At the end of my last post, I mentioned that I ended up reusing Scala’s build-in List collection to implement the exercises instead of writing a generic abstract Stack and sample implementations of those. Since then, I’ve spent some time learning about how to implement generic collections in Scala. I came up with a Stack trait, a la Okasaki’s Stack signature, and three implementations: the first two are straightforward translations of the SML structures given in the book, and the third is a more Scala-idiomatic implementation.

On an organizational note, I’ve also started a PFDScala repository on GitHub for this blog series, so I can have a centralized place to put all of the code I write for the exercises and examples. I’ll still put the relevant snippets in gists so I can embed them here.

A Generic Trait for Stacks

Scala traits are very powerful. They’re basically like mixin classes from Ruby, in that your class can extend from multiple traits without having the troubles that plague C++-style multiple inheritance.

Abstract traits can be used like Java interfaces. This is just what we need to write a generic interface for Stacks:

For those of you unfamiliar with Scala, this is similar to a Java interface or a SML signature: we’re defining a type-parameterized trait/interface/signature with a bunch of method signatures that classes which extend this trait must implement themselves. What’s up with the weird type annotations, though?

In Scala, annotating a type parameter with “+” marks it as a covariant type parameter. This means that if we have a type C[+T], a type with a single covariant type parameter, and if type A is a subtype of type B, then type C[A] is a subtype of C[B]. We can similarly annotate a type parameter with “-” for the opposite effect.

The “>:” type operator is the supertype relationship: the type on the left of it has to be a supertype of the type on the right of it.

So why do we need to annotate our types like this here? In Scala, by default, types are invariant in their type parameters. This means if we had a List[Integer], and List’s type parameter was invariant, then that list would not be a subtype of a List[Number], even though it seems like it would be fine because a List of Integers is just a specialized List of Numbers. The same thing goes for our Stack type, so we mark its type parameter up as covariant.

However, to keep compile-time type checking sound, Scala has to impose some restrictions on the types of methods in classes with covariant type parameters, due to the problem of mutability: a mutable array of type T is actually contravariant in its type parameter, because if you had an Array of type Any, updating one of its cells to be a subtype of Any like String is not always legal. So, in our Stack example, we can no longer simply say

def cons(x: T) : Stack[T]

because the x is appearing in a contravariant position, meaning that it has the potential to mutate state in a way that could break type safety.

Luckily, we can get around this problem of contraviariant position by imposing a bound on the type of cons’s parameter: we say

def cons(x: U >: T) : Stack[U]

to ensure that the input type of cons is a supertype of the stack’s type. This prevents any type safety issues caused by the potential contravariance of the formal parameter, and allows users to generalize the type of a Stack by consing a more general type onto it. For example, one could cons a String onto a Stack[Int] and get out a Stack[Any].

Implementation 1: Using builtin Lists as the backing store

This class uses the builtin List implementation to provide the underlying storage for the Stack. It also uses Scala’s companion object feature to provide a static factory method for creating new ListStacks.

Implementation 2: Using a custom List case class as the backing store

This class uses a custom implementation of List as the backing store. It’s the same idea as the first one, except we use a set of case classes to match on instead of just wrapping List’s functions.

This shows off Scala’s case classes a little bit: they’re just like datatypes in SML, except a bit more verbose to specify. The abstract class LIST is like the name of the datatype, and the case classes that inherit from it are like the datatype’s constructors. Making LIST sealed restricts classes from extending it unless they’re in the same file: this allows the compiler to detect non-exhaustive matches.

Implementation 3: Implementing the Stack functions within case classes

This implementation is a bit different from the other two: instead of storing the actual Stack data as a data member inside of a single StackImpl class, this puts the implementation inside of case classes that extend the implementation class, which is made abstract.

This strategy seems the most idiomatic of the implementations I wrote. It defines its own internal datatype like the custom List implementation, without the mess of having to match on each of the cases of the custom List every time it’s necessary to operate on the data. I think it produces the most compact and readable code out of all three implementations.

Conclusion

That was a long-winded post for what was just defining a simple custom collection interface and a few simple implementations. Again, all of this code is available in the PFDScala GitHub repo. Next time we’ll proceed with the next section, 2.2. Hopefully it will be easier to implement the examples and exercise solutions with the improved understanding of Scala’s type system that I gathered by writing this post.