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Ternary/Universum.hs

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+module Ternary.Universum (Universum(..), universum, aFromStatements) where
+import           Data.List     (nub)
+import           Ternary.Term  (Item (..), Term (..), Vee)
+
+data Universum = Aristotle
+               | Empty
+               | Default
+
+universum :: (Eq a) => Universum -> [Vee a] -> [Vee a]
+universum Aristotle facts =
+    [Term True (Item [Term x v])
+         | v <- aFromStatements facts,
+           x <- [False, True]]
+universum Empty _ = []
+universum Default xs = xs
+
+aFromStatements :: (Eq a) => [Vee a] -> [a]
+aFromStatements = nub . concatMap (extract . getItem . getVee)
+  where
+    getItem  (Item x) = x
+    getVee (Term _ i) = i
+    extract terms = [v | (Term _ v) <- terms]