tpolecat

case class Box[A](a:A) { 
  def len1(implicit ev: A <:< Seq[_]) = a.length
  def len2(implicit ev: A => Seq[_]) = a.length
}

Box(List(1,2,3)).len1 // ok
Box(List(1,2,3)).len2 // ok

Box("foo").len1 // doesn't compile

tpolecat

package util

import shapeless._
import shapeless.TypeOperators._
import shapeless.Nat._
import shapeless.LTEq._
import scala.annotation.implicitNotFound

@implicitNotFound("Cannot prove that ${A} is evenly divisble by ${B}")
trait Div[A <: Nat, B <: Nat]

tpolecat

object Alg {
  
  sealed trait Num { def toInt: Int }
  sealed trait Big extends Num
  sealed trait Small extends Num
  
  object Big {
    
    def fromInt(n:Int): Option[Big] =
      if (n >= 5) Some(new Big { def toInt = n }) else None

tpolecat

// Scala 2.10.2 and Shapeless 2.0.0-SNAPSHOT

import shapeless._
import shapeless.nat._

sealed trait Month {
  type ND <: Nat
  def apply(n: Nat)(implicit ev: LTEq[n.N, ND]) = "foo"
}

tpolecat

> console
[info] Starting scala interpreter...
[info] 
Welcome to Scala version 2.10.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_13).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import shapeless._
import shapeless._

tpolecat

package time.calendar

import scalaz.Enum
import scalaz.Ordering
import shapeless._
import shapeless.Nat._

import shapeless._
import shapeless.Nat._
import shapeless.LTEq._

tpolecat

object TPayne extends App {

  val m = Map(
    "2" -> List(List("1")),
    "3" -> List(List("9"), List("5", "6", "7", "8")),
    "1" -> List(List("8", "9"), List("2"), List("2", "3")))

  def combos[A, B](xs: Map[A, List[B]]): List[Map[A, B]] =

tpolecat

scala> trait X { type A }
defined trait X

scala> implicit def convert(a:X)(implicit ev: a.A =:= String): Int = 42
convert: (a: X)(implicit ev: =:=[a.A,String])Int

scala> (new X {}) : Int
<console>:10: error: Cannot prove that a.A =:= String.
              (new X {}) : Int
               ^

tpolecat

scala> trait X { type A }
defined trait X

scala> implicit def convert(a:X)(implicit ev: a.A =:= String): Int = 42
convert: (a: X)(implicit ev: =:=[a.A,String])Int

scala> (new X {}) : Int
<console>:10: error: Cannot prove that a.A =:= String.
              (new X {}) : Int
               ^

tpolecat

package angles

import spire.algebra._
import scala.math.Pi

object Angle {
  def apply(n: Double): Angle =
    if (n < 0.0) new Angle(n % 360.0 + 360.0) else new Angle(n % 360.0)
  def fromRadians(n: Double): Angle =
    Angle(n * 360.0 / (2.0 * Pi))