24 lines
554 B
Haskell
24 lines
554 B
Haskell
{-# 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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, modifyAt
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) where
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import Control.Monad.State (MonadState(..))
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class 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 -> a -> b -> m a
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modifyAt :: (MonadState a m, Box m i a b) => i -> (b -> m b) -> m ()
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modifyAt i f = do
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a <- get
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r i a >>= f >>= w i a >>= put
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instance (Box m i a b, Box m j b c) => Box m (i, j) a c
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