How are Scala Futures chained together with flatMap?

Question

I\'m working on using Futures for the first time in Scala and am working through an example of using the flatMap combinator; I\'ve been following this discussion:

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Answer 1

I'm face the same question... And i found useful this general explanation about for-comprehesion. May be this helps:

For-Comprehensions

A for-comprehension is syntactic sugar for map, flatMap and filter operations on collections.

The general form is for (s) yield e

• s is a sequence of generators and filters
• p <- e is a generator
• if f is a filter
• If there are several generators (equivalent of a nested loop), the last generator varies faster than the first
• You can use { s } instead of ( s ) if you want to use multiple lines without requiring semicolons
• e is an element of the resulting collection

Example 1:

  // list all combinations of numbers x and y where x is drawn from
  // 1 to M and y is drawn from 1 to N
  for (x <- 1 to M; y <- 1 to N)
    yield (x,y)

is equivalent to

(1 to M) flatMap (x => (1 to N) map (y => (x, y)))

Translation Rules

A for-expression looks like a traditional for loop but works differently internally

• for (x <- e1) yield e2 is translated to e1.map(x => e2)
• for (x <- e1 if f) yield e2 is translated to for (x <- e1.filter(x => f)) yield e2
• for (x <- e1; y <- e2) yield e3 is translated to e1.flatMap(x => for (y <- e2) yield e3)

This means you can use a for-comprehension for your own type, as long as you define map, flatMap and filter

Example 2:

for {  
  i <- 1 until n  
  j <- 1 until i  
  if isPrime(i + j)  
} yield (i, j)

is equivalent to

for (i <- 1 until n; j <- 1 until i if isPrime(i + j))
    yield (i, j)

is equivalent to

(1 until n).flatMap(i => (1 until i).filter(j => isPrime(i + j)).map(j => (i, j)))