Add remaining haskell book exercises

Haskell-book/26/MaybeT/.gitignore

@@ -0,0 +1,3 @@
+.stack-work/
+MaybeT.cabal
+*~

Haskell-book/26/MaybeT/Setup.hs

@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain

Haskell-book/26/MaybeT/package.yaml

@@ -0,0 +1,23 @@
+name:                MaybeT
+version:             0.1.0.0
+license:             BSD3
+author:              "Eugen Wissner"
+maintainer:          "belka@caraus.de"
+copyright:           "2018 Eugen Wissner"
+
+dependencies:
+- base >= 4.7 && < 5
+
+library:
+  source-dirs: src
+
+tests:
+  MaybeT-test:
+    main:                Spec.hs
+    source-dirs:         test
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - MaybeT

Haskell-book/26/MaybeT/src/Either.hs

@@ -0,0 +1,56 @@
+module Either where
+
+import Control.Monad (liftM)
+import MonadTrans
+import MonadIO
+
+newtype EitherT e m a =
+    EitherT { runEitherT :: m (Either e a) }
+
+-- 1
+instance Functor m => Functor (EitherT e m) where
+    fmap f (EitherT x) = EitherT $ (fmap . fmap) f x
+
+-- 2
+instance Applicative m => Applicative (EitherT e m) where
+    pure x = EitherT $ pure $ pure x
+
+    (EitherT f) <*> (EitherT a) = EitherT $ (<*>) <$> f <*> a
+
+-- 3
+instance Monad m => Monad (EitherT e m) where
+    return = pure
+
+    (EitherT em) >>= f = EitherT $ do
+        v <- em
+        case v of
+            Left y -> return $ Left y
+            Right y -> runEitherT (f y)
+
+
+-- 4
+-- transformer version of swapEither.
+-- Hint: write swapEither first, then swapEitherT in terms of the former.
+swapEither :: Either e a -> Either a e
+swapEither (Left x) = Right x
+swapEither (Right y) = Left y
+
+swapEitherT :: (Functor m)
+            => EitherT e m a
+            -> EitherT a m e
+swapEitherT (EitherT x) = EitherT $ fmap swapEither x
+
+-- 5. Write the transformer variant of the either catamorphism.
+eitherT :: Monad m
+        => (a -> m c)
+        -> (b -> m c)
+        -> EitherT a m b
+        -> m c
+eitherT f g (EitherT x) = x >>= (either f g)
+
+instance MonadTrans (EitherT e) where
+    lift = EitherT . liftM Right
+
+instance (MonadIO m)
+  => MonadIO (EitherT e m) where
+      liftIO = lift . liftIO

Haskell-book/26/MaybeT/src/Identity.hs

@@ -0,0 +1,43 @@
+module Identity where
+
+import MonadIO
+import MonadTrans
+
+newtype Identity a =
+    Identity { runIdentity :: a }
+    deriving (Eq, Show)
+
+instance Functor Identity where
+    fmap f (Identity a) = Identity (f a)
+
+instance Applicative Identity where
+    pure = Identity
+    (Identity f) <*> (Identity a) = Identity (f a)
+
+newtype IdentityT f a =
+    IdentityT { runIdentityT :: f a }
+    deriving (Eq, Show)
+
+instance (Functor m)
+  => Functor (IdentityT m) where
+      fmap f (IdentityT fa) = IdentityT (fmap f fa)
+
+instance (Applicative m)
+  => Applicative (IdentityT m) where
+      pure x = IdentityT (pure x)
+
+      (IdentityT fab) <*> (IdentityT fa) =
+          IdentityT (fab <*> fa)
+
+instance (Monad m)
+  => Monad (IdentityT m) where
+      return = pure
+
+      (IdentityT ma) >>= f = IdentityT $ ma >>= runIdentityT . f
+
+instance (MonadIO m)
+  => MonadIO (IdentityT m) where
+      liftIO = IdentityT . liftIO
+
+instance MonadTrans IdentityT where
+    lift = IdentityT

Haskell-book/26/MaybeT/src/Maybe.hs

@@ -0,0 +1,40 @@
+module Maybe where
+
+import Control.Monad
+import MonadIO
+import MonadTrans
+
+newtype MaybeT m a =
+    MaybeT { runMaybeT :: m (Maybe a) }
+
+-- compare to the instance for MaybeT
+instance (Functor m)
+      => Functor (MaybeT m) where
+        fmap f (MaybeT ma) =
+            MaybeT $ (fmap . fmap) f ma
+
+instance (Applicative m)
+      => Applicative (MaybeT m) where
+        pure x = MaybeT (pure (pure x))
+
+        (MaybeT fab) <*> (MaybeT mma) = MaybeT $ (<*>) <$> fab <*> mma
+
+instance (Monad m)
+      => Monad (MaybeT m) where
+        return = pure
+
+        -- (>>=) :: MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b
+        (MaybeT ma) >>= f = MaybeT $ do
+            -- ma :: m (Maybe a)
+            -- v :: Maybe a
+            v <- ma
+            case v of
+                Nothing -> return Nothing
+                Just y -> runMaybeT (f y)
+
+instance MonadTrans MaybeT where
+    lift = MaybeT . liftM Just
+
+instance (MonadIO m)
+  => MonadIO (MaybeT m) where
+      liftIO = lift . liftIO

Haskell-book/26/MaybeT/src/MonadIO.hs

@@ -0,0 +1,5 @@
+module MonadIO where
+
+class (Monad m) => MonadIO m where
+    -- | Lift a computation from the 'IO' monad.
+    liftIO :: IO a -> m a

