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// smallest expression of monad i can find
trait MBrace[C[X] <: MBrace[C,X],A] {
def nest( a : A ) : C[A]
def flatten[T <: C[C[A]]]( bsq : T ) : C[A]
}
// a monad that is a Seq
trait MBraceSeq[C[X] <: MBrace[C,X] with Seq[X],A] extends MBrace[C,A]
// one of the simplest witnesses of monad i can find
case class MSequitor[A]( a_ : A* ) extends Seq[A] with MBrace[MSequitor,A]
{
override def nest( a : A ) = new MSequitor[A]( a )
override def flatten[T <: MSequitor[MSequitor[A]]]( bsq : T ) : MSequitor[A] = {
(new MSequitor[A]( ) /: bsq)( {
( acc : MSequitor[A], e : MSequitor[A] ) => ( acc ++ e ).asInstanceOf[MSequitor[A]]
} )
}
override def length = a_.length
override def iterator = a_.iterator
override def apply( n : Int ) = a_.apply( n )
}
// type arguments [MSequitor,A] do not conform to trait MBraceSeq's type parameter bounds [C[_] <: MBrace[C,A] with Seq[A],A]
// a statement of the instance relation
class MBraceSequitor[A] extends MBraceSeq[MSequitor,A] {
val empty : MSequitor[A] = new MSequitor[A]( )
override def nest( a : A ) = empty.nest( a )
override def flatten[T <: MSequitor[MSequitor[A]]]( bsq : T ): MSequitor[A] = empty.flatten( bsq )
}
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