我正在尝试实现类型安全的二项式堆。为此,我有两种数据类型,它们的元素类型由它们的类型编码索引参数化:
data Zero
data Succ a = Succ
{-| A GADT representation of lists with type-encoded length.
Unlike the most common implementation, the type paramater
for values is of kind * -> *. Each element type is
parametrized by its index (counting from the end of the vector),
so the type-encoded numbers decrease from start to end.
-}
data Vec n a where
Nil :: Vec Zero a
Cons :: a n -> Vec n a -> Vec (Succ n) a
{-| A GADT representation of lists whose values are of kind
* -> *. Each element type is parametrized by its by its
index (counting from the start of the vector), so the
type-encoded numbers increase from start to end.
Unlike Vec the type-encode number here doesn't represent
the length, it can be arbitrary (just increasing).
-}
data RVec n a where
RNil :: RVec n a
RCons :: a n -> RVec (Succ n) a -> RVec n a
Vec
使用递减的数字参数对值进行编码,其中最后一个元素始终由 参数化Zero
。RVec
使用增加的数字参数对值进行编码,没有其他限制(这就是为什么RNil
可以产生RVec
任何数字的原因)。
这允许我(例如)拥有一个高度增加/减少的树木列表,由类型系统检查。在实现了我的大部分项目之后,我意识到我需要一个看似简单的函数,但我无法实现:
vreverse :: Vec n a -> RVec Zero a
它应该简单地颠倒给定列表的顺序。有任何想法吗?提前致谢。