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| {-# OPTIONS_GHC -Wall #-} | |
| {-# LANGUAGE FunctionalDependencies #-} | |
| {-# LANGUAGE TypeFamilies #-} | |
| -- If there were an opportunity to fix the IsList type-class... | |
| import Control.Lens | |
| import Data.List.NonEmpty hiding (fromList) | |
| class FromList l f | l -> f where | |
| type Item l | |
| fromList :: [Item l] -> f l | |
| -- | | |
| -- >>> fromList [1,2,3] :: Identity [Int] | |
| -- Identity [1,2,3] | |
| -- >>> fromList [] :: Identity [Int] | |
| -- Identity [] | |
| instance FromList [a] Identity where | |
| type Item [a] = a | |
| fromList = pure | |
| -- | | |
| -- >>> fromList [] :: Maybe (NonEmpty Int) | |
| -- Nothing | |
| -- >>> fromList [1] :: Maybe (NonEmpty Int) | |
| -- Just (1 :| []) | |
| -- >>> fromList [1,2,3] :: Maybe (NonEmpty Int) | |
| -- Just (1 :| [2,3]) | |
| instance FromList (NonEmpty a) Maybe where | |
| type Item (NonEmpty a) = a | |
| fromList [] = Nothing | |
| fromList (h:t) = Just (h :| t) | |
| data MustBeTwo a = | |
| IsZero | IsOne | IsMoreThanTwo | IsTwo a | |
| deriving (Eq, Ord, Show, Functor) | |
| -- | | |
| -- >>> fromList [] :: MustBeTwo (Int, Int) | |
| -- IsZero | |
| -- >>> fromList [1] :: MustBeTwo (Int, Int) | |
| -- IsOne | |
| -- >>> fromList [1,2] :: MustBeTwo (Int, Int) | |
| -- IsTwo (1,2) | |
| -- >>> fromList [1,2,3] :: MustBeTwo (Int, Int) | |
| -- IsMoreThanTwo | |
| -- >>> fromList [1,2,3,4] :: MustBeTwo (Int, Int) | |
| -- IsMoreThanTwo | |
| instance FromList (a, a) MustBeTwo where | |
| type Item (a, a) = a | |
| fromList [] = IsZero | |
| fromList [_] = IsOne | |
| fromList [a1, a2] = IsTwo (a1, a2) | |
| fromList (_:_:_:_) = IsMoreThanTwo | |
| -- | | |
| -- >>> fromList [] :: Maybe (Maybe Int) | |
| -- Just Nothing | |
| -- >>> fromList [1] :: Maybe (Maybe Int) | |
| -- Just (Just 1) | |
| -- >>> fromList [1,2] :: Maybe (Maybe Int) | |
| -- Nothing | |
| -- >>> fromList [1,2,3] :: Maybe (Maybe Int) | |
| -- Nothing | |
| instance FromList (Maybe a) Maybe where | |
| type Item (Maybe a) = a | |
| fromList [] = Just Nothing | |
| fromList [a] = Just (Just a) | |
| fromList (_:_:_) = Nothing | |
| class FromList l f => IsList l f | l -> f where | |
| isList :: Iso' (f l) [Item l] | |
| fromList' :: IsList l f => [Item l] -> f l | |
| fromList' = review isList | |
| -- | | |
| -- >>> view isList (Identity [1,2,3 :: Int]) | |
| -- [1,2,3] | |
| -- >>> review isList [1,2,3] :: Identity [Int] | |
| -- Identity [1,2,3] | |
| instance IsList [a] Identity where | |
| isList = iso runIdentity fromList | |
| -- | | |
| -- >>> view isList (Nothing :: Maybe (NonEmpty Int)) | |
| -- [] | |
| -- >>> view isList (Just (1 :| [2,3])) | |
| -- [1,2,3] | |
| -- >>> review isList [] :: Maybe (NonEmpty Int) | |
| -- Nothing | |
| -- >>> review isList [1,2,3] :: Maybe (NonEmpty Int) | |
| -- Just (1 :| [2,3]) | |
| instance IsList (NonEmpty a) Maybe where | |
| isList = iso (maybe [] (\(h:|t) -> h:t)) fromList | |
| -- not possible | |
| -- instance IsList (a, a) MustBeTwo where | |
| -- not possible | |
| -- instance IsList (Maybe a) Maybe where |
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