Language Design Deal Breakers
sebastiansylvan.wordpress.com
sebastiansylvan.wordpress.com
Dynamic typing is easy to implement, and that’s about as far as its value goes, as far as I’m concerned. If your static type system is getting in your way, you need a better static type system, not a dynamic one. The same goes for dynamic scope—much as I like having it available in Perl and Emacs Lisp, it’s best used sparingly.
Memory safety is also kind of a no-brainer. I enjoy working at all levels of abstraction, and having control over memory layout is great, but I don’t miss it when working in a higher-level language. Compilers and runtimes for statically typed languages can usually be trusted on matters of value representation.
Efficient storage reclamation is something I’m particularly concerned with. In the absence of mutability and laziness, refcounting is an excellent GC replacement because you can’t get cycles. :)
I came to that choice from working with Haskell—it’s wonderful, but it suffers from performance issues due to the GC–laziness combination. Both immensely useful features, to be sure, but they come at a cost. OCaml hits a certain sweet spot: it uses GC, but also eager evaluation by default; has convenient access to mutable values; and the runtime uses unboxed primitives.
Windows support is…well, an unfortunate truth. The state of programming on Windows isn’t great outside the Microsoft stack—MinGW and Cygwin notwithstanding, as “using Windows to emulate Unix” isn’t the same as “using Windows”. Language designers have a sad tendency to neglect Windows as a platform but I can’t quite tell why. Can it be just because they work primarily with Linux, Unix, or OS X?
Refcounting is an extremely poor choice for memory management on modern machines. Even putting aside the issue that it can't handle cycles: constantly writing to memory to modify reference counts can be a huge performance hit (you often have to do it even when you are accessing objects in a read-only fashion). Also it requires extra memory on a per-object basis. Also it doesn't play nicely with concurrent threads. Also it's not cache friendly.
I'd expect any halfway-decent modern GC implementation to be significantly more efficient than reference counting.
About the only valid justification I can see nowadays for not using a GC is a hard-realtime latency requirement. Yep, GC pauses suck. But for efficient memory management in nontrivial systems, GCs are definitely the state of the art.
Tell that to Linus:
Data structures that have visibility outside the single-threaded
environment they are created and destroyed in should always have
reference counts. In the kernel, garbage collection doesn't exist (and
outside the kernel garbage collection is slow and inefficient), which
means that you absolutely _have_ to reference count all your uses."
Just to balance your downsides of refcounting, some downsides of GC: requires halting the program periodically and trashing all the caches, which is inconvenient and sometimes impossible. And performance-wise it's unpredictable and can require lots of tuning, as Java programmers who work on allocation-heavy workloads can attest.
Garbage collection has a lot going for it too, but any garbage collecting algorithm will suffer from at least one of the problems you mention, in varying degrees. Reads and writes should not require memory management code, like updating a refcount? That rules out a large class of GC concurrent/generational algorithms that depend on read and write barriers. No extra memory on a per-object basis? This rules out many incremental mark-and-sweep algorithms that store the object color. (True, the color may be implemented with a side table, but so also may reference counts.) Play nicely with concurrent threads? This rules out a large class of stop-the-world collectors. Cache friendly? This rules out semispace copying collectors, which may even page in memory from disk during a collection.
I'd say the evidence shows that GC performs best when there's lots of memory available and low interactivity, so it's a good choice for server-type applications. For interactive applications with limited memory, especially mobile apps/games, GC is more problematic, and it's not so clear. I don't know if the GCs in XNA or Android are "halfway-decent", but they both have significant overhead that causes real performance issues.
And I’m well aware of modern GC implementations, but one of my goals is deterministic memory behaviour, a feature lacking in other functional languages. I swear I know what I’m doing. :)
[1]: Read: will be; the current implementation is interpreted but I’m working with a friend on an x86_64 VM target.
I would argue that ITAs software (http://www.itasoftware.com/solutions/travelers/flightsearch....) is larger than most software efforts, and it is written in LISP. Not statically typed.
Great Windows support
The other points are largely flame bait too. A competent developer can manage just fine in java, python, c++ or a host of other languages that fail his tests.
A classic case or the bad worker who blames his tools. My response to this kind of blog post is grow up, deal with the reality of the languages that are in every day use and stop whining about things like null pointers!
We live with the flaws of old tools because they have other, more important value to us—a new language has no value without adoption, so it had better improve on its predecessors in order to convince people to adopt it.
I had the same reaction, but given that his code has to run on other peoples' desktops, it makes sense.
So let's remove null pointer exceptions and lets require that anything that could return failure be wrapped in an option type. That way you can determine if there is Some x or None inside of the possibly-None value. Well, then you need to deal with how to fail in the case of possibly-None.
Even the best functional programmers I've seen fall back to some kind of exit-or-exception semantic in the None case. So we've kind of re-invented the null pointer exception, by having cases where the easiest thing to do in our program in response to some failure is to just abort the program.
