haskell - 读取比率时如何检测零分母？

Question

我想从字符串中读取比率，但我不希望我的程序在分母为零时崩溃。如何检测零分母并避免错误？只需使用 readMaybe不起作用:

Prelude Text.Read> readMaybe "1 % 0" :: Maybe Rational
Just *** Exception: Ratio has zero denominator

我创造了这个远非完美的解决方案:

readMaybeRational :: String -> Maybe Rational
readMaybeRational s =
  case ((readMaybe $ drop 1 $ dropWhile (/='%') s) :: Maybe Int)
    of Just 0 -> Nothing
       _ -> readMaybe s

但我不知道如何很好地处理嵌套的比率:

"Just (1 % 0)"

如果我可以覆盖 Ratio 的 Read 实例，我可以让 readMaybe 在分母为零时返回 Nothing:

instance (Integral a, Read a) => Read (Ratio a) where
  readPrec =
    parens
    ( prec ratioPrec
      ( do x <- step readPrec
           expectP (L.Symbol "%")
           y <- step readPrec
           -- is y 0? If so, do something here
           return (x % y)
      )
    )

但我很确定我不能那样做。

Answer 1

我认为您最好的解决方案是 newtype包装 Ratio ， 像这样:

import Control.Monad
import GHC.Read
import GHC.Real
import qualified Text.Read.Lex as L
import Text.ParserCombinators.ReadPrec

newtype SaneReadRatio a = SaneReadRatio (Ratio a)
type SaneReadRational = SaneReadRatio Integer

instance (Integral a, Read a) => Read (SaneReadRatio a) where
  readPrec =
    parens
    ( prec ratioPrec
      ( do x <- step readPrec
           expectP (L.Symbol "%")
           y <- step readPrec
           guard (y /= 0)
           return (SaneReadRatio (x % y))
      )
    )

  readListPrec = readListPrecDefault
  readList     = readListDefault

通过 SaneReadRational 读取数据来使用它代替 Rational ，然后使用 coerce来自 Data.Coerce结果，这将把它改回底层 Rational无论它在你的字体中埋藏的多么深。