Haskell-book/26/MaybeT/src/MonadTrans.hs

@@ -0,0 +1,7 @@
+module MonadTrans where
+
+class MonadTrans t where
+    -- | Lift a computation from
+    -- the argument monad to
+    -- the constructed monad.
+    lift :: (Monad m) => m a -> t m a

Haskell-book/26/MaybeT/src/Reader.hs

@@ -0,0 +1,35 @@
+module Reader where
+
+import MonadIO
+import MonadTrans
+
+newtype ReaderT r m a =
+    ReaderT { runReaderT :: r -> m a }
+
+instance (Functor m)
+      => Functor (ReaderT r m) where
+    fmap f (ReaderT rma) =
+        ReaderT $ (fmap . fmap) f rma
+
+instance (Applicative m)
+      => Applicative (ReaderT r m) where
+    pure a = ReaderT (pure (pure a))
+
+    (ReaderT fmab) <*> (ReaderT rma) =
+        ReaderT $ (<*>) <$> fmab <*> rma
+
+instance (Monad m)
+      => Monad (ReaderT r m) where
+    return = pure
+
+    (ReaderT rma) >>= f =
+        ReaderT $ \r -> do
+            a <- rma r
+            runReaderT (f a) r
+
+instance MonadTrans (ReaderT r) where
+    lift = ReaderT . const
+
+instance (MonadIO m)
+  => MonadIO (ReaderT r m) where
+      liftIO = lift . liftIO

Haskell-book/26/MaybeT/src/State.hs

@@ -0,0 +1,41 @@
+module State where
+
+import MonadIO
+import MonadTrans
+
+newtype StateT s m a =
+    StateT { runStateT :: s -> m (a, s) }
+
+-- 1
+instance (Functor m)
+      => Functor (StateT s m) where
+    fmap f (StateT m) = StateT $ \s -> fmap first $ m s
+            where first = uncurry (\t1 t2 -> ((f t1), t2))
+            
+-- 2
+-- Links:
+--   http://stackoverflow.com/questions/18673525/is-it-possible-to-implement-applicative-m-applicative-statet-s-m
+--   https://github.com/NICTA/course/issues/134
+instance (Monad m)
+      => Applicative (StateT s m) where
+    pure x = StateT $ (\s -> pure (x, s))
+    StateT g <*> StateT h = StateT $ \s -> keepFirst <$> g s <*> h s
+        where keepFirst (f, s') (x, _) = (f x, s')
+  
+
+-- 3
+instance (Monad m)
+      => Monad (StateT s m) where
+    return = pure
+
+    (StateT sma) >>= f =
+        StateT $ \s -> do
+            a <- sma s
+            runStateT (f $ fst a) s
+
+instance MonadTrans (StateT s) where
+    lift c = StateT $ \s -> c >>= (\x -> return (x, s))
+
+instance (MonadIO m)
+  => MonadIO (StateT s m) where
+      liftIO = lift . liftIO

Haskell-book/26/MaybeT/stack.yaml

@@ -0,0 +1,66 @@
+# This file was automatically generated by 'stack init'
+#
+# Some commonly used options have been documented as comments in this file.
+# For advanced use and comprehensive documentation of the format, please see:
+# https://docs.haskellstack.org/en/stable/yaml_configuration/
+
+# Resolver to choose a 'specific' stackage snapshot or a compiler version.
+# A snapshot resolver dictates the compiler version and the set of packages
+# to be used for project dependencies. For example:
+#
+# resolver: lts-3.5
+# resolver: nightly-2015-09-21
+# resolver: ghc-7.10.2
+# resolver: ghcjs-0.1.0_ghc-7.10.2
+# resolver:
+#  name: custom-snapshot
+#  location: "./custom-snapshot.yaml"
+resolver: lts-11.6
+
+# User packages to be built.
+# Various formats can be used as shown in the example below.
+#
+# packages:
+# - some-directory
+# - https://example.com/foo/bar/baz-0.0.2.tar.gz
+# - location:
+#    git: https://github.com/commercialhaskell/stack.git
+#    commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+# - location: https://github.com/commercialhaskell/stack/commit/e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+#   extra-dep: true
+#  subdirs:
+#  - auto-update
+#  - wai
+#
+# A package marked 'extra-dep: true' will only be built if demanded by a
+# non-dependency (i.e. a user package), and its test suites and benchmarks
+# will not be run. This is useful for tweaking upstream packages.
+packages:
+- .
+# Dependency packages to be pulled from upstream that are not in the resolver
+# (e.g., acme-missiles-0.3)
+# extra-deps: []
+
+# Override default flag values for local packages and extra-deps
+# flags: {}
+
+# Extra package databases containing global packages
+# extra-package-dbs: []
+
+# Control whether we use the GHC we find on the path
+# system-ghc: true
+#
+# Require a specific version of stack, using version ranges
+# require-stack-version: -any # Default
+# require-stack-version: ">=1.6"
+#
+# Override the architecture used by stack, especially useful on Windows
+# arch: i386
+# arch: x86_64
+#
+# Extra directories used by stack for building
+# extra-include-dirs: [/path/to/dir]
+# extra-lib-dirs: [/path/to/dir]
+#
+# Allow a newer minor version of GHC than the snapshot specifies
+# compiler-check: newer-minor

Haskell-book/26/MaybeT/test/Spec.hs

@@ -0,0 +1,2 @@
+main :: IO ()
+main = putStrLn "Test suite not yet implemented"