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I'll echo all of this (minus scalaz, we don't use that).

I'll give an example of the power of Scala's type system. When I first convinced one of our founders to give Scala a try, he used it to write the first pass of our analytics service. He'd never written Scala before but once it compiled, it ran correctly on the first try, that's the power of its type system.

Not to be too big of an ass, but this could just be the skill of your founder. Or do you believe it is impossible to have compiling code that does not do as its author intended?

Obviously it's possible to write code that compiles while still not meeting requirements. My point is that the by leveraging the type system he was able to offload a lot of those checks to compile time rather than runtime.

It's not impossible, but it's harder in Scala. Actually the closest comparison I could make would be Python, where I had a similar experience - as soon as my code stopped producing errors, it was doing what I wanted it to (the difference being that the errors were at runtime rather than compile time). Whereas in Java and C and even ML I've written programs that compiled correctly but did something very different from what they were meant to.

Having never used Scala before, does its type system provide details about errors that other languages do not? Watching a compile fail because of an incorrect type is not something I have run into very often, if ever, in other languages that provide similar type systems.

I mean mistakes can happen and if the type system can catch it for free, so to speak, that is great. I'm not about to argue that it is completely useless. But in terms of probabilities of what kind of bugs might be introduced, type errors seem to be on the low end.

To infer that Scala's type system is powerful enough to catch the more common types of mistakes programmers make, that is quite an impressive claim. I am curious to know more.

In Haskell, and I assume Scala, you can name types of the same form, so I might say a TimeSeries is a list of floating point values. If I use that data type, you can't accidentally pass a different kind of list of floating points even though they're the same type.

Yes, you can do this in scala with type aliases

Type aliases don't solve that problem. You need to use something like Shapeless' newtype.

Type aliases do not solve this. The closest you can get is using the new value classes & those have some very big caveats

A simple example: consider scalaz's NonEmptyList[T] class, which turns a failure to populate a list into a compile error rather than a runtime error.

A more complicated example. In one scala system I worked on we had a master/slave postgres db system. We were replacing a legacy RoR system with a Scala system, and in the RoR system there were all sorts of data inconsistency errors caused by people writing to master and reading from the slaves before replication occurred.

In the Scala system which replaced it, we set up a monadic action system. A function which hit the database would have a type signature either PReader[T] or PWriter[T] - this was the only way to access a java.sql.Connection short of manually calling jdbc. Combining the two forms yields a PWriter:

    for {
      _ <- putObjectToDB(args)
      obj <- getObjectFromDB(args)
    yield (render(obj)) // is a PWriter[_]

The whole operation runs in a single transaction against the correct database, and any attempt to do the wrong thing is a compile error.

It's partly culture; Scala makes types that little bit easier to use (e.g. case classes, type inference), so things that you might pass as an untyped Map[String, Object] in other languages, you tend to pass as a case class instead. e.g. using spray in scala, the type system will catch if you misspell a HTTP header, because, as the article complains about, they're represented as types.

(Though of course there is an escape hatch, you can use RawHeader(name, value) if you need to).

Scala has dependent type support, so, yes, it is quite more powerful than most languages in widespread use. You can look at the Shapeless library to see that at work.

However, that is not really what's going on... though Scala type system, even without dependent types, is strictly more powerful that Java's, it really comes down to the ease of use: Scala programmers tend to use types liberally (which was one of the criticisms of the article), and that is part of the "magic".

Part of this is how easy it is to define a type: it can be as short as a single line. So you'll see way less String types in Scala, for instance, or "one type to do it all" like the article wanted for HTTP. Instead, you get many types, in which invalid states are not possible and, therefore, getting the types right is enough to ensure there are no invalid states.

Likewise, the partial type inference offered by Scala makes it possible to not worry about all these types: the compiler knows what type you got, and knows whether it can be used the way you are using it, so you don't have to keep looking up types to get things accomplished, but still get to have the types.

Another part is that Scala support higher order types, something not supported by Java, which makes them more useful. That's what allows monads & companions come in and make an appearance -- it's not that you can't write a monad in Java, it's just that you can't abstract over them.

The final part is emphasis on immutability. When you stick to immutable objects representing valid states, the space for errors not caught by the type system is greatly reduced.

Let's get back to the headers, then. First, yes, you are allowed to add arbitrary headers (which is supported by Spray through RawHeader, something it took me 30 seconds to find out, even though I have never used Spray and have little familiarity with it), but section 14 of RFC 2616 has a nice list of standard headers and the rules they are expected to abide by, so let's have them.

And if you have a header, and it is a X-Forwarded-For, isn't it reasonable to expect the value to be a valid remote address? It is supposed to be a valid remote address, after all. So, if you ask for this header, should you simply get a string that may or may not be actually valid, or should the type system give you an Option that may contain Some valid header, or None, therefore ensuring that you don't just assume you are getting the right thing? And if you get something and retrieve the value, is it preferable to just get a string, that might lead to some ad hoc parsing of the address, or some actual type where you can easily retrieve information in a safe manner?

The Scala community will usually think that yes, that's a good idea, and design the APIs accordingly.

> That the author complains about having types rather than a simple map for keys shows a misunderstanding of what they are there for - the idea is to allow the compiler to prove at run-time what is valid code and what isn't.

There's some evidence from what the author wrote he's aware of the benefits of type systems. The question he raises is whether the developers who wrote Spray modeled the problem domain well by choosing this particular set of types (his opinion is clearly no).

> the author makes a strange comment that H-M 'enabled' Monads, and the comment is written in a way that implies that Scala does not have Monads.

Given that the author immediately follows his "enabled" statement with the idea that monads can be used in a type-free language, I think it's most likely he means that H-M has been the ground where the use of monads has been largely developed/defined, not that monads can't be used in either Scala or even something with a considerably less expressive type system.

I agree with everything above. At Vonjour we use Scala. From a business standpoint, our only reluctance was that hiring was going to be more difficult. However, we have received higher quality candidates that are eager to learn applying for our positions.

> Coming to Scala from experience including C, Java and Haskell, I intially found Scala quite difficult.

I think because it allows you to do OOP and FP, Scala becomes difficult since it is so easy to mix the styles. Similar to C and C++, this is very powerful, but also very dangerous. The main learning curve is programming in a way so you don't shoot yourself in the foot while using its flexibility.

May you expand on the type of systems you are building with Scala? I have previous work experience in a multinational bank that ran Java or C# on everything.