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{-# LANGUAGE NamedFieldPuns #-}
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{-# LANGUAGE FunctionalDependencies #-}
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{-# LANGUAGE FlexibleInstances #-}
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{-# LANGUAGE FlexibleContexts #-}
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{-# LANGUAGE UndecidableInstances #-}
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module PDF.Box (
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Box(..)
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, I(..)
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, at
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2020-02-26 17:14:43 +01:00
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, atAll
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, edit
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2020-02-23 22:24:59 +01:00
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) where
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import Control.Monad.Fail (MonadFail(..))
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import Control.Monad.State (MonadState(..))
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import Data.Map (Map)
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import qualified Data.Map as Map (insert, lookup, member)
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import Prelude hiding (fail)
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newtype I = I Int
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class Monad m => Box m i a b | m a i -> b where
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r :: i -> a -> m b
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w :: i -> b -> a -> m a
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at :: Box m i a b => i -> (b -> m b) -> a -> m a
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at i f a = r i a >>= f >>= flip (w i) a
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atAll :: (Traversable t, Monad m, Box m i a (t b)) => i -> (b -> m b) -> a -> m a
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atAll i f = at i $ (mapM f)
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edit :: MonadState a m => (a -> m a) -> m ()
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edit f = get >>= f >>= put
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instance MonadFail m => Box m I [a] a where
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r (I i) = getAt i
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where
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getAt _ [] = fail $ "Index out of bounds " ++ show i
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getAt 0 (x:_) = return x
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getAt k (_:xs) = getAt (k+1) xs
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w (I i) newX = setAt i
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where
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setAt _ [] = fail $ "Index out of bounds " ++ show i
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setAt 0 (_:xs) = return (newX:xs)
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setAt k (x:xs) = (x:)<$>(setAt (k-1) xs)
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instance (Ord k, MonadFail m) => Box m k (Map k a) a where
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r k = maybe (fail "Unknown key") return . Map.lookup k
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w k a aMap
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| Map.member k aMap =
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return $ Map.insert k a aMap
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| otherwise = fail "Unknown key"
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