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import Control.Applicative
import Control.Monad (guard)
import Control.Arrow ((&&&))
import Data.List (unfoldr)
 
g = sum 
          . filter even
          . unfoldr ( id &&& \x ->
                   x`div`2 <$ guard(even x)
               <|> x*3 + 1 <$ guard( x/=1 )
             )
 
main = print $ g 201

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:8:23:
    Couldn't match expected type `Maybe (c0, a0)'
                with actual type `(c1, c'0)'
    Expected type: a0 -> Maybe (c0, a0)
      Actual type: a0 -> (c1, c'0)
    In the first argument of `unfoldr', namely
      `(id
        &&&
          \ x
            -> x `div` 2 <$ guard (even x) <|> x * 3 + 1 <$ guard (x /= 1))'
    In the second argument of `(.)', namely
      `unfoldr
         (id
          &&&
            \ x
              -> x `div` 2 <$ guard (even x) <|> x * 3 + 1 <$ guard (x /= 1))'

Standard output is empty