Reader Monad.

category-theory/reader-monad.hs

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+import Control.Monad.Instances
+
+-- Monad instance for functions (also called Reader Monad) 
+--instance Monad ((->) r) where  
+--    return x = \_ -> x  
+--    h >>= f = \w -> f (h w) w
+
+-- example using the function monad (in fact it's the Reader Monad itself)
+addStuff :: Int -> Int  
+addStuff = do  
+    a <- (*2)  
+    b <- (+10)  
+    return (a+b)
+
+a1 = addStuff 2
+
+-- redefined using let in
+addStuffB :: Int -> Int  
+addStuffB x = let  
+    a = (*2) x  
+    b = (+10) x  
+    in a+b