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import Prelude
import Control.Monad
import Data.List
data Customer = BetsyBeard | CoraCarey | DonnaDrake | EdwinEllis deriving (Eq,Show)
type Size = Integer
type Price = Integer
-- triple pairing
data Tripling = Tripling { customer :: Customer, size :: Size, price :: Price } deriving (Show)
customers = [BetsyBeard, CoraCarey, DonnaDrake, EdwinEllis]
prices = [550, 775, 1500, 2000]
sizes = [55, 60, 65, 70]
-- each solution is 4 triples
type Solution = [Tripling]
answers :: [Solution]
answers = do
solution@[betsy, cora, donna, edwin] <- [zipWith3 Tripling customers szs prs
| szs <- permutations sizes
, prs <- permutations prices]
-- Clue 1
Just twoThousand <- [find (\x -> price x == 2000) solution]
Just fiveFifty <- [find (\x -> price x == 550) solution]
guard (size twoThousand < size fiveFifty)
-- Clue 2
guard (price donna == 550)
-- Clue 3
Just fiftyFive <- [find (\x -> size x == 55) solution]
guard (price fiftyFive == 775 || price fiftyFive == 550)
-- Clue 4
guard (size cora == 70)
-- Clue 5
guard (size edwin + 5 == size betsy)
return solution
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