|If-statements in Smalltalk
||[Mar. 3rd, 2008|01:32 pm]
I did warn you...
A while back, markdominus wrote an article about the Design Patterns movement, in which he claimed that design patterns are basically signs of weaknesses in programming languages; if a language forces you to implement the same pattern of code over and over in different programs, that's a sign that your language is insufficiently high-level with respect to problems that your users actually encounter, so language designers should focus on patterns as places where their languages could be improved. This prompted some reaction in the tech blogosphere, including this response from Ralph Johnson (2017 edit: this now 404s, alas), one of the authors of the original Design Patterns book. Dominus responded here, correcting some of Johnson's misunderstandings and commenting further; in fact, though, they're mostly in agreement (and I'm pretty much in agreement with them both, FWIW).
Everyone up to speed? Good.
Something Dominus didn't seem to get was Johnson's distinction between things that are in the language and things that are in the library. Johnson seems to think that there's a three-tier hierarchy:
1) Language features
2) Library code (available to every application)
3) User code (must be written for each application).
He claims that it can be a good tradeoff to leave something as a pattern, because the alternative might be to overcomplicate the standard library or, even worse, the language itself - he seems to have visions of a language with decorator, abstractFactory etc. as keywords (2017 edit: I now mostly work in Python, which does have decorators built into the language - and I see they were proposed in 2003, five years before I wrote this...). Dominus largely refutes this claim in his response. But more interestingly, Dominus thinks that the distinction between 1 and 2 is basically artificial - is C's printf (or Haskell's standard prelude) part of the language or the library? It might matter to language implementers, but to the average programmer, the distinction's pretty unimportant. Now, this is a good point, but I think I can see what Johnson's getting at. Remember that Johnson's a Smalltalker. Let's consider (finally) how Smalltalk handles if-statements, which in most languages are built-in syntactic elements of the language.
In Smalltalk, booleans (ie, True or False) are objects: specifically, they're instantiations of the abstract base class Boolean, or rather of its two subclasses True and False. So every boolean has type True or False, and no actual member data. Bool has two virtual functions, ifTrue: and ifFalse:, which take as their argument a block of code. Both True and False override these functions; True's version of ifTrue: calls the code it's passed, and False's version does nothing (and vice-versa for ifFalse:). 2017 edit: there's also a single method called ifTrue:ifFalse: which takes two blocks, and calls the right one. Here's an example:
Those things in square brackets are essentially anonymous functions, by the way. Except they're objects, because everything is an object in Smalltalk. Now, what's happening there is that we call a's "<" method, with argument b; this returns a boolean. We call its ifTrue: and ifFalse: methods (2017 edit: or rather, its ifTrue:ifFalse: method), passing as arguments the code we want executed in either case. The effect is the same as that of the Ruby code
a < b
ifTrue: [^'a is less than b']
ifFalse: [^'a is greater than or equal to b']
but what other languages do with special syntax, Smalltalk does as a special case of method-dispatch. (Code samples from here). This completely blew me away when someone (pdcawley?) first told me about it. Haskell does something similar, of course. [Edit: no it doesn't - see comments. But it could. I've also got some details of the Smalltalk wrong.]
if a < b then
"a is less than b"
"a is greater than or equal to b"
Now, back to Johnson. Smalltalk's designers had three choices: bake special syntax for if-statements into the language, implement it in Smalltalk library code in terms of more general concepts that were part of the language, or force the programmer to do the work him/herself every time (using, I dunno, computed gotos or something). They made the unusual choice to go for the second option. While this doesn't matter if all you want are if-statements, it affects the language in other ways: the powerful abstraction mechanism needed to do this is available to the user to define new features.
This, I think, is the real distinction between "language" and "library": if the feature in question could have been implemented by your users using a combination of other features, it might as well be part of the library. Smalltalk's if-statements pass this test, as do (say) Lisp's looping constructs. Haskell's monads fail, because of do-notation: if a Haskell programmer wanted to add similar syntax for (say) comonads, would they be able to? I don't think so, and so it follows that monads are actually baked into the language. Several of Perl's built-in functions do things that user functions wouldn't be able to do, and thus should count as part of the language, even though they feel like part of a library.
So here's where it applies to design patterns: if your programmers find themselves having to implement essentially the same code over and over again, it's because you don't provide powerful enough abstraction mechanisms for programmers to write library modules that would do the job for them. Programmers are (rightly) lazy, and don't do work many times if they can do it once and then use that solution over and over again. So the fact that it's a design pattern means that it can't be in the library, and your language needs fixing: either by special-casing extra language features in or (preferably) by making your abstraction features more powerful, so the necessary library code can be written.