parseExpr.hs

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 *> pure 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"