Writing array fill with a macro

Question

I am trying to write a function which fills an array with a given value using scala macros. For instance a call to:

val ary = Array( 0, 1, 2 )
fill3( ary, 50+25 )

should be expanded to:

val ary = Array(0, 1, 2 )
{
  val $value = 50+25
  ary(0) = $value
  ary(1) = $value
  ary(2) = $value       
}

Here is my first attempt:

def fill3( ary: Array[Int], x: Int ) = macro fill_impl3

def fill_impl3( c: Context )
( ary: c.Expr[Array[Int]], x: c.Expr[Int]): c.Expr[Unit] = {
  import c.universe._        
  def const(x:Int) = Literal(Constant(x))

  //Precompute x
  val valName = newTermName("$value")
  val valdef = ValDef( Modifiers(), valName, TypeTree(typeOf[Int]), x.tree )

  val updates = List.tabulate( 3 ){
  i => Apply( Select( ary.tree, "update"), List( const(i), ??? ) )
  }

  val insts = valdef :: updates
  c.Expr[Unit](Block(insts:_*))
}

But here I'm stuck for two reasons:

I don't know hot to get the precomputed value ($value)
I need several of these function for Arrays of size 3, 4, 6, 9 and 27. Is there a way to dry the definitions, or should I write fill3, fill4, fill6, etc.

Is there the right way to proceed ? How can I solve my two problems ?

EDIT: I realized my initial question was stupid because the size must be known at compile time...

I just updated the question to clarify this point. I simplified the problem with a fixed size. — paradigmatic
– paradigmatic, Commented Aug 18, 2012 at 12:29
And is the value known at compile time or can it be some arbitrary expression? — Kim Stebel
– Kim Stebel, Commented Aug 18, 2012 at 12:37
@kimstebel 1. Learning macros. 2. I am rewriting a numerical intensive code that uses lots of computations on small size arrays (size known at compile time). I would like to see if I can gain some speed-up by unrolling all loops to avoid incrementing counters and the bound check. I plan to write vector operations this way. — paradigmatic
– paradigmatic, Commented Aug 18, 2012 at 12:37

Kim Stebel · Accepted Answer · 2012-08-18 13:00:02Z

2

def fill(size:Int, ary: Array[Int], x: Int ) = macro fill_impl

def fill_impl( c: Context )
(size:c.Expr[Int], ary: c.Expr[Array[Int]], x: c.Expr[Int]): c.Expr[Unit] = {
  import c.universe._        
  def const(x:Int) = Literal(Constant(x))

  val Literal(Constant(arySize:Int)) = size.tree

  //Precompute x
  val valName = newTermName("$value")
  val valdef = ValDef( Modifiers(), valName, TypeTree(typeOf[Int]), x.tree )

  val updates = List.tabulate( arySize ){
  i => Apply( Select( ary.tree, "update"), List( const(i), Ident(valName) ) )
  }

  val insts = valdef :: updates
  c.Expr[Unit](Block(insts:_*))
}

answered Aug 18, 2012 at 13:00

Kim Stebel

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3 Comments

Kim Stebel Over a year ago

that is strange, I am using M6 too. Is your project available somewhere?

Kim Stebel Over a year ago

how do you compile that without splitting it into subprojects?

paradigmatic Over a year ago

I will split it in subprojects, but currently I stupidly comment the main method and when then macro compiles I uncomment it...

0__ · Accepted Answer · 2012-08-18 13:15:25Z

0

You can try to figure it out by using reify along with printing the raw tree of its result:

def fill_impl3( c: Context )
( ary: c.Expr[Array[Int]], x: c.Expr[Int]): c.Expr[Unit] = {
  import c.universe._
  val r = reify {
     val $value = x.splice
     val $arr  = ary.splice
     $arr(0)   = $value
     $arr(1)   = $value
     $arr(2)   = $value
  }
  println( showRaw( r.tree ))
  r
}

Which gives something like

val vt = newTermName("$value")
val at = newTermName("$arr")
val ut = newTermName("update")
Block(List(
  ValDef(Modifiers(), vt, TypeTree(), ...),
  ValDef(Modifiers(), at, TypeTree(), ...),
  Apply(Select(Ident(at), ut), List(Literal(Constant(0)), Ident(vt))),
  Apply(Select(Ident(at), ut), List(Literal(Constant(1)), Ident(vt)))),
  Apply(Select(Ident(at), ut), List(Literal(Constant(2)), Ident(vt)))
)

edited Aug 18, 2012 at 13:15

answered Aug 18, 2012 at 13:10

0__

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1 Comment

Daniel C. Sobral Over a year ago

Do not use $ on identifiers! They are reserved for the compiler, and reifying already protects you from identifier clash.

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