Foreach vs Map in Scala [duplicate]

Question

i tried two ways for making a list out of something

var response = List[RS_TxnNested]
consumertxnlