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ReaderT instances for Semialign and Align #196

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ReaderT instances for Semialign and Align #196

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7 changes: 7 additions & 0 deletions semialign/src/Data/Semialign/Internal.hs
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Expand Up @@ -80,6 +80,7 @@ import Data.Semigroup (Option (..))

import Data.These
import Data.These.Combinators
import Control.Monad.Trans.Reader (ReaderT (..))

oops :: String -> a
oops = error . ("Data.Align: internal error: " ++)
Expand Down Expand Up @@ -454,6 +455,12 @@ instance SemialignWithIndex Int ZipList
instance ZipWithIndex Int ZipList
instance RepeatWithIndex Int ZipList

instance Semialign m => Semialign (ReaderT r m) where
align ma mb = ReaderT $ getCompose $ align (Compose (runReaderT ma)) (Compose (runReaderT mb))

instance Align m => Align (ReaderT r m) where
nil = ReaderT $ Prelude.const nil

-------------------------------------------------------------------------------
-- semigroups
-------------------------------------------------------------------------------
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1 change: 1 addition & 0 deletions these-tests/test/Tests/Semialign.hs
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Expand Up @@ -91,6 +91,7 @@ alignProps = testGroup "Align"
#ifdef MIN_VERSION_lattice
-- note: with e.g. N5 (which isn't distributive lattice) distributivity laws fail!
, semialignLaws (CZip :: CSemialign (Const M2))
, semialignLaws (CAlign :: CSemialign (ReaderT Int []))
#endif
]

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