safe.scala

// zip two lists, call map, function types inferred
scala> SafeList(1, 2, 3, 4) zip SafeList[Double](1, 2, 3, 4) map (_ + _)
res0: psp.core.Foreach[Double] = 2.0 :: 4.0 :: 6.0 :: 8.0 :: Nil

// zip three lists, call map, function types inferred
scala> SafeList(1, 2, 3, 4) zip SafeList[Double](1, 2, 3, 4) zip SafeList[Long](1, 2, 3, 4) map (_ + _ + _)
res1: psp.core.Foreach[Double] = 3.0 :: 6.0 :: 9.0 :: 12.0 :: Nil

// Arity mismatch - compile time error
scala> SafeList(1, 2, 3, 4) zip SafeList[Double](1, 2, 3, 4) zip SafeList[Long](1, 2, 3) map (_ + _ + _)
<console>:18: error: type mismatch;
 found   : psp.core.SafeList.L3[Long]
 required: psp.core.SafeList[psp.core.SafeList._4,?]

// Three tails on list of length three, okay
scala> SafeList(1, 2, 3).tail.tail.tail
res3: psp.core.SafeList[psp.core.SafeList._3#Prev#Prev#Prev,Int] = Nil

// Tail on empty list - compile time error
scala> SafeList(1, 2, 3).tail.tail.tail.tail
<console>:18: error: type mismatch;
 found   : psp.core.SafeList[psp.core.SafeList._3#Prev#Prev#Prev,Int]
 required: ?{def tail: ?}