Type-safe refactoring with ad-hoc polymorphism

Adhoc.hs

module Adhoc where

data ClientIdentifier
  = Admin
  | User String
  deriving (Eq, Show)

data Settings = Settings
  {settingAllowList :: [ClientIdentifier]}
  deriving (Show)

clientIsAllowed :: Settings -> ClientIdentifier -> Bool
clientIsAllowed settings client =
  clientIsInAllowList (settingAllowList settings) client

clientIsInAllowList :: Foldable t => t ClientIdentifier -> ClientIdentifier -> Bool
clientIsInAllowList allowList client =
  foldr checkOne False allowList
 where
  checkOne :: ClientIdentifier -> Bool -> Bool
  checkOne _ True = True
  checkOne otherClient False = client == otherClient