I think that this is fine from a memory safety perspective. The terrible thing about the 'NULL' pointer is that it is a sentinel that is overlaid into the domain of valid addresses to the machine. When we add a layer that checks if the address is valid without relying on its literal value, then we gain a lot of safety in terms of not generating a hardware exception (that is difficult to reason about) or scribbling on some other object (which could be impossible to reason about). Terminating the program, to me, is a fine tradeoff to avoid corrupting and modifying the program in an unknowable way...
The difference here is that the programmer is making a conscious choice to violate the option type by writing a partial function which escapes it. It also makes very plain and clear (via type annotations) which functions are allowed to fail and which are not (pure functions). With NULL pointers you're basically making every pointer in the entire program an option type with the potential to crash if you haven't written the boilerplate to handle it.
Oh and by the way, Haskell's Maybe and Either types are monads which can be handled very elegantly via do notation, simple combinators or monad transformers (MaybeT and ErrorT).
Maybe I should write more Haskell code, but this has definitely been my experience with OCaml...
If an intermediate calculation results in Nothing, then the final value will be Nothing. Handling this is as simple as:
foo :: Maybe Int -> Int
foo Nothing = 0
foo (Just x) = x
It's just that if it's the right thing to do, you should have an active hand in making it do it.
Is the big difference between like:
m.foo_meth() // may throw NullPointerException
and
m = getSomeM()
match m with Some x -> x.foo_meth() | None -> raise NullPointerException
that in the second case, the exception raising is explicit and in the first it is implicit? I kind of get that I guess, it just seems that it's a similar level of verbosity to deal with it whether or not you allow null pointers and you will, I think, always bottom out to some similar kind of code pattern for error handling...
(a) in a case where failure is supposed to be possible, you are strongly pushed to check for null when it first comes up, rather than just ferrying around the returned object and perhaps getting a NullPointerException five levels into a subsequent function call;
(b) in a case where failure is not supposed to be possible, which is probably the majority of cases where NullPointerExceptions come up in practice (since failure is more commonly represented by exceptions or error codes than just null), you have the type system verifying that there is no path through your code that accidentally fails to initialize a variable or otherwise introduces a null.
edit: Of course, initialization is not always predictable enough for the type system to verify it, but here functional-ish type systems can help: if your class data looks like "bool connected; Socket socket;", it's possible that you could accidentally end up with connected == true but socket == null, but if you write it as (pseudocode) "state : data Disconnected | Connected (socket : Socket)", with Socket non-nullable, you're assured that it's impossible.
Even the best functional programmers I've seen fall
back to some kind of exit-or-exception semantic in
the None case.
First, consciously raising an exception is far, far better than having an exception raised without you being aware of it. I can search OCaml code for all of the instances of raise, failwith, and _exn, and end up with a reasonable understanding of when my code might conceivably fail. I can't do the same thing in Java, because many of the functions I'm calling might return null.
And it's really not that hard to deal with optional values when you have them. Sure, it's slightly more verbose than not having to deal with null, but in practice you have to write a bunch of if statements checking if things are null everywhere anyway.
Also, most languages don't have good abstractions for dealing with nulls despite them being everywhere, so they're always painful. Languages like OCaml and Haskell do have good abstractions, so you can often avoid a lot of the verbosity: if you have an optional value and really do want to raise an exception (usually only in scripts/smaller programs), wrap your value in Option.value_exn. If you have a reasonable default value, call Option.value ~default:(...). If you have a bunch of optional values together, you can sequence them together with Option.(>>=), because Option is a monad.
Second, there are alternatives to raising exceptions in the middle of big programs. Consider the Or_error library (see https://github.com/janestreet/core/blob/master/lib/or_error....), which is a part of Core. Rather than having a type like Option everywhere:
type 'a option = Some 'a | None
type 'a or_error = Ok 'a | Error Error.t
What this lets you do is write your code inside the Or_error monad. Because it's a monad, it's very easy to sequence error-generating operations together, and you don't have to check for errors unless you explicitly decide you want to.
Often, you can do something useful like log the error to a log file or display an alert, rather than killing off the entire program. Sometimes, of course, an error is bad enough that it should cause the program to die, and then you can make it terminate with Or_error.ok_exn. But that's relatively rare. If you're trying to write robust systems, many errors are things you can deal with intelligently, and having those errors encoded in the type system ensures you don't forget to do it.
I think I underestimate the benefits that come with reducing the surprise in code. I get a similar amount of distaste lots of matching on option types and throwing to comparing to null and throwing. or_error definitely would help with that distaste I think.
you still have to give a lot of consideration to exactly what the error means, I think. in a lot of programs I write, the presence of an error cascades a lot and doing things like unwinding transactions and ensuring that the fault leaves a component in a consistent state is much more difficult than identifying all of the locations in a component where a failure could arise. maybe I'm doing something much higher level incorrectly but I've read a lot of other peoples code and it either has 10x the code to deal with, for example, many potential null-return scenarios or "works most of the time" and presents extremely critical consistency problems if a "stars align" error occurs ...
