completed the execises on Course.Monad

src/Course/Monad.hs

@@ -6,7 +6,7 @@
 module Course.Monad(
   Monad(..)
 , join
-, (>>=)  
+, (>>=)
 , (<=<)
 ) where
 
@@ -68,8 +68,7 @@ infixr 1 =<<
   f (a -> b)
   -> f a
   -> f b
-(<*>) =
-  error "todo: Course.Monad#(<*>)"
+(<*>) ff fa = (=<<) (<$> fa) ff
 
 infixl 4 <*>
 
@@ -82,8 +81,7 @@ instance Monad Id where
     (a -> Id b)
     -> Id a
     -> Id b
-  (=<<) =
-    error "todo: Course.Monad (=<<)#instance Id"
+  (=<<) f (Id a) = f a
 
 -- | Binds a function on a List.
 --
@@ -94,8 +92,9 @@ instance Monad List where
     (a -> List b)
     -> List a
     -> List b
-  (=<<) =
-    error "todo: Course.Monad (=<<)#instance List"
+  (=<<) _ Nil = Nil
+  (=<<) f (a:.Nil) = f a
+  (=<<) f (a:.as) = f a ++ (=<<) f as
 
 -- | Binds a function on an Optional.
 --
@@ -106,20 +105,18 @@ instance Monad Optional where
     (a -> Optional b)
     -> Optional a
     -> Optional b
-  (=<<) =
-    error "todo: Course.Monad (=<<)#instance Optional"
-
+  (=<<) f (Full a) = f a
+  (=<<) _ Empty = Empty
 -- | Binds a function on the reader ((->) t).
 --
 -- >>> ((*) =<< (+10)) 7
 -- 119
 instance Monad ((->) t) where
   (=<<) ::
-    (a -> ((->) t b))
-    -> ((->) t a)
-    -> ((->) t b)
-  (=<<) =
-    error "todo: Course.Monad (=<<)#instance ((->) t)"
+    (a -> (->) t b)
+    -> (->) t a
+    -> (->) t b
+  (f =<< g) t = f (g t) t
 
 -- | Flattens a combined structure to a single structure.
 --
@@ -138,8 +135,7 @@ join ::
   Monad f =>
   f (f a)
   -> f a
-join =
-  error "todo: Course.Monad#join"
+join  = (=<<) id
 
 -- | Implement a flipped version of @(=<<)@, however, use only
 -- @join@ and @(<$>)@.
@@ -152,8 +148,7 @@ join =
   f a
   -> (a -> f b)
   -> f b
-(>>=) =
-  error "todo: Course.Monad#(>>=)"
+(>>=) fa ff = (=<<) ff fa
 
 infixl 1 >>=
 
@@ -168,8 +163,7 @@ infixl 1 >>=
   -> (a -> f b)
   -> a
   -> f c
-(<=<) =
-  error "todo: Course.Monad#(<=<)"
+(<=<) bfc afb a = (=<<) bfc (afb a)  
 
 infixr 1 <=<