Thursday, December 1, 2011

A rant about Scala vs. Haskell

After two weeks of non-stop Scala programming I now feel comfortable working with implicits, abstract member types, path-dependent types, variance annotations, existential types and everything else. We even used the CPS transformation plugin as a Cont-Monad replacement, already.

Scala's type system is powerful but also complex right from the start. With Haskell you can use most basic libraries without enabling GADTs, RankNTypes, OverlappingInstances and TypeFamilies or having to learn about them. And you can take your time to learn all these concepts. The containers API is really simple compared to Scala's collections API. On the other hand in Haskell you have to learn about monads quite early. (Oh, I miss this warm fuzzy feeling in Scala!)

We're using Scala for integration with existing Java code and libraries and that's working really well. As a "stand alone" language I still prefer Haskell and I doubt that's going to change with more Scala experience. For me as a functional programmer subtyping doesn't add much more than Java compatibilty. Maybe I have to change but IMHO it makes API design and the type system too complex. I'm still overwhelmed by the "default" design possibilities: type parameters or abstract member types? Sealed case classes or open subtyping? Implicit parameters and conversions (aka type classes) offer even more design choices! Maybe I'm just more experienced with Haskell but mostly I just decide between "data" and "class" and with higher-order functions that's enough 95% of the time. For the missing 5% we have extensions (and FlexibleX + Rank2Types + TypeFamilies + ExistentialQuantification is enough for me).

Often people complain about GHC's error messages but they're really great compared to scalac's. I also miss STM. Actors are nice, but using react doesn't feel natural and we can't just use "receive" with thousands of threads "without thinking" just like in Haskell. (We're now using react with the CPS transform plugin to hide the fact that internally everything is implemented using continuations.)

I don't miss lazyness, yet. I've used call-by-name and lazy vals on some occasions and I think I prefer strict by default with optional lazyness annotations. I just can't think "the lazy way" - even after 18 months of full-time Haskell coding.

I do miss control over side effects. When writing a higher order function I always get scared when I realize that I can only expect the caller to provide a pure function but the function could be doing all sorts of things behind my back!

One thing I'd like as a GHC extensions is explicit dictionary passing. It's nice that implicit parameters can be used implicitly but also explicitly. Of course it's risky, too, because nobody can stop you from using a different Ord instance every time you're inserting into a Set. "unsafeCoerce" seemed so evil - until erasure forced me to use foo.asInstanceOf[...] (Of course an exception is not as bad as a segfault.) Sometimes I know what I want and I'm willing to take the risk. As long as all those features are not enabled by default I think hard enough before enabling {-# LANGUAGE IncoherentInstances #-}.

Scala might be the best way to live between worlds. But you have to understand both worlds and you're still in between. It's not twice as good. If I have to live in OO world I prefer having a functional world in addition so I'd chose Scala over Java at any time. But if I can choose freely I still prefer the pure Haskell world.

1 comment:

phil said...

I find it very hard to compare Haskell and Scala, I've been working with Scala for a while now and come from a C++/Java OO background. I'm worried that if I move to Haskell, I don't have OO.

Do you think there is any loss to go to Haskell and loose OO?

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