Sunday, December 02, 2007

Metaprogramming and conjuring spells

"A computational process is indeed much like a sorcerer's idea of a spirit. It cannot be seen or touched. It is not composed of matter at all. However, it is very real. It can perform intellectual work. It can answer questions. It can affect the world by disbursing money at a bank or by controlling a robot arm in a factory. The programs we use to conjure processes are like a sorcerer's spells. They are carefully composed from symbolic expressions in arcane and esoteric programming languages that prescribe the tasks we want our processes to perform."
- Abelson and Sussman — SICP


Among the plethora of metaphors that plague our field, I find computing as incantation of spells one of the least annoying. Some fellow with a long beard writes odd-looking prose that will later, through some magical process, turn lead into gold or help vanquish some inept demon. Man, that show sucked, I don't know why anyone would watch that shit, specially the reruns Tuesday to Saturday at 04AM and Monday to Friday at 12PM on channel 49. Well, anyway, what's interesting about the analogy is that it shows the dual nature of software: it is both a magical process and a bunch of scribbled lines. To mix metaphors a bit, we can say that software is, at the same time, a machine and the specification for that machine.

This is what makes metaprogramming possible and, also, what makes it unnecessary. By the way, when I write "metaprogramming", I'm specifically thinking about mutable meta-structures, changing the tires while the car is running metaprogramming, not mere introspection. Throwing away that mundane car analogy and getting back to our wizardry metaphor, metaprogramming would be like a spell that modifies itself while being casted. This begs the question, beyond fodder for bad TV Show scripts, what would this be useful for? My answer: for very little, because if the wizard programmer already has all the information needed for the metaprogram, then he might as well just program it... To make this a little less abstract, take a simple example of Ruby metaprogramming:
01  class DrunkenActress
02 attr_writer :blood_alcohol_level
03 end
04
05 shannen_doherty = DrunkenActress.new
06 shannen_doherty.blood_alcohol_level = 0.13
By the way, I'm not a Rubyist, so please let me know if the example above is wrong. The only line with meta-stuff is line 2, where a method named "attr_writer" is being called with an argument of :blood_alcohol_level. This call will change the DrunkenActress class definition to add accessor methods for the blood_alcohol_level attribute. We can see that it worked in line 6, where we call the newly defined setter.

But the programmer obviously already knows that the DunkenActress class needs to define a blood_alcohol_level attribute, so we see that meta-stuff is only applied here to save a few keystrokes. And that is not a bad motivation in itself, more concise code often is easier to understand. Then again, there are other ways to eliminate this kind of boilerplate without recursing to runtime metaprogramming, such as macros or even built-in support for common idioms (in this case, properties support like C# or Scala).

There may be instances where the cleanest way to react to information available only in runtime is trough some metaprogramming facility, but I have yet to encounter them. Coming back to Rubyland, Active Record is often touted as a poster child for runtime metaprogramming, as it extracts metadata from the database to represent table columns as attributes in a class. But those attributes will be accessed by code that some programmer will write — and that means, again, that the information consumed by the metaprogram will need to be available in development time. And indeed it is, in the database. So ActiveRecord metaprogramming facilities are just means to delegate the definition of some static structure to an external store, with no real dynamicity involved. If it were not so, this kind of thing would be impossible. Also note that recent Rails projects probably use Migrations to specify schema info in yet another static format.

To summarize, runtime mutable metaprogramming is like that bad purple translucent special effect, it is flashy, but ultimately useless. Anyway, that's my current thinking in the matter, but I still need to read more on staging.

[EDIT: corrected a mistake relating to the code sample]

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