This may not cover your use cases, but for cascading errors you can often write code something like the following:
match
foo_err ()
>>=? bar_err
>>=? baz_err
[... etc ...]
with
| Error e -> Log.error e
| Ok result ->
[...]
If you're interested in talking to more people about this kind of thing, the Core mailing list might be a good place to try: https://groups.google.com/forum/?fromgroups#!forum/ocaml-cor...
exception safety, maybe, but that's a superset of memory safety?
It's C-fast.
A NULL (usually defined as 0) is a valid memory location, especially if you are doing certain types of systems work--the fact that we've used it as a sentinel value in C/C++ is a separate problem entirely.
Maybe we need to use a taxonomy where languages that do not express anything about layout in memory (as bytes, quibits, or whatever) are kept separate from other languages.
The issue, of course, is that eventually you need to move out from algorithm space and start dealing with real hardware, which speaks bytes. Then again, a model where you only interface over JSON blobs might obviate the need for that, provided you don't talk to anything that speaks in something other than numbers and human-readable strings.
What the author is talking about is the idea that a pointer dereference operation will never result in a failure (exception, segfault, panic, etc). That's a separate concern from control over memory layout.
Somehow, this reads very strangely to me. Are you saying something like "I think you are talking about the same issue from a different point of view"?
The compiler checks types... but there's no generics. Given that the author is coming from C++, they probably want a type system that can represent 'vector of ints' vs 'vector of strings'.
Nil doesn't cause null pointer exceptions... but it causes silent bugs, where you thought something was happening but it wasn't. The real problem (all types have an extra misbehaved value) is still present.
Automatic reference counting gives you memory safety... but it can't handle cycles automatically, and obviously it doesn't apply to the C subset.
In Objective-C, it could be a silent bug or just intended behavior to send messages to a nil object. It is explained here: http://developer.apple.com/library/ios/#documentation/cocoa/...
I've also accidentally had a nil where it wasn't expected, and lost time figuring out why particular effects weren't occurring. More time than I would have lost if the mistake had caused an immediate exception.
The same type of mistake is still possible (unexpected value), though the consequences are different, so I don't think it makes sense to say nil solves the null problem.
There is a lot of responsibility on the shoulders of language designers. There is the notion of technical debt that we all deal with as programmers, but the meta-debt the language designer accumulates is astounding.
Let's meditate a while. As we read this, some young, well meaning C++ programmer is writing another String class.
I don't even know if talking about Tony Hoare's NULL story is really that relevant. After all, that's a single feature of a programming language. In the context of C++, talking about NULLs at the language level is like arguing about the colour of your Edsel.
var list = new List<int>();
There is terrific functional support through LINQ.
It is a great language. I just don't want to work on windows and it only has full support there. The library support and community are also pretty bad.
Wouldn't Strongtalk and the research around it be the counter example to this claim?
Flame-bait. I happen to like static typing-- and I'd be excited to see more progress with optional static typing in Clojure-- but I disagree.
First of all, you can pretty easily get principled strong typing with a language like Clojure. One of the first things I write on a large Clojure project is defn-typed. If types were a major part of what I was doing (e.g. theorem proving) that wouldn't be acceptable, but for web programming it's generally enough. It's pretty easy using a few macros to make it so that you get almost all of the guarantees (from unit testing) that you'd get from compile-time type checking.
Remember: dynamic typing doesn't mean you don't have types. You can include types in your contract system. It means that type sanity isn't checked at compile-time. For certain problems that require extremely high levels of correctness, you need static error-checking/analysis because runtime unit testing isn't good enough. For every large system? No, not the case. What you need, in general, are good programmers and sane development practices; those matter a lot more than static vs. dynamic.
My experience is that static typing performs best when you're trying to solve a known problem extremely well, and that's not an uncommon thing in software. Dynamic typing is better-suited to a more chaotic world where you don't know the problem yet, because it makes it easier to "poke" at things in the REPL and build prototypes.
Also, improperly used, static and dynamic typing both fall down disastrously on large or distributed systems. For example, if you have circular dependencies in a large Scala project ("don't do that", you might say; but we're talking multi-developer now, with the full spectrum of skill) you're going to see gargantuan compile times because incremental compiles will fail you, and while Scala's compiler is an extremely powerful machine, it's not fast.
All this said, Haskell is still a very cool language and I think everyone who wants to be a serious computer scientist needs to get some exposure to what it has to offer. Real World Haskell is a great book, for starters. I also think Scala is great if you have strong developers who won't be tempted to OOP things up.
static typing systems work with good programmers and sane development practices as a force multiplier. dynamic typing systems become something that requires you to have good programmers.
It's not as good as static analysis at catching all type errors-- if that's what you need, I wouldn't recommend any dynamic language-- but it does mean that most type errors will be caught in unit testing, with minimal recurring work.