Add remaining haskell book exercises

1 files changed, 30 insertions, 0 deletions

diff --git a/Haskell-book/26/Embedded/src/OuterInner.hs b/Haskell-book/26/Embedded/src/OuterInner.hs
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index 0000000..edc31c1
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+++ b/Haskell-book/26/Embedded/src/OuterInner.hs
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+module OuterInner where
+
+import Control.Monad.Trans.Except
+import Control.Monad.Trans.Maybe
+import Control.Monad.Trans.Reader
+
+-- We only need to use return once
+-- because it's one big Monad
+embedded :: MaybeT
+            (ExceptT String
+                    (ReaderT () IO))
+            Int
+--embedded = return 1
+embedded = MaybeT . ExceptT . ReaderT $ return . (const (Right (Just 1)))
+
+-- We can sort of peel away the layers one by one:
+maybeUnwrap :: ExceptT String
+               (ReaderT () IO) (Maybe Int)
+maybeUnwrap = runMaybeT embedded
+
+-- Next
+eitherUnwrap :: ReaderT () IO
+                (Either String (Maybe Int))
+eitherUnwrap = runExceptT maybeUnwrap
+
+-- Lastly
+readerUnwrap :: ()
+             -> IO (Either String
+                           (Maybe Int))
+readerUnwrap = runReaderT eitherUnwrap
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