Created
June 6, 2012 18:08
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Very simple phantom types example
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| trait FlightStatus | |
| trait Flying extends FlightStatus | |
| trait Landed extends FlightStatus | |
| case class Plane[Status <: FlightStatus]() | |
| def land(p:Plane[Flying])=Plane[Landed]() | |
| def takeOff(p:Plane[Landed])= Plane[Flying]() | |
| val plane = new Plane[Landed]() | |
| /* | |
| scala> val flying=takeOff(plane) | |
| flying: Plane[Flying] = Plane() | |
| scala> val landed=land(flying) | |
| landed: Plane[Landed] = Plane() | |
| scala> takeOff(flying) | |
| <console>:15: error: type mismatch; | |
| found : Plane[Flying] | |
| required: Plane[Landed] | |
| takeOff(flying) | |
| ^ | |
| scala> land(landed) | |
| <console>:17: error: type mismatch; | |
| found : Plane[Landed] | |
| required: Plane[Flying] | |
| land(landed) | |
| ^ | |
| */ |
I still don't see the phantoms. Oh, maybe they are invisible...
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Credit to
http://blog.rafaelferreira.net/2008/07/type-safe-builder-pattern-in-scala.html
and
http://james-iry.blogspot.com/2010/10/phantom-types-in-haskell-and-scala.html