如何实现返回递归匹配类型的递归Scala 3函数？
我无法实现返回递归匹配类型的函数。例如，我从std lib中取了元组附加类型，并尝试实现一个简单的附加函数。
// Tuple append match type, copied from std lib
type Append[X <: Tuple, Y] <: Tuple = X match {
case EmptyTuple => Y *: EmptyTuple
case x *: xs => x *: Append[xs, Y]
}
// Types work just fine
val x: Append[(String, Int), Long] = ("", 1, 2L)
// My simple function implementation that does not compile
def append[X <: Tuple, Y](x: X, y: Y): Append[X, Y] = x match
case _: EmptyTuple => y *: EmptyTuple
case x *: xs => x *: append(xs, y)
[E007] Type Mismatch Error:
case _: EmptyTuple => y *: EmptyTuple
^^^^^^^^^^^^^^^
Found: Y *: EmptyTuple.type
Required: Append[X, Y]
where: X is a type in method append with bounds <: Tuple
Note: a match type could not be fully reduced:
trying to reduce Append[X, Y]
failed since selector X
does not match case EmptyTuple => Y *: EmptyTuple
and cannot be shown to be disjoint from it either.
Therefore, reduction cannot advance to the remaining case
case x *: xs => x *: Append[xs, Y]
longer explanation available when compiling with `-explain`
[E007] Type Mismatch Error:
case x *: xs => x *: append(xs, y)
^^^^^^^^^^^^^^^^^^
Found: Any *: Append[Tuple, Y]
Required: Append[X, Y]
where: X is a type in method append with bounds <: Tuple
Note: a match type could not be fully reduced:
trying to reduce Append[Tuple, Y]
failed since selector Tuple
does not match case EmptyTuple => Y *: EmptyTuple
and cannot be shown to be disjoint from it either.
Therefore, reduction cannot advance to the remaining case
case x *: xs => x *: Append[xs, Y]
longer explanation available when compiling with `-explain`
I fail to implement a function that returns a recursive match type. As an example, I took the tuple Append type from the std lib and tried to implement a simple append function.
// Tuple append match type, copied from std lib
type Append[X <: Tuple, Y] <: Tuple = X match {
case EmptyTuple => Y *: EmptyTuple
case x *: xs => x *: Append[xs, Y]
}
// Types work just fine
val x: Append[(String, Int), Long] = ("", 1, 2L)
// My simple function implementation that does not compile
def append[X <: Tuple, Y](x: X, y: Y): Append[X, Y] = x match
case _: EmptyTuple => y *: EmptyTuple
case x *: xs => x *: append(xs, y)
[E007] Type Mismatch Error:
case _: EmptyTuple => y *: EmptyTuple
^^^^^^^^^^^^^^^
Found: Y *: EmptyTuple.type
Required: Append[X, Y]
where: X is a type in method append with bounds <: Tuple
Note: a match type could not be fully reduced:
trying to reduce Append[X, Y]
failed since selector X
does not match case EmptyTuple => Y *: EmptyTuple
and cannot be shown to be disjoint from it either.
Therefore, reduction cannot advance to the remaining case
case x *: xs => x *: Append[xs, Y]
longer explanation available when compiling with `-explain`
[E007] Type Mismatch Error:
case x *: xs => x *: append(xs, y)
^^^^^^^^^^^^^^^^^^
Found: Any *: Append[Tuple, Y]
Required: Append[X, Y]
where: X is a type in method append with bounds <: Tuple
Note: a match type could not be fully reduced:
trying to reduce Append[Tuple, Y]
failed since selector Tuple
does not match case EmptyTuple => Y *: EmptyTuple
and cannot be shown to be disjoint from it either.
Therefore, reduction cannot advance to the remaining case
case x *: xs => x *: Append[xs, Y]
longer explanation available when compiling with `-explain`
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评论（2）
在返回比赛类型方面，编译器非常挑剔。您的第二种情况需要进行少量修改以使其在没有演员的情况下工作（ scastie ）
：
：（x *：xs），编译器不明白您的函数的第二种情况与匹配类型的第二种情况相对应。也许将来会更聪明。The compiler is very fussy when it comes to returning match types. Your second case needs a small modification to get it to work without the cast (Scastie):
Without the
: (x *: xs), the compiler doesn't understand that your function's second case corresponds with the second case of the match type. Perhaps in the future it'll be smarter.附加函数中的铸件可以做到这一点。我无法内联它，但这暂时就足够了。scastie
A cast in the
appendfunction did the trick. I couldn't manage toinlineit, but this is sufficient for now.Scastie