类型不匹配在斯卡拉理解 [英] Type Mismatch on Scala For Comprehension
问题描述
< console>:6:error:type mismatch;
found:List [(Int,Int)]
required:Option [?]
for(first< - Some(1); second< - List(1,2,3 ))yield(first,second)
如果我用List切换Some, (第一个< - 列表(1,2,3);第二个< - 一些(1))yield(第一个,第二个)
res41:List [(Int,Int)] = List((1,1),(2,1),(3,1))
这也可以正常工作:
for(first< ;一些(1);第二个< - 一些(2))收益(第一,第二)
理解转换为对 map
或 flatMap
方法的调用。例如这个:
$ b $ pre $ for(x < - List(1); y < - List(1,2, 3))yield(x,y)
变成:
List(1).flatMap(x => List(1,2,3).map(y =>(x,y)))
因此,第一个循环值(在本例中, List(1) code>)将会收到
flatMap
方法调用。在 List > > List > List
上返回 flatMap
,结果的理解当然是 List
。 (这对我来说是新的:理解并不总是导致流,甚至不一定在 Seq
s。)
现在,看看如何在 Option
中声明 flatMap
:
def flatMap [B](f:(A)⇒选项[B]):选项[B]
记住这一点。让我们来看看错误的理解( Some(1)
)被转换为一系列map调用:
(1).flatMap(x => List(1,2,3).map(y =>(x,y)))
现在很容易看到 flatMap
调用的参数是根据需要返回 List
,但不是 Option
的东西。
为了解决这个问题,您可以执行以下操作:
for(x < - (1).toSeq; y< - List(1,2,3))yield(x,y)
编译得很好。值得注意的是, Option
不是 Seq
的子类型,正如通常所假设的那样。 $ b
Why does this construction cause a Type Mismatch error in Scala?
for (first <- Some(1); second <- List(1,2,3)) yield (first,second)
<console>:6: error: type mismatch;
found : List[(Int, Int)]
required: Option[?]
for (first <- Some(1); second <- List(1,2,3)) yield (first,second)
If I switch the Some with the List it compiles fine:
for (first <- List(1,2,3); second <- Some(1)) yield (first,second)
res41: List[(Int, Int)] = List((1,1), (2,1), (3,1))
This also works fine:
for (first <- Some(1); second <- Some(2)) yield (first,second)
For comprehensions are converted into calls to the map
or flatMap
method. For example this one:
for(x <- List(1) ; y <- List(1,2,3)) yield (x,y)
becomes that:
List(1).flatMap(x => List(1,2,3).map(y => (x,y)))
Therefore, the first loop value (in this case, List(1)
) will receive the flatMap
method call. Since flatMap
on a List
returns another List
, the result of the for comprehension will of course be a List
. (This was new to me: For comprehensions don't always result in streams, not even necessarily in Seq
s.)
Now, take a look at how flatMap
is declared in Option
:
def flatMap [B] (f: (A) ⇒ Option[B]) : Option[B]
Keep this in mind. Let's see how the erroneous for comprehension (the one with Some(1)
) gets converted to a sequence of map calls:
Some(1).flatMap(x => List(1,2,3).map(y => (x, y)))
Now, it's easy to see that the parameter of the flatMap
call is something that returns a List
, but not an Option
, as required.
In order to fix the thing, you can do the following:
for(x <- Some(1).toSeq ; y <- List(1,2,3)) yield (x, y)
That compiles just fine. It is worth noting that Option
is not a subtype of Seq
, as is often assumed.
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