snip/snip.hs


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import Control.Monad.Writer 
import Control.Monad.State

import Data.Tuple.Extra (dupe,first,second)
import System.Environment (getArgs)
import Control.Parallel.Strategies (parMap,rseq,rdeepseq)

main = do
        mainMonTra
        mainParMap

-- monad transformers in action
mainMonTra = print $ 
       runState (
               runWriterT  (
                   get >>= \a -> 
                   tell ["foo"] >> 
                   put (a*a) >> 
                   tell ["bar"] >> 
                   tell [show a]
                   )
               ) 5

-- parallel Map arguments: sparks maps
mainParMap = do 
          (sparks:max:_) <- fmap (map read) getArgs
          let ranges = uncurry zip . second tail . dupe $ [0,(max`div`sparks)..max]
          let sums   = parMap rdeepseq (\(a,b)->sum [a+1..b]) ranges
          print $ show (sum sums::Integer)