Trait

org.scalactic.Or

GOOD

Related Doc: package Or

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trait GOOD[G] extends AnyRef

Trait providing a concise type lambda syntax for Or types partially applied on their "good" type.

This trait is used to curry the type parameters of Or, which takes two type parameters, into a type (this trait) which takes one parameter, and another (its type member) which takes the other. For example, type Or[G, B] (which can be written in infix form as G Or B) can be expressed in curried form as Or.GOOD[G]#BAD[B]. Leaving off the final BAD type parameter yields a "type lambda," such as Or.GOOD[Int]#BAD.

For example, consider this method that takes two type parameters, a type constructor named Context and a type named A:

scala> def example[Context[_], A](ca: Context[A]) = ca
example: [Context[_], A](ca: Context[A])Context[A]

Because List takes a single type parameter, it fits the shape of Context, it can be simply passed to example--i.e., the compiler will infer Context as List:

scala> example(List(1, 2, 3))
res0: List[Int] = List(1, 2, 3)

But because Or takes two type parameters, G for the "good" type and B for the "bad" type, it cannot simply be passed, because the compiler doesn't know which of G or B you'd want to abstract over:

scala> example(Good(3))
<console>:26: error: no type parameters for method example: (ca: Context[A])Context[A] exist
    so that it can be applied to arguments (org.scalactic.Good[Int,Nothing])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : org.scalactic.Good[Int,Nothing]
 required: ?Context[?A]
              example(Good(3))
              ^
<console>:26: error: type mismatch;
 found   : org.scalactic.Good[Int,Nothing]
 required: Context[A]
              example(Good(3))
                          ^

You must therefore tell the compiler which one you want with a "type lambda." Here's an example:

scala> example[({type L[B] = Int Or B})#L, String](Good(3))
res1: org.scalactic.Or[Int,String] = Good(3)

The alternate type lambda syntax provided by this trait is more concise and hopefully easier to remember and read:

scala> example[Or.GOOD[Int]#BAD, String](Good(3))
res15: org.scalactic.Or[Int,String] = Good(3)

You can read Or.GOOD[Int]#BAD as: an Or with its "good" type fixed to Int and its "bad" type left unspecified.

Source
Or.scala
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Type Members

  1. type BAD[B] = Or[G, B]

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    Type member that provides a curried alias to G Or B.

    Type member that provides a curried alias to G Or B.

    See the main documentation for trait GOOD for more detail.

Value Members

  1. final def !=(arg0: Any): Boolean

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  2. final def ##(): Int

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  3. final def ==(arg0: Any): Boolean

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  5. def clone(): AnyRef

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  8. def finalize(): Unit

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  9. final def getClass(): Class[_]

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  10. def hashCode(): Int

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  11. final def isInstanceOf[T0]: Boolean

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  12. final def ne(arg0: AnyRef): Boolean

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  13. final def notify(): Unit

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  14. final def notifyAll(): Unit

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  15. final def synchronized[T0](arg0: ⇒ T0): T0

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  17. final def wait(): Unit

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