问题
I want to extract from Iterable[Try[Int]]
a list of all valid values (Iterable[Int]
)
val test = List(
Try(8),
Try(throw new RuntimeException("foo")),
Try(42),
Try(throw new RuntimeException("bar"))
)
The following is the way to print all valid values from test
:
for {
n <- test
p <- n
} println(p)
// Output
// 8
// 42
However, when I tried to save the valid values to list I have received an error:
val nums: Seq[Int] = for {
n <- list
p <- n // Type mismatch. Required: IterableOnce[Int], found Try[Int]
} yield(p)
println(nums)
How to fix the error and why it was raised?
回答1:
Try collect
test.collect { case Success(value) => value }
// res0: List[Int] = List(8, 42)
In a for-comprehension format that corresponds to
for { Success(p) <- test } yield p
Both make use of Constructor Patterns which under the hood perform isInstanceOf
type test followed by asInstanceOf
type cast. Verbosly that corresponds to something like
test
.filter (_.isInstanceOf[Success[Int]])
.map (_.asInstanceOf[Success[Int]].value)
The following for-comprehension does not work because the monads in it have to align
for {
n <- test // List monad
p <- n // does not align with Try monad
} yield (p)
The above for-comprehension desugars to
test.flatMap((n: Try[Int]) => n.map((p: Int) => p))
and looking at the signature of flatMap
we see it expects a function
Try[Int] => IterableOnce[Int]
whilst we provide
Try[Int] => Try[Int]
because n.map((p: Int) => p)
returns Try[Int]
. Now the following for-comprehension is a whole different beast
for {
n <- test
p <- n
} println(p)
because of the absence of yield
it desugars to
test.foreach((n: Try[Int]) => n.foreach((p: Int) => println(p)))
where foreach
expects a function of type
Try[Int] => Unit
which we indeed provide because n.foreach((p: Int) => println(p))
indeed returns Unit
.
回答2:
You can also try:
val nums: Seq[Int] = list.map(_.toOption).flatten
or
val nums: Seq[Int] = list.flatMap(_.toOption)
来源:https://stackoverflow.com/questions/61763561/for-comprehension-yield-raises-type-mismatch-compiler-error