无法理解Semigroupal.product
和Semigroupal.tuple2
之间的实际差异。这是一个简短的例子:
import cats.Semigroupal
import cats.data.Validated
import cats.data.Validated.Invalid
import cats.instances.list._ // for Monoid
type AllErrorsOr[A] = Validated[List[String], A]
def bothInvalid = {
Semigroupal[AllErrorsOr].product(
Validated.invalid(List("Error 1")),
Validated.invalid(List("Error 2"))
)
}
def bothInvalidTuple = {
Semigroupal.tuple2(
Validated.invalid(List("Error 1")),
Validated.invalid(List("Error 2"))
)
}
def bothValid = {
Semigroupal[AllErrorsOr].product(
Validated.valid(10),
Validated.valid(20)
)
}
def bothValidTuple = {
Semigroupal.tuple2(
Validated.valid(10),
Validated.valid(20)
)
}
对于残疾人,bothInvalid
和bothInvalidTuple
都给出相同的结果。使用有效值,仅编译第一个值。我得到的错误:
错误:(40,23)找不到参数半群的隐含值:cats.Semigroupal [[+ A] cats.data.Validated [Nothing,A]] Semigroupal.tuple2(
似乎(如果我没错)Scala试图找到Monoid
来组合Nothing
,但不是List[String]
。如何使用tuple2
?
只是推断了一些仿制药。尝试明确指定它们
type AllErrorsOr[A] = Validated[List[String], A]
def bothInvalid: AllErrorsOr[(Int, Int)] = {
Semigroupal[AllErrorsOr].product[Int, Int](
Validated.invalid(List("Error 1")),
Validated.invalid(List("Error 2"))
)
}
def bothInvalidTuple: AllErrorsOr[(Int, Int)] = {
Semigroupal.tuple2[AllErrorsOr, Int, Int](
Validated.invalid(List("Error 1")),
Validated.invalid(List("Error 2"))
)
}
def bothValid: AllErrorsOr[(Int, Int)] = {
Semigroupal[AllErrorsOr].product[Int, Int](
Validated.valid(10),
Validated.valid(20)
)
}
def bothValidTuple: AllErrorsOr[(Int, Int)] = {
Semigroupal.tuple2[AllErrorsOr, Int, Int](
Validated.valid(10),
Validated.valid(20)
)
}