{-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleContexts, FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses, UndecidableInstances #-} {- | The HList library (C) 2004, Oleg Kiselyov, Ralf Laemmel, Keean Schupke Variants, i.e., labelled sums. One approach to their implementation would be to consider both the favoured label and the corresponding value as dynamics upon variant construction. Since we are too lazy to programme some Typeable instances for non-ghc systems (NB: in GHC, Typeable is derivable), we rather model variants as (opaque) records with maybies for the values. Only one value will actually hold non-Nothing, as guaranteed by the constructor. See VariantP.hs for a different approach to open sums. -} module Data.HList.Variant where import Data.HList.FakePrelude import Data.HList.Record import Data.HList.HList -- -------------------------------------------------------------------------- -- | Variant types on the basis of label-maybe pairs. newtype Variant mr = Variant mr -- -------------------------------------------------------------------------- -- | Turn proxy sequence into sequence of Nothings data HMaybeF = HMaybeF instance ((Tagged l (Proxy t) ~ a, b ~ Tagged l (Maybe t))) => ApplyAB HMaybeF a b where applyAB _ _ = Tagged Nothing hMaybied x = hMap HMaybeF x -- -------------------------------------------------------------------------- -- | Public constructor -- it seems we can blame 'hUpdateAtLabel' (not 'HMap') for needing the asTypeOf? mkVariant x y (Record v) = let r1 = Record (hMaybied v) in case hUpdateAtLabel x (Just y) r1 `asTypeOf` r1 of Record t -> Variant t -- -------------------------------------------------------------------------- -- | Public destructor unVariant x (Variant v) = hLookupByLabel x (Record v) -- -------------------------------------------------------------------------- -- | Variants are opaque instance Show (Variant v) where show _ = "<Cannot show Variant content!>"