« Return to Thread: [scala] higher-kinded types
[scala] Re: higher-kinded types
by Meredith Gregory
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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
--
L.G. Meredith
Managing Partner
Biosimilarity LLC
1219 NW 83rd St
Seattle, WA 98117
+1 206.650.3740
http://biosimilarity.blogspot.com
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.meredith@...> 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
« Return to Thread: [scala] higher-kinded types
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