All,
The following code works without going through the M-P-O construction. It
enjoys approximately the same brevity as a Haskell type class.
// smallest expression of monad i can find
trait MBrace[C[_] <: MBrace[C,A],A] {
def nest( a : A ) : C[A]
def flatten[T <: C[C[A]]]( bsq : T ) : C[A]
}
// one of the simplest witnesses of monad i can find
class MBraceSeq[A]( a_ : A* ) extends Seq[A] with MBrace[MBraceSeq,A] {
override def nest( a : A ) = new MBraceSeq[A]( a )
override def flatten[T <: MBraceSeq[MBraceSeq[A]]]( bsq : T ) :
MBraceSeq[A] = {
(new MBraceSeq[A]( ) /: bsq)( {
( acc : MBraceSeq[A], e : MBraceSeq[A] ) => ( acc ++ e
).asInstanceOf[MBraceSeq[A]]
} )
}
override def length = a_.length
override def elements = a_.elements
override def apply( n : Int ) = a_.apply( n )
}
Best wishes,
--greg
On Wed, Jun 24, 2009 at 3:49 PM, Meredith Gregory
<lgreg.mered...@gmail.com>wrote:
> All,
>
> Am i correct in concluding that the solution in The Moors-Piessens-Odersky
> paper only works with collections that have been clever enough to have used
> type members rather that type parameters? Or, is there a trick to making the
> vast majority of the collections types that are parametrically typed look as
> if they have type members? (See example below.)
>
> Best wishes,
>
> --greg
>
> // Paraphrasing the basic Moors-Piessens-Odersky construction
> trait TypeCtor1 { type E }
> trait Brace[A] extends TypeCtor1 {
> type C <: TypeCtor1
> def nest( a : A ) : C{type E = A}
> def flatten( bsq : C{type E=C{type E=A}} ) : C{type E=A}
> }
>
> // Now, how to make a version of BraceList since List is parametrically
> typed?
>
>
> --
> L.G. Meredith
> Managing Partner
> Biosimilarity LLC
> 1219 NW 83rd St
> Seattle, WA 98117
>
> +1 206.650.3740
>
> http://biosimilarity.blogspot.com
>
--
L.G. Meredith
Managing Partner
Biosimilarity LLC
1219 NW 83rd St
Seattle, WA 98117
+1 206.650.3740
http://biosimilarity.blogspot.com
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