相关疑难解决方法(0)

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

有些情况:

fiveFoo :: Foo Int
fiveFoo = Foo 5

falseFoo :: Foo Bool
falseFoo = Foo False

我如何定义任何函数b -> Foo a,例如:

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

这里 …