haskell - 多态数据类型的函数

Question

数据 Foo a 定义如下:

data Foo a where
  Foo :: (Typeable a, Show a) => a -> Foo a
  -- perhaps more constructors

instance Show a => Show (Foo a) where
  show (Foo a) = show a

有一些情况:

fiveFoo :: Foo Int
fiveFoo = Foo 5

falseFoo :: Foo Bool
falseFoo = Foo False

如何从 b -> Foo a 定义任何函数，例如:

getFoo :: (Show a, Typeable a) => String -> Foo a
getFoo "five" = fiveFoo
getFoo "false" = falseFoo

这里 getFoo 不使用 Couldn't match type ‘a’ with ‘Bool’ 进行类型检查。

我在这里唯一感兴趣的是 a 属于 Show 类，因此我可以使用 getFoo ，例如:

main = getLine >>= (print . getFoo)

Answer 1

也许您想省略 Foo 中的类型参数。

data Foo where
  Foo :: (Typeable a, Show a) => a -> Foo

instance Show Foo where
  show (Foo a) = show a

fiveFoo :: Foo
fiveFoo = Foo (5 :: Int) -- (Foo 5) doesn't work because of ambiguity

falseFoo :: Foo
falseFoo = Foo False

getFoo :: String -> Foo
getFoo "five" = fiveFoo
getFoo "false" = falseFoo

print $ getFoo "five" -- prints '5'
print $ getFoo "false" -- prints 'False'