Created
November 13, 2013 17:04
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DeMorgan
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{-# LANGUAGE NoMonomorphismRestriction #-} | |
import Text.ParserCombinators.Parsec | |
import Text.ParserCombinators.Parsec.Token hiding (parens) | |
import Text.ParserCombinators.Parsec.Expr | |
import Control.Applicative hiding ((<|>)) | |
import Control.Monad | |
import Prelude hiding (not) | |
data Expr = Not Expr | And Expr Expr | Or Expr Expr | Var Char | SubExpr Expr deriving Eq | |
not :: Expr -> Expr | |
not (Not e) = e | |
not (And e1 e2) = Or (not e1) (not e2) | |
not (Or e1 e2) = And (not e1) (not e2) | |
not (Var c) = Not (Var c) | |
not (SubExpr e) = not e | |
instance Show Expr where | |
show (Not e) = "NOT " ++ show e | |
show (And e1 e2) = show e1 ++ " AND " ++ show e2 | |
show (Or e1 e2) = show e1 ++ " OR " ++ show e2 | |
show (Var c) = [c] | |
show (SubExpr e) = "(" ++ show e ++ ")" | |
parseExpr :: String -> Either ParseError Expr | |
parseExpr = parse expr "" | |
where expr = buildExpressionParser operators term <?> "compound expression" | |
term = parens expr <|> variable <?> "full expression" | |
operators = [ [Prefix (string "NOT" >> spaces >> return Not)] | |
, [binary "AND" And] | |
, [binary "OR" Or] ] | |
where binary n c = Infix (string n *> spaces *> pure c) AssocLeft | |
variable = Var <$> (letter <* spaces) <?> "variable" | |
parens p = SubExpr <$> (char '(' *> spaces *> p <* char ')' <* spaces) <?> "parens" | |
main = mapM_ printNotExpr . lines =<< readFile "inputs.txt" | |
where printNotExpr e = case parseExpr e of | |
Right x -> print $ not x | |
Left e -> error $ show e |
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The problem pointed out by sandermak could be fixed either by his suggestion or by changing the
Show
instance to defineshowsPrec
instead ofshow
as done in my fork