切换循环发电机指令时的返回类型不同

Question

I have issue with these lines from the book "Scala for the Impatient", which I confirmed myself:

for (c <- "Hello"; i <- 0 to 1) yield (c + i).toChar
 // Yields "HIeflmlmop"
for (i <- 0 to 1; c <- "Hello") yield (c + i).toChar
 // Yields Vector('H', 'e', 'l', 'l', 'o', 'I', 'f', 'm', 'm', 'p')

The first yields a String and the second yields a Vector ? Yet I would expect them to return the same value.

Answer 1

In general the firs collection in the for comprehension determines the return type of yield .

Because:

In scala a for comprehension is syntactic sugar for Collection .map , .flatMap and so on. So the collection you start with is the collection you end up with.

The equivalent with map and flatMap for your code would be:

"Hello".flatMap(c => (0 to 1).map(i => (c + i).toChar))

or

(0 to 1).flatMap(i => "Hello".map(c => (c + i).toChar))