ReaderT instances for Semialign and Align

Author: Manuel Bärenz

semialign/src/Data/Semialign/Internal.hs

@@ -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: " ++)
@@ -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
 -------------------------------------------------------------------------------

these-tests/test/Tests/Semialign.hs

@@ -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
     ]