Functional Programming

Using Maybe Monad for checks

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{-# LANGUAGE LambdaCase, RecordWildCards #-}

import Data.Maybe (isJust, mapMaybe)

import Data.List (nub)

data Group = X25519 | Secp384

           deriving (Eq)

data KeyShareData = KeyShareClientHello [Group]

                  | KeyShareHelloRetryRequest Group

data Ext = SupportedGroups [Group]

         | KeyShares KeyShareData

check :: [Ext] -> [Ext] -> Bool

check eCH eHRR = isJust $ do

  sgCH <- exactlyOne $ mapMaybe (\case SupportedGroups gs -> Just gs; _ -> Nothing) eCH

  continueIf $ nub sgCH == sgCH -- no duplicates allowed

  ksCH <- (\case KeyShareClientHello gs -> Just gs; _ -> Nothing) =<<

    exactlyOne (mapMaybe (\case KeyShares ks -> Just ks; _ -> Nothing) eCH)

  ksHRR <- (\case KeyShareHelloRetryRequest g -> Just g; _ -> Nothing) =<<

    exactlyOne (mapMaybe (\case KeyShares ks -> Just ks; _ -> Nothing) eHRR)

  continueIf $ nub ksCH == ksCH -- no duplicates allowed

  continueIf $ ksHRR `elem` sgCH -- group must be supported

  continueIf $ ksHRR `notElem` ksCH -- key share must not have been already sent

exactlyOne :: [a] -> Maybe a

exactlyOne [x] = Just x

exactlyOne _ = Nothing

continueIf :: Bool -> Maybe ()

continueIf True = Just ()

continueIf False = Nothing

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