1_problem.hs

-- In a hypothetical scenario where:
-- * I have packs of beverages
-- * Each pack belongs to a Brand (Coke or Pepsi) and contains a Product
-- * Each product belongs to a Brand (Coke or Pepsi)
-- * A Coke pack filled with Coke cans is correct
-- * A Coke pack filled with Pepsi cans is incorrect
-- 
-- Is there a way that I can make the type system check that for me during compilation time?
-- I know I could have smart constructors that would build the correct pack with the correct content but I'd like to know if there are other and smarter -- ways of achieving it.

module Beverages where
data Brand = Coke | Pepsi deriving (Show)
data Product brand = Bottle brand | Can brand deriving (Show)
data Pack brand product = Pack brand product deriving (Show)

-- valid
coke_bottle_pack = Pack Coke (Bottle Coke)
coke_can_pack = Pack Coke (Can Coke)
pepsi_bottle_pack = Pack Pepsi (Bottle Pepsi)
pepsi_can_pack = Pack Pepsi (Can Pepsi)

-- must be invalid - compilation should fail but it does not
coke_pack_with_pepsi_bottles = Pack Coke (Bottle Pepsi)
coke_pack_with_pepsi_cans = Pack Coke (Can Pepsi)
pepsi_pack_with_coke_bottles = Pack Pepsi (Bottle Coke)
pepsi_pack_with_coke_cans = Pack Pepsi (Can Coke)

2_user_submitted.hs

{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}

-- The person who wrote it does not recommended to to this

data Brand = Coke | Pepsi
data SBrand brand where
  SCoke :: SBrand 'Coke
  SPepsi :: SBrand 'Pepsi

data Product = Bottle | Can
data SProduct brand product where
  SBottle :: SBrand brand -> SProduct brand 'Bottle
  SCan :: SBrand brand -> SProduct brand 'Can

data Pack brand product = Pack (SBrand brand) (SProduct brand product)

-- valid
coke_bottle_pack = Pack SCoke (SBottle SCoke)
coke_can_pack = Pack SCoke (SCan SCoke)
pepsi_bottle_pack = Pack SPepsi (SBottle SPepsi)
pepsi_can_pack = Pack SPepsi (SCan SPepsi)

-- invalid: compilation fails
coke_pack_with_pepsi_bottles = Pack SCoke (SBottle SPepsi)
coke_pack_with_pepsi_cans = Pack SCoke (SCan SPepsi)
pepsi_pack_with_coke_bottles = Pack SPepsi (SBottle SCoke)
pepsi_pack_with_coke_cans = Pack SPepsi (SCan SCoke)

3_gadts.hs

{-# LANGUAGE GADTs #-}

module Beverages where

data Coke
data Pepsi

data Brand a where
  Coke :: Brand Coke
  Pepsi :: Brand Pepsi

data Product brand = Can brand | Bottle brand deriving Show

data Pack brand product where
  Pack :: a -> Product a -> Pack a (Product a)

-- valid
coke_bottle_pack = Pack Coke (Bottle Coke)
coke_can_pack = Pack Coke (Can Coke)
pepsi_bottle_pack = Pack Pepsi (Bottle Pepsi)
pepsi_can_pack = Pack Pepsi (Can Pepsi)

-- invalid: compilation fails
coke_pack_with_pepsi_bottles = Pack Coke (Bottle Pepsi)
coke_pack_with_pepsi_cans = Pack Coke (Can Pepsi)
pepsi_pack_with_coke_bottles = Pack Pepsi (Bottle Coke)
pepsi_pack_with_coke_cans = Pack Pepsi (Can Coke)

4.simple_adt.hs

module Beverages where

data Coke = Coke deriving (Show)
data Pepsi = Pepsi deriving (Show)

data Product brand = Bottle brand | Can brand deriving (Show)
data Pack brand = Pack brand (Product brand) deriving (Show)

-- valid
coke_bottle_pack = Pack Coke (Bottle Coke)
coke_can_pack = Pack Coke (Can Coke)
pepsi_bottle_pack = Pack Pepsi (Bottle Pepsi)
pepsi_can_pack = Pack Pepsi (Can Pepsi)

-- invalid: compilation fails
coke_pack_with_pepsi_bottles = Pack Coke (Bottle Pepsi)
coke_pack_with_pepsi_cans = Pack Coke (Can Pepsi)
pepsi_pack_with_coke_bottles = Pack Pepsi (Bottle Coke)
pepsi_pack_with_coke_cans = Pack Pepsi (Can Coke)