for comprehension

If a type has flatMap and map (or it has flatMap and map as its extension methods), you can use for comprehension instead of using nested flatMaps and map.

`flatMap` and `map`

In for comprehension, <-s (left arrows) are flatMap except for the last one. The last <- is map.

for {
  b <- foo(a)
  c <- bar(b)
  d <- baz(c)
} yield a + b + c + d

is equivalent to

foo(a).flatMap { b =>
  bar(b).flatMap { c =>
    baz(c).map { d =>
      a + b + c + d
    }
  }
}

for {
  b <- foo(a)
  c <- bar(b)
} yield a + b + c

is equivalent to

foo(a).flatMap { b =>
  bar(b).map { c =>
    a + b + c
  }
}

for {
  b <- foo(a)
} yield a + b

is equivalent to

foo(a).map { b =>
  a + b
}

So the following one is meaningless

for {
  b <- foo(a)
} yield b

because it's equivalent to

foo(a).map { b =>
  b
}

which does nothing.

So you can simply do this instead.

foo(a)

flatMap and map​

`flatMap` and `map`