Sunday, 13 June 2010

Scala Frustrations

I'm learning Scala at the moment. I went looking for other "Java++" type languages after programming in Groovy for some time, and was very hopeful that Scala was what I was looking for - a type safe fully object based Java-like language, with clever stuff to save you typing the types. And there are lots of bits of it I really like - the var/val way of specifying whether a variable is final is nice, the object concept that allows you to have singletons that implement interfaces/traits, the removal of operators in favour of allowing them as method names. All good.

However, I'm finding it frustrating. One of the things Scala claims is that it has fewer special cases than Java; notably it has no primitives and no operators. Yet I keep on coming up against some pretty fundamental, and to me highly surprising, special cases.

First there's the whole notion that if you name a method ending in :, and you want to call it in operator notation (instance methodname argument rather than instance.methodname(argument)) then you call it by placing it to the left of the object on which you are calling it - so argument methodname: instance. To me that's a humungous special case. At the moment it seems to me that it's been added solely in order to make prepending to Lists more natural, because Scala lists are immutable and so the time taken to append apparently grows linearly whereas the time taken to prepend is constant. I can't help but feel that's an insufficient reason to introduce a massive syntax divergence from the norm. Is there really no other way? Would an immutable List backed by a mutable LinkedList (so appending returns a new view over the same List) really perform so badly when appending? And if it does, could it not be just a case where developers need to be cautious, as with String concatenation in Java?

Then there's the operator precedence rules. Scala doesn't have operators, right? Only methods? Wrong. It has a few method names which are special cased to have a different precedence order because they are normally operators in other languages. Nice. So for instance (2 + 2 * 5) == 12, whereas ((2).+(2).*(5)) == 20, despite the fact that they are the same methods in the same order on the same objects - Scala just specifies that 2 + 2 * 5 is treated as 2 + (2 * 5). I understand this one, I'm sure it's to align Scala with mathematical convention, but I still disagree. Having a bunch of methods who behave differently in terms of order of evaluation to others based on their name is a huge and confusing special case.

Then there's the Tuple behaviour. You can declare a tuple as so:

val aTuple = (12, "astring")

but if you want to reference the first element, it's one index based! aTuple._1 == 12, aTuple._2 == "astring". I find this staggering - the whole of the rest of Scala, as a descendant of C/C++/Java, is 0 index based. Apparently it's becase Haskell uses 1 index base for its tuples, but surely this is a case where you should ignore Haskell's arbitrary conventions and be internally consistent. Hands up anyone caught out by JDBC's one index based behaviour?

Then there's the behaviour of + and += on mutable and immutable Maps and Sets. On an immutable Map +(Tuple) returns a new Map instance with the Tuple added, as you'd expect. If you call += it attempts to assign the resulting new Map instance to the variable, also as you'd expect - so

val aMap = Map(1 -> "one")
aMap += (2 -> "two") // cannot compile - reassigning aMap to a new instance

will fail to compile, because you are trying to reassign aMap. So far, so good

On a mutable Map the behaviour of +(Tuple) is consistent - it returns a new Map instance with the new entry, it does not mutate the original one. Again, good. However, the behaviour of += is completely inconsistent - it mutates the map on which it is called, but does not reassign the variable! So

val aMap = scala.collection.Map( -> "one")
aMap += (2 -> "two") // compiles!

does compile - it isn't reassigning the aMap variable, just mutating the underlying instance. To me that's crazy; += screams "variable reassignment", it does not scream "mutate this instance, keep same assignment". I'd be happy if the + method mutated a mutable instance and returned a new instance on an immutable type, or alternatively if it was absolutely consistent and the + method always returned a new instance leaving the instance on which it was called unaltered and some other method (Groovy uses <<) were responsible for mutating - but either way, the += operation should do exactly the same as the + operation with the caveat that it reassigns the variable to the result of the operation. It should not, in my opinion, ever be possible to call += on an instance stored in a val declared, and hence final, variable. The same confusing behaviour can be observed on Sets.


It's true that I'm very new to Scala, and in some ways this blog post is injudicious - perhaps if I stick with Scala in a year or two I'll look back and be baffled by my lack of understanding and folly in speaking up knowing so little. But so far I'm finding this sort of thing significantly reducing my enthusiasm for going further.

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