Is there any fundamental limitations that stops Scala from implementing pattern matching over functions?

Question

In languages like SML, Erlang and in buch of others we may define functions like this:

fun reverse [] = []
|   reverse x :: xs  = reverse xs @ [x];

Answer 1

I don't know SML or Erlang, but I know Haskell. It is a language without method overloading. Method overloading combined with such pattern matching could lead to ambiguities. Imagine following code:

def f(x: String) = "String "+x
def f(x: List[_]) = "List "+x

What should it mean? It can mean method overloading, i.e. the method is determined in compile time. It can also mean pattern matching. There would be just a f(x: AnyRef) method that would do the matching.

Scala also has named parameters, which would be probably also broken.

I don't think that Scala is able to offer more simple syntax than you have shown in general. A simpler syntax may IMHO work in some special cases only.

Answer 2

There are at least two problems:

1. [ and ] are reserved characters because they are used for type arguments. The compiler allows spaces around them, so that would not be an option.
2. The other problem is that = returns Unit. So the expression after the | would not return any result

The closest I could come up with is this (note that is very specialized towards your example):

// Define a class to hold the values left and right of the | sign
class |[T, S](val left: T, val right: PartialFunction[T, T])

// Create a class that contains the | operator
class OrAssoc[T](left: T) {
  def |(right: PartialFunction[T, T]): T | T = new |(left, right)
}

// Add the | to any potential target
implicit def anyToOrAssoc[S](left: S): OrAssoc[S] = new OrAssoc(left)

object fun {

  // Use the magic of the update method
  def update[T, S](choice: T | S): T => T = { arg =>
    if (choice.right.isDefinedAt(arg)) choice.right(arg)
    else choice.left
  }
}

// Use the above construction to define a new method
val reverse: List[Int] => List[Int] =
  fun() = List.empty[Int] | {
    case x :: xs => reverse(xs) ++ List(x)
  }

// Call the method
reverse(List(3, 2, 1))

Answer 3

It really depends on what you mean by fundamental.

If you are really asking "if there is a technical showstopper that would prevent to implement this feature", then I would say the answer is no. You are talking about desugaring, and you are on the right track here. All there is to do is to basically stitch several separates cases into one single function, and this can be done as a mere preprocessing step (this only requires syntactic knowledge, no need for semantic knowledge). But for this to even make sense, I would define a few rules:

• The function signature is mandatory (in Haskell by example, this would be optional, but it is always optional whether you are defining the function at once or in several parts). We could try to arrange to live without the signature and attempt to extract it from the different parts, but lack of type information would quickly come to byte us. A simpler argument is that if we are to try to infer an implicit signature, we might as well do it for all the methods. But the truth is that there are very good reasons to have explicit singatures in scala and I can't imagine to change that.
• All the parts must be defined within the same scope. To start with, they must be declared in the same file because each source file is compiled separately, and thus a simple preprocessor would not be enough to implement the feature. Second, we still end up with a single method in the end, so it's only natural to have all the parts in the same scope.
• Overloading is not possible for such methods (otherwise we would need to repeat the signature for each part just so the preprocessor knows which part belongs to which overload)
• Parts are added (stitched) to the generated match in the order they are declared

So here is how it could look like:

def reverse[T](lst: List[T]): List[T] // Exactly like an abstract def (provides the signature)
// .... some unrelated code here...
def reverse(Nil) = Nil
// .... another bit of unrelated code here...
def reverse(x :: xs ) = reverse(xs) ++ List(x)

Which could be trivially transformed into:

def reverse[T](list: List[T]): List[T] = lst match {
  case Nil     => Nil
  case x :: xs => reverse(xs) ++ List(x)
}
// .... some unrelated code here...
// .... another bit of unrelated code here...

It is easy to see that the above transformation is very mechanical and can be done by just manipulating a source AST (the AST produced by the slightly modified grammar that accepts this new constructs), and transforming it into the target AST (the AST produced by the standard scala grammar). Then we can compile the result as usual.

So there you go, with a few simple rules we are able to implement a preprocessor that does all the work to implement this new feature.

---

If by fundamental you are asking "is there anything that would make this feature out of place" then it can be argued that this does not feel very scala. But more to the point, it does not bring that much to the table. Scala author(s) actually tend toward making the language simpler (as in less built-in features, trying to move some built-in features into libraries) and adding a new syntax that is not really more readable goes against the goal of simplification.

Answer 4

In SML, your code snippet is literally just syntactic sugar (a "derived form" in the terminology of the language spec) for

val rec reverse = fn x =>
    case x of [] => []
            | x::xs  = reverse xs @ [x]

which is very close to the Scala code you show. So, no there is no "fundamental" reason that Scala couldn't provide the same kind of syntax. The main problem is Scala's need for more type annotations, which makes this shorthand syntax far less attractive in general, and probably not worth the while.

Note also that the specific syntax you suggest would not fly well, because there is no way to distinguish one case-by-case function definition from two overloaded functions syntactically. You probably would need some alternative syntax, similar to SML using "|".