camera. The one-sentence summary: the GF1 perfect for a DSLR newbie like me, the micro four-thirds lens system it uses looks like it has enough legs that your lens investments will be good for the future, and learning photography with the GF1 is great and deserves a Unicode snowman: ☃.

The reason you want the camera is to use the 20mm f/1.7 lens. For the non-photography geeks, that means that it’s not a zoom lens, i.e. you can’t zoom in and out with it, and the f/1.7 means that it can take pretty good shots in low light without a flash. All the reviews of it are right: that lens is what makes the camera so fantastic. Do not even bother with 14-45mm kit lens. The 20mm lens is fast enough that you can shoot with it at night, and the focal length’s versatile enough to take both close-up/portrait shots (whee food porn), and swing out a bit wider for landscape photos or group photos. It’s no wide-angle nor zoom and it’s not a super-fast f/1.4, but it’s versatile enough and so tiny that you will end up using it almost all the time. It feels weird to get a DSLR and only have one lens for it, but the pancake 20mm lens is so damn good that it’s all you really need. The only thing it really can’t do at all is go super-zoomalicious, for wildlife/distance shots.

The 20mm non-zoom (a.k.a. “prime”) lens has another advantage: it teaches you to compose. Despite all the technology and all the geek speak, photography is ultimately about your composition skills. Prime lenses force you to move around to find the perfect framing for the shot you’re trying to do; I think learning with a prime lens moves your composition skills along much faster than it would if you were using a standard zoom lens. If you’re a photography beginner, like me, shoot with a prime. It’s a totally different experience than shooting with a zoom, and you learn a lot more. Plus, primes are cheap: the Canon 50mm f/1.8 is USD$100, and Canon’s near top-of-the-line 50mm f/1.4 is USD$350. The Canon 35mm f/2, for something that’s similar in focal length to the Panasonic 20mm prime, is USD$300. (You need to multiply the 20mm by 2 to convert between micro four-thirds and 35mm framing, so the actual focal length of the 20mm in traditional camera speak is 20mm*2=40mm.)

After playing it for a few months, you realise that the GF1 is a fun camera to use. The combination of the 20mm prime lens, the super-fast focus, the size, and the great UI design just begs you to take pictures with it. You feel like you’re wielding a real camera rather than a toy: one that wants you to shoot with it. It’s not imposing like a bigger DSLR so it doesn’t feel like the camera is with you all the time, but it’s not so small that you feel like you’re just snipping super-casually with something that’s cheap. And did I mention the excellent UI? It’s excellent. The better controls are a good reason to get the GF1 over its rivals, the Olympus EP series.

One big bonus: I’ve found that the full-auto mode (“iAuto” as Panasonic brands it) very rarely gets stuff wrong. This is useful if you hand the camera over to someone else who doesn’t know how to use DSLRs so that they can take a picture of you… or, like me, if you just don’t know quite what aperture/shutter speeds to use for the particular shot you’re taking. The full-auto just adds to the joy of using it. I usually shoot in full-auto or aperture priority mode, but honestly, I could probably shoot on full-auto mode all the time. I can’t recall a single occasion where it didn’t guess f/1.7 or a landscape correctly.

Do follow DPreview and Craig Mod’s advice and shoot RAW, not JPEG. Honestly, I’d prefer to shoot JPEG if I could, but RAW lets you turn some bad shots into good shots. I use it because gives you a second chance, not because I want to maximise picture quality. Here’s one photo that was accidentally taken with the wrong settings: I had the camera on full-manual mode, and didn’t realise that the shutter speed and ISO settings were totally incorrect.

However, since I shot it in RAW, I could lift up the exposure up two stops, pulled up the shadows and pulled down the highlights, and here’s the result:

Seriously, that’s just frickin’ amazing. Sure, that photo might not be super-awesome: it’s a little grainy, and it looks a bit post-processed if you squint right, but it’s still a photo of a precious memory that I wouldn’t have otherwise had, and you know what? That photo’s just fine. If I shot JPEG, I would’ve had no choice but to throw it away. RAW’s a small pain in the arse since the file sizes are far bigger and you need to wait a long time for your computer to do the RAW processing if you’ve taken hundreds of photos, but boy, it’s worth it.

I did finally buy a wide-angle lens for the GF1—the Olympus 9-18mm f/4-5.6—and have been using it a lot for landscape shots. I bought the Olympus 9-18mm over the Panasonic 7-14 f/4.0 because it was cheaper, and also smaller. I figured that if I was getting a GF1, it was because I wanted something compact, so I wanted to keep the lenses as small as possible. (Otherwise, if you don’t care about size, then a full-blown Canon or Nikon DSLR would probably serve you much better.) I’ve always wanted a wide-angle lens from the first day that I asked “how do those real estate agents make those rooms look so bloody large?”, so now I have one, woohoo. The next lens on my shopping will probably be the Panasonic 45-200mm. (Never mind the quality, feel the price!) Here’s a shot taken with the Olympus 9-18mm; click through to see the original picture on Flickr.

The main thing that I wish for in a future version of the camera would be image stabilisation. Panasonic follow the Canon path and put image stabilisation in the lens, rather than in the body. I think Olympus made the right decision by putting image stabilisation in the body for their compact DSLRs; you can keep the lenses smaller that way, and you then get image stabilisation with all your lenses instead of the ones that only support it explicitly, e.g. the 20mm f/1.7 prime doesn’t have image stabilision, boo. In-body image stabilisation just seems more in-line with the size reduction goal for micro four-thirds cameras. I’d love to get my hands on an Olympus EP for a week and shoot with the 20mm to see if image stabilisation makes a difference when it’s dark and the environment is starting to challenge the f/1.7 speeds.

The only other thing I wish for would be a better sensor. The GF1’s great up to ISO 800: at ISO 1600+, it starts getting grainy. 1600 is acceptable, and you can do wondrous things with the modern noise reduction algorithms that are in Lightroom if you really need to save a shot. Shoot at ISO 3200+ though, and it’s just too grainy. This is the main advantage that more traditional DSLRs have: their larger sensors are simply better than the GF1’s. I’ve seen shots taken with a Nikon D50 at ISO 6400 in the dark because a slower f/4 lens was being used, and bam, the shot comes out fine. Don’t even try to compare this thing to a Canon 5D Mk II. The GF1 just can’t do high ISO. Here’s an ISO 3200 shot, which is just starting to get a little too grainy. It’s fine for Facebook-sized images, but if you click through to the original, you’ll see it’s noisy.

But y’know, despite the two nitpicks above, the GF1 is a fine camera, and the 20mm f/1.7 lens is an amazing do-it-all lens that’s absolutely perfect to learn with. There’s really nothing else out there like it except for the Olympus EP range (the EP-1, EP-2 and EPL-1), which you should definitely consider, but get it with the 20mm f/1.7 lens if you do. I’ve had a total blast learning photography with the GF1, and I’ve captured hundreds of memories in the past year that made the investment completely worthwhile. I don’t think I’m at the point yet where I feel like I need another camera yet, but it feels good knowing that the micro four-thirds format will be around for a while so that I can use my existing lenses with future cameras I buy. If you’re interested in learning photography, the GF1 is a fantastic starting point.

Update: Thom Hogan did a comparison between the most popular mirrorless cameras: the Olympus E-PL1, the Panasonic GF1, Samsung NX100, and Sony NEX-5. It’s written for people who know photography rather than for novices, but basically, the GF1 came out on top, with the E-PL1 being recommended if you can live with the worse screen and the far worse UI. That’s pretty much exactly my opinion, too.

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