Phantom type - Haskell and Cryptocurrencies

Phantom type

Overview

A phantom type is a parametrised type whose parameters do not all appear on the right-hand side of its definition

code:phantom.hs

newtype Const a b = Const { getConst :: a }

Here Const is a phantom type, because the b parameter doesn't appear after the = sign.

The motivation behind using phantom types is to specialize the return type of data constructors.

Phantom type can be used for Smart constructors or GADTs that encodes information about how and where a value can be used

Example code

code:phantom.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

{-# LANGUAGE KindSignatures #-}

{-# LANGUAGE DataKinds #-}

data LengthUnit = Kilometer | Mile

newtype Distance (a :: LengthUnit) = Distance Double

deriving (Num, Show)

marathonDistance :: Distance 'Kilometer

marathonDistance = Distance 42.195

distanceKmToMiles :: Distance 'Kilometer -> Distance 'Mile

distanceKmToMiles (Distance km) = Distance (0.621371 * km)

marathonDistanceInMiles :: Distance 'Mile

marathonDistanceInMiles = distanceKmToMiles marathonDistance

Using GADT

code:gadtphantom.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

{-# LANGUAGE KindSignatures #-}

{-# LANGUAGE DataKinds #-}

{-# LANGUAGE GADTs #-}

{-# LANGUAGE StandaloneDeriving #-}

data Kilometer

data Mile

data Distance a where

KilometerDistance :: Double -> Distance Kilometer

MileDistance :: Double -> Distance Mile

deriving instance Show (Distance a)

marathonDistance :: Distance Kilometer

marathonDistance = KilometerDistance 42.195

distanceKmToMiles :: Distance Kilometer -> Distance Mile

distanceKmToMiles (KilometerDistance km) = MileDistance (0.621371 * km)

marathonDistanceInMiles :: Distance Mile

marathonDistanceInMiles = distanceKmToMiles marathonDistance

https://gyazo.com/174a87cb331cd69d5adf874500764dbb

Useful links

https://www.youtube.com/watch?v=cyOvlaweUtw