Arithmetic.hs

module Model.Arithmetic (modelEval) where

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import           Calculator.Parser.Base             (parseNumber)

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import           Control.Applicative                ((<*))
import           Text.ParserCombinators.Parsec
import           Text.ParserCombinators.Parsec.Expr

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modelEval :: String -> String
modelEval inp =
    case parse (spaces >> expr <* spaces <* eof) "Expression" inp of
      Left _  -> ""
      Right n -> show n

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expr :: Parser Double
expr = buildExpressionParser table factor <?> "Expression"

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table :: [[ Operator Char st Double ]]
table = [ [ op "^" (**) AssocRight ]
        , [ op "*" (*) AssocLeft, op "/" (/) AssocLeft ]
        , [ op "+" (+) AssocLeft, op "-" (-) AssocLeft ]
        ]
  where op s f = Infix (try (spaces >> string s >> spaces) >> return f)

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parseFactor :: Parser Double
parseFactor = do
  _ <- char '('
  x <- expr
  _ <- char ')'
  return x

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factor :: Parser Double
factor = parseFactor <|> parseNumber

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