GHC doesn't do subtyping. I suspect that is the main reason why Scala is slow - it's certainly the reason why the code is more complex.
What Ocaml does really well, compared to both Haskell and Scala is type-inference, but it's kind of a weak language IMHO as far as functional programming is concerned.
I also don't get all this complaining, because in practice Scala compilation speed is not a problem. After working with Scala in production for a couple of months now, I'm beginning to suspect that all this is just hyperbole perpetuated by people that never worked with Scala a single day in their lives.
Admittedly, we do make heavy use of the type system.
Incremental compilation suffers from the fragile base class problem. In a well structured app with proper encapsulation and rigid (and type annotated) interfaces, it helps a lot.
The project I'm working on right now is about 15kloc, but it's split in 3 sub-projects. Coupled with incremental (and continuous) compilation, this compilation speed issue hasn't been a problem for me.
Also, if you look around, many open-source projects built with Scala follow the same technique. It's quite sane too.
For example, if you delete AbstractFooBase.unusedMethod, sbt will recompile every file everywhere that references a concrete Foo instance.
(In principle it should only need to recompile files which reference unusedMethod, but it's not that smart yet.)
* Launching the interpreter takes 10 seconds; 50 times slower than Haskell.
* Compiling a small module takes 15 seconds; 10 times slower than Haskell.
* Running 100 command-line unit tests on a small compiled module takes 1.5 minutes; 100 times slower than Haskell.
I also found Scala's type system requires many type annotations even in trivial code. For example, here Scala fails to infer the type of `optBar`, apparently because `None` is also a type:
scala> class Foo {
| def setBar(bar: Int) = optBar = Some(bar)
| def getBar = optBar.get
| var optBar = None
| }
<console>:8: error: type mismatch;
found : Some[Int]
required: object None
def setBar(bar: Int) = optBar = Some(bar)
^ scala> class Foo {
| def setBar(bar: Int) = optBar = Some(bar)
| def getBar = optBar.get
| var optBar: Option[Int] = None
| }
defined class Foo
While OCaml's type system may not integrate objects cleanly, in everyday use type annotations are not necessary, perhaps because objects aren't commonly used.
Similarly, while it's possible to confuse Haskell's type inference by using advanced language extensions, in everyday use one tends not to run into such problems.
With SBT you're only going to do that once per work session. Plus the startup time isn't as bad as you make it sound. I've had a much more painful experience with Ruby (Rails).
> Compiling a small module takes 15 seconds
That hasn't been my experience. I just did a test for a project of mine and it took 5 seconds. Without line-counts such metrics are useless (how small is that module anyway?). Also many Scala projects are composed of multiple subprojects that are efficiently managed by SBT, so a root compile triggers compilation for all these sub-modules, but context-switching between them is what increases compilation cost.
> Running 100 command-line unit tests on a small compiled module
When in the world are you ever going to do that? Like seriously?
> Scala fails to infer the type of `optBar`
In practice it's good to annotate the signature of public functions. Haskell developers do that too, because really, when a developer looks at the definition of "optBar" are you seriously going to suggest that they should be looking at other parts of the code to infer what it returns?
IMHO, this is a feature, as what you want is even worse than what you get with a dynamic language, but YMMV.
> Scala's implicits can pose a significant barrier to understanding other people's code
I haven't met a single library or project of significant size that isn't making use of implicits in one form or another. Whether they are called global state, singletons or parameters injected through DI, we are really talking about implicit parameters.
The difference is that Scala gives you the means to document these implicits in the signature (referential transparency FTW). They are also statically type-safe, can be overridden and have the extra benefit that they make possible some powerful techniques, like type-classes, something that Ocaml isn't capable of.
> I just did a test for a project of mine and it took 5 seconds. Without line-counts such metrics are useless (how small is that module anyway?).
Obviously, comparing absolute times between two unknown systems isn't going to make sense. I was comparing how long it takes to accomplish the same thing on my machine in Haskell and Scala.
> In practice it's good to annotate the signature of public functions. Haskell developers do that too, because really, when a developer looks at the definition of "optBar" are you seriously going to suggest that they should be looking at other parts of the code to infer what it returns?
I agree, annotating type signatures of top-level functions is a good practice. However, in the above example `optBar` is an internal variable in class `Foo`. The point here is I found Scala's type inference to be incapable of dealing even with trivial code.
> The difference is that Scala gives you the means to document these implicits in the signature (referential transparency FTW). They are also statically type-safe, can be overridden and have the extra benefit that they make possible some powerful techniques, like type-classes, something that Ocaml isn't capable of.
Haskell does type classes without the need for implicits. Also, true referential transparency requires control over side effects, which is something neither OCaml or Scala are capable of, but Haskell is.
If you really think about it, type-classes are all about passing around a global vtable implicitly. If anything, Scala's implicits are a superset of Haskell's type-classes and allows for techniques that Haskell isn't capable of.
We could debate all day about what's better. If you're going to make the case that Haskell's type-classes are easier to reason about, then I can make the case that a dynamic language beats Haskell on reasoning about it any day of the week. In the end it's all about tradeoffs.
> true referential transparency requires control over side effects, which is something neither OCaml or Scala are capable of, but Haskell is
With all due respect, that's just bullshit. First of all because there is no such thing as "true referential transparency", second because most techniques that are available in Haskell for dealing with side-effects are also available in Scala. As examples, Scala's library is filled with monadic types and the Play framework makes use of Iteratees for composing HTTP responses, something which makes Comet/WebSocket a peach to deal with.
A referentially transparent expression is guaranteed not to have side effects. A Haskell function which doesn't advertise in its type that it may have side effects, by being in the IO monad, can be trusted to be referentially transparent. This cannot be said about OCaml or Scala functions.
First, I'm not a Haskell expert, but from what I understand there are no vtable passed around nor attached to "objects" in Haskell.
Type-classes are used to find an implementation given an instance. Without extensions you cannot do this:
> lst :: Show a => [a]
> lst = [1, "hello"]
So Haskell isn't the slightest dynamic here.