Order of generators in Scala "for comprehension" affects answer -

i'm new scala, , i'm working through scala impatient cay horstmann. going until got comprehension, , came against cryptic passage (section 2.6, advanced loops , comprehensions):

[start quote]

the generated collection compatible first generator.

for (c <- "hello"; <- 0 1) yield (c + i).tochar   // yields "hieflmlmop" (i <- 0 1; c <- "hello") yield (c + i).tochar   // yields vector('h', 'e', 'l', 'l', 'o', 'i', 'f', 'm', 'm', 'p') 

[end quote]

indeed, running in repl, see result of first code snippet has type string, , second code snippet has type scala.collection.immutable.indexedseq[char].

why types differ? think understand second line of code, don't understand why first line doesn't have type scala.collection.immutable.indexedseq[char]. magic happening string rather vector? author mean "compatible first generator"?

both flatmap , map trying build object of same type if possible. first example effectively:

"hello".flatmap { c => (0 1).map { => (c + i).tochar } } 

since calling string#flatmap (stringops#flatmap exact), try build string, , it's possible because internal collection returns collection of chars (try remove tochar , see different).

in second example:

(0 1).flatmap { => "hello".map { c =>  yield (c + i).tochar }} 

it's impossible generate valid range, range#flatmap falls vector.

another interesting example:

map(1 -> 2, 3 -> 4).map(_._1) //> list(1, 3) 

normally map#map try generate map, since don't have pairs it's impossible, falls list

update

you can use trick if want generate different default (e.g. want generate list of chars instead):

for {    _ <- list(none) // list type doesn't matter   c <- "hello"   <- 0 1 } yield (c + i).tochar //> list(h, i, e, f, l, m, l, m, o, p)