英文:
Read either Double or Int
问题
read function requires type annotation. However I want to make it read int as int, double as double when either int or double is provided. Can we do that?
英文:
read function requires type annotation. However I want to make it read int as int , double as double when either int or double is provided. Can we do that?
Thanks.
答案1
得分: 3
你可以使用 readMaybe 首先尝试将其解析为 Int，如果失败，再尝试将其解析为 Double。 (请注意，反过来是行不通的，因为整数也是有效的双精度浮点数。)
readDoubleOrInt :: String -> DoubleOrInt
readDoubleOrInt s = case readMaybe s of
Just i -> R i
Nothing -> L $ read s
顺便提一下，最好根本不使用 read，而仅使用 readMaybe / readsPrec，还要定义适合处理可能失败情况的实例。上述的失败处理版本也可以使用 Alternative 组合子来很好地编写：
readMaybeDoubleOrInt :: String -> Maybe DoubleOrInt
readMaybeDoubleOrInt s = R <$> readMaybe s <|> L <$> readMaybe s
英文:
You can use readMaybe to first attempt parsing it as an Int, if that fails you read it as a Double. (Note the other way around wouldn't work, because integers are also valid doubles.)
readDoubleOrInt :: String -> DoubleOrInt
readDoubleOrInt s = case readMaybe s of
Just i -> R i
Nothing -> L $ read s
BTW it's actually better not to use read at all, but only readMaybe / readsPrec, and also defining your instance suitable to handle possible failures. The failure-handling version of the above can also nicely be written using Alternative combinators:
readMaybeDoubleOrInt :: String -> Maybe DoubleOrInt
readMaybeDoubleOrInt s = R<$>readMaybe s <|> L<$>readMaybe s
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