Learn

For-comprehensions are syntax sugar for chained map, flatMap, and filter. They make nested operations on "box" types readable.

Without for-comp:

result1.flatMap { a =>
    result2(a).flatMap { b =>
        result3(a, b).map { c =>
            c + a
        }
    }
}

With for-comp:

for {
    a <- result1
    b <- result2(a)
    c <- result3(a, b)
} yield c + a

Same syntax works for any "box" type that has map/flatMap:

• Option, Try, Either, Future, List, Vector, custom monads

With filter (only for types where withFilter makes sense):

for {
    n <- 1 to 10
    if n % 2 == 0
    sq = n * n           // local binding (not a generator)
} yield sq
// Vector(4, 16, 36, 64, 100)

Multiple generators give Cartesian products — like nested loops:

for {
    x <- 1 to 3
    y <- 1 to 3
    if x != y
} yield (x, y)
// pairs where x != y

Without yield — just runs side effects:

for (line <- io.Source.stdin.getLines()) {
    println(line.toUpperCase)
}

For-comprehensions are how Scala folds nested logic flat. Whenever you see 3+ levels of nested flatMap, switch to a for-comp.