我想编写一个安全版本的 toEnum
:
safeToEnum :: (Enum t, Bounded t) => Int -> Maybe t
一个简单的实现:
safeToEnum :: (Enum t, Bounded t) => Int -> Maybe t
safeToEnum i =
if (i >= fromEnum (minBound :: t)) && (i <= fromEnum (maxBound :: t))
then Just . toEnum $ i
else Nothing
main = do
print $ (safeToEnum 1 :: Maybe Bool)
print $ (safeToEnum 2 :: Maybe Bool)
它不起作用:
safeToEnum.hs:3:21:
Could not deduce (Bounded t1) from the context ()
arising from a use of `minBound' at safeToEnum.hs:3:21-28
Possible fix:
add (Bounded t1) to the context of an expression type signature
In the first argument of `fromEnum', namely `(minBound :: t)'
In the second argument of `(>=)', namely `fromEnum (minBound :: t)'
In the first argument of `(&&)', namely
`(i >= fromEnum (minBound :: t))'
safeToEnum.hs:3:56:
Could not deduce (Bounded t1) from the context ()
arising from a use of `maxBound' at safeToEnum.hs:3:56-63
Possible fix:
add (Bounded t1) to the context of an expression type signature
In the first argument of `fromEnum', namely `(maxBound :: t)'
In the second argument of `(<=)', namely `fromEnum (maxBound :: t)'
In the second argument of `(&&)', namely
`(i <= fromEnum (maxBound :: t))'
据我了解该消息,编译器无法识别 minBound
和 maxBound
应该生成与 safeToEnum 结果类型完全相同的类型
尽管有显式类型声明 (::t
)。知道如何修复它吗?
已解决
camccann 和 Dave 的解决方案都有效(尽管 Dave 的解决方案需要调整)。谢谢你们两位(但我只能接受一位)。使用 ScopedTypeVariables 的工作示例:
{-# LANGUAGE ScopedTypeVariables #-}
safeToEnum :: forall t . (Enum t, Bounded t) => Int -> Maybe t
safeToEnum i =
if (i >= fromEnum (minBound :: t)) && (i <= fromEnum (maxBound :: t))
then Just . toEnum $ i
else Nothing
最佳答案
这里不需要作用域类型变量,您只需要向 GHC 明确表示您希望所有 Enum
内容都是相同的类型。通过将它们全部传递给一个显式采用相同类型的各种 Enum
的函数,可以很容易地做到这一点。这是一种方法:
enumIfBetween :: (Enum a) => a -> a -> Int -> Maybe a
enumIfBetween a z x = let a' = fromEnum a
z' = fromEnum z
in if a' <= x && x <= z'
then Just $ toEnum x
else Nothing
safeToEnum i = enumIfBetween minBound maxBound i
main = do
print $ (safeToEnum 1 :: Maybe Bool)
print $ (safeToEnum 2 :: Maybe Bool)
在 GHCi 中尝试:
> main
Just True
Nothing
使用相同原理的更通用的解决方案是标准库函数 asTypeOf
,它具有与 const
相同的行为,但要求两个参数为相同类型:
safeToEnum :: (Enum t, Bounded t) => Int -> Maybe t
safeToEnum i = let r = toEnum i
max = maxBound `asTypeOf` r
min = minBound `asTypeOf` r
in if i >= fromEnum min && i <= fromEnum max
then Just r
else Nothing
这个版本也有效。
请记住,ScopedTypeVariables
是一种语言扩展,因此不一定可以在编译器之间移植。在实践中,几乎每个人都使用 GHC,但通常首选在可能的情况下坚持使用标准基本语言(即 Haskell 98)。在这种情况下,ScopedTypeVariables 确实有点过分了; Haskell wiki suggests asTypeOf
as a portable replacement对于这种场景。
关于haskell - 安全且多态的 toEnum,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/2743858/