I can't count the number of times I've created overloaded methods to destructure a function argument and call a version that takes arguments individually. This is most prevalent in code that takes a Point or Vector , like so:

case class Vector ( x : Float , y : Float ) class Transform { def translate ( tx : Float , ty : Float ) = ... def translate ( t : Vector ) = translate ( t . x , t . y ) // ... }

but there are plenty of other situations where I end up doing the same thing. Here are some examples from Scaled:

case class Region ( start : Loc , end : Loc ) class BufferV { // ... def region ( start : Loc , until : Loc ) : Seq [ Line ] = ... def region ( r : Region ) : Seq [ Line ] = region ( r . start , r . end ) // ... } class Buffer extends BufferV { // ... def delete ( start : Loc , until : Loc ) : Seq [ Line ] def delete ( r : Region ) : Seq [ Line ] = delete ( r . start , r . end ) def replace ( start : Loc , until : Loc , lines : Ordered [ LineV ]) : Loc def replace ( r : Region , lines : Ordered [ LineV ]) : Loc = replace ( r . start , r . end , lines ) def transform ( start : Loc , until : Loc , fn : Char => Char ) : Loc def transform ( r : Region , fn : Char => Char ) : Loc = transform ( r . start , r . end , fn ) // ... }

Some from a game I'm working on:

public Facility facility ( Coord coord ) { return _facs [ index ( coord )]; } public Facility facility ( int x , int y ) { return _facs [ index ( x , y )]; } public Stack stack ( Coord coord ) { return _stacks [ index ( coord )]; } public Stack stack ( int x , int y ) { return _stacks [ index ( x , y )]; } public boolean scouted ( Coord coord ) { return _scouted [ index ( coord )]; } public boolean scouted ( int x , int y ) { return _scouted [ index ( x , y )]; }

You get the idea.

If we have ADTs (Scala calls them case classes), or value classes/structs, it seems perfectly reasonable to automatically destructure one passed to a function which takes arguments of the same type and in the same order.

val tx = new Transform (...) val dt = Vector (...) tx . translate ( dt ) // desugars into tx.translate(dt.x, dt.y)

This should work even if there are additional arguments to the function:

val buffer : Buffer = ... val r = Region ( start , end ) buffer . replace ( r , Seq ( Line ( "Yes" ), Line ( "we" ), Line ( "can" ))) // desugars into buffer.replace(r.start, r.until, Seq(...))

I'm inclined to match purely on declaration order and type, because name matching seems likely to fall prey to minor quibbles like translate taking tx and ty whereas Vector declares x and y . However, this does restrict us to only desugaring "struct-like" types, which are unambiguously meant to be an ordered bundle of values.

I would like to allow arbitrary types to opt-into this behavior, not least because my Region example up there was cheating. The actual definition of Region is:

trait Region { def start : Loc def end : Loc def contains ( loc : Loc ) : Boolean = ( start <= loc ) && ( loc < end ) def isEmpty = start >= end }

For a variety of reasons, I didn't want it to be a case class, but I sure wish I didn't have to create dozens of overloaded methods to manually destructure a Region .

I use another pattern in my Java geometry library which would fail in these circumstances as well:

interface XY { float x (); float y (); } class Point implements XY { public float x , y ; public float x () { return x ; } public float y () { return y ; } // ... public Point set ( float x , float y ) { ... } public Point set ( XY p ) { return set ( p . x (), p . y ()); } } class Vector implements XY { public float x , y ; public float x () { return x ; } public float y () { return y ; } // ... public Vector set ( float x , float y ) { ... } public Vector set ( XY p ) { return set ( p . x (), p . y ()); } }

The XY interface makes it easy to express that you don't care whether your x,y pair is actually a Point or a Vector (or any other damned thing that has an x and y), which is very useful. But how do I know that it's OK to destructure an object that happens to implement XY ?

Probably some kind of annotation like @Value which, when placed on a class or interface means that all public methods represent destructurable values:

@Value interface XY { float x (); float y (); }

and when placed on fields/methods means that those fields/methods comprise the value part of the class:

trait Region { @value def start : Loc @value def end : Loc def contains ( loc : Loc ) : Boolean = ( start <= loc ) && ( loc < end ) def isEmpty = start >= end }

I can already imagine situations where this would cause trouble. Imagine I have two traits, both annotated with @Value which are equivalent from the perspective of the types of their fields and their order:

@value trait XY { def x : Float def y : Float } @value trait PolarVec { def angle : Float def length : Float } case class Impulse ( x : Float , y : Float , angle : Float , length : Float ) extends XY with PolarVec

If I pass an Impulse to def translate (tx :Float, ty :Float) how do I know which value to use when destructuring (and for that matter, does a value trait override the value nature of the case class itself)?

Maybe the answer is just to fail, report the ambiguity, and be happy that the solution helps the programmer 90% of the time. One can always be explicit and write translate(imp.x, imp.y) .

Update: 7/27

Having slept on this little exposition, it occurred to me this morning that I should at least mention Scala's extractors and how they provide a general mechanism which could also be used to accomplish this destructuring.

In Scala, each type has a companion singleton which can define an extractor for a type in the form of an unapply method. For example, the Region trait above can define:

object Region { // .. def unapply ( r : Region ) = Some (( r . start , r . end )) // .. }

This allows one to write code like so:

def foo ( r : Region ) = r match { case ( Loc ( 0 , 0 ), end ) => // starts at zero, end is bound to r.end case ( start , end ) => // start is bound to r.start, end to r.end } def bar ( r : Region ) { val Region ( start , end ) = r // start is bound to r.start and end to r.end }

This abstraction is great and unifies pattern matching, destructuring assignment and my proposed feature of automatic destructuring of function arguments, but in Scala it comes at substantial performance cost.

Scala is syntactically concise, which makes it very easy to violate programmer expectations. That unapply method is actually doing:

def unapply ( r : Region ) = new Some ( new Tuple2 [ Loc , Loc ]( r . start , r . end ))

and the destructuring assignment under the hood, looks like:

val v : Option [ Tuple2 [ Loc , Loc ]] = Region . unapply ( r ) if ( v . isEmpty ) throw new MatchError val t : Tuple2 [ Loc , Loc ] = v . get () val start = t . _1 () val end = t . _2 ()

We're wrapping the data of interest into two temporary objects and then immediately unwrapping them. Yes, between the Scala optimizer and the JVM optimizer this profligacy may eventually be optimized away, but it also may not, and I should not have to worry that using a handy feature like passing a Point to a method which takes two floats might incidentally create two objects on the heap and throw them away. Stick that into an inner loop that's doing math calculations and you've just "abstracted away" 50% of your performance.

So the abstraction is good, the implementation is bad. I would definitely use one abstraction for pattern matching, destructuring assignment and this auto-destructuring of arguments, but I would do it in such a way that it was closer to zero cost. Scala already cheats and automatically optimizes this abstraction for its own ADT types (case classes) to dramatically reduce the cost, but I'd prefer to design the mechanism to be cheap for everyone rather than make user code a performance second class citizen.

The @Value annotation described above is one ham-fisted way of accomplishing that. One could also imagine a metaprogramming facility where you provide the moral equivalent of an unapply method, but it's written in code that communicates to the compiler the number, type and order of your type's value components so that when it sees:

val Region ( start , end ) = r

it can generate:

val start = r . start val end = r . end