flatMap and for-comprehensions
flatMap changes each item into a small list and then flattens all of them into one, like unpacking small boxes into one big tray. It is the key to chaining steps that each produce results.
In this page:
Syntax
val result = collection.flatMap(x => other_collection(x))
val same = for {
x <- collection
y <- other_collection(x)
} yield yflatMap and for-comprehensions
flatMap(f) maps each element to a new collection (or Option) and then flattens all those results into a single collection, avoiding the nested collections a plain map would leave behind. This is essential when a transformation itself produces zero, one, or many results per input element.
A for comprehension with yield is syntactic sugar that the compiler translates into a chain of map/flatMap/filter calls, which is why both approaches solve the same class of problems.
Note:
A for comprehension with multiple <- generators is exactly equivalent to nested flatMap/map calls -- the compiler desugars it that way.
Example: flatMap and for-comprehensions
object Main extends App {
val words = List("hello", "world")
val letters = words.flatMap(w => w.toList)
println(letters)
val pairs = for {
x <- List(1, 2)
y <- List("a", "b")
} yield (x, y)
println(pairs)
}Login to try C/C++/Java/PHP code in the editor
Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
Common Mistakes
- Using
mapwhen the function returns a collection, ending up with a nestedList[List[Char]]instead of the flat result fromflatMap. - Passing a function to
flatMapthat returns a single value rather than a collection orOption, which gives a type error. - Mixing collection types in a
forcomprehension, such as aListand anOption, and getting a confusing type error.
Chapter Summary
- Higher-order functions take or return functions, including
map,filter, andreduce. - Anonymous functions, closures, currying, and partial application create flexible functions.
- Function composition and
flatMapwith for-comprehensions combine operations.
🔒
Chapter Quiz — Complete all 8 topics to unlock
0/8 topics done
Complete these topics first: