为什么这种构造会在Scala中导致类型不匹配错误?
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)
如果我用列表切换Some,它可以很好地编译:
for (first <- List(1,2,3); second <- Some(1)) yield (first,second)
res41: List[(Int, Int)] = List((1,1), (2,1), (3,1))
这也可以正常工作:
for (first <- Some(1); second <- Some(2)) yield (first,second)