GOOD
Related Doc:
package Or
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.
Trait providing a concise type lambda syntax for
Ortypes 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, typeOr[G, B](which can be written in infix form asG Or B) can be expressed in curried form asOr.GOOD[G]#BAD[B]. Leaving off the finalBADtype parameter yields a "type lambda," such asOr.GOOD[Int]#BAD.For example, consider this method that takes two type parameters, a type constructor named
Contextand a type namedA:Because
Listtakes a single type parameter, it fits the shape ofContext, it can be simply passed toexample--i.e., the compiler will inferContextasList:But because
Ortakes two type parameters,Gfor the "good" type andBfor the "bad" type, it cannot simply be passed, because the compiler doesn't know which ofGor 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:
You can read
Or.GOOD[Int]#BADas: anOrwith its "good" type fixed toIntand its "bad" type left unspecified.