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--------------------------------------------------------------------------------
-- | Various arrow utility functions
module Hakyll.Core.Util.Arrow
( ArrowMap (..)
, constA
, sequenceA
, unitA
) where
--------------------------------------------------------------------------------
import Control.Arrow (Arrow, ArrowChoice, arr, (&&&), (>>^))
--------------------------------------------------------------------------------
-- | Additional arrow typeclass for performance reasons.
class ArrowChoice a => ArrowMap a where
mapA :: a b c -> a [b] [c]
--------------------------------------------------------------------------------
instance ArrowMap (->) where
mapA = map
--------------------------------------------------------------------------------
constA :: Arrow a => c -> a b c
constA = arr . const
--------------------------------------------------------------------------------
sequenceA :: Arrow a => [a b c] -> a b [c]
sequenceA = foldr reduce $ constA []
where
reduce xa la = xa &&& la >>^ arr (uncurry (:))
--------------------------------------------------------------------------------
unitA :: Arrow a => a b ()
unitA = constA ()
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