plp/haskell/haskell5.hs

module EsHs4 where

import Control.Applicative
import Control.Monad

--Logger
type Log = [String]
newtype Logger a = Logger { runLogger :: (a, Log) }

--Define an instance of Show for Logger
instance Show a => Show (Logger a) where
        show (Logger a) = show a

--Define an instance of Functor for logger
class Functor f where
        fmap :: (a -> b) -> f a -> f b

loggerMap :: ( a -> b) -> (Logger a) -> (Logger b)
loggerMap f lg =
        let (v, l) = runLogger lg
            n = f v
        in Logger (n, l)

instance Functor Logger where
        fmap = loggerMap

loggerApp :: Logger (a -> b) -> Logger a -> Logger b
loggerApp lf lg =
        let (f, s) runLogger lf
            nl = loggerMap f lg
            (n, l) = runLogger nl
        in Logger (n, l ++ s)


instance Applicative Logger where
        pure a = Logger (a, [])
        (<*>) = loggerApp

instance Monad Logger where
        m >>= f = let(a, w) = runLogger m
                     n = f a
                     (b, x) = runLogger n
                  in Logger (b, w ++ x)

--Define a function that takes a number, add one and log the op:

logPlusOne :: (Num a) => a -> Logger a
logPlusOne a = Logger (a+1, ["added one"])

logMultiplyTwo :: (Num a) => a -> Logger a
logMultiplyTwo = Logger (a*2, ["Multiplied by two"])

--Define logOps

logOps :: (Num a) => Logger a -> Logger a
logOps lg = do
        v <- lg
        p1 <- logPlusOne v
        m2 <- logMultiplyTwo p1
        return m2

-- Define a record function to record things in the log
record :: String -> Logger()
record s = Logger ((), [s])