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Nullable.scala
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Nullable.scala
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/*
* Copyright 2020 Daniel Spiewak
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package parseback
import Nullable.{ False, Maybe, True }
// a Kleene algebra
private[parseback] final class Nullable(val value: Byte) extends AnyVal {
def ||(that: Nullable): Nullable = (this: @unchecked) match {
case True => True
case Maybe => if (that == True) True else Maybe
case False => that
}
def &&(that: Nullable): Nullable = (this: @unchecked) match {
case True => that
case Maybe => if (that == False) False else Maybe
case False => False
}
def toBoolean: Boolean = (this: @unchecked) match {
case True => true
case Maybe => sys.error("not intended to be called")
case False => false
}
override def toString = (this: @unchecked) match {
case True => "True"
case Maybe => "Maybe"
case False => "False"
}
}
private[parseback] object Nullable {
val True = new Nullable(1)
val Maybe = new Nullable(0)
val False = new Nullable(-1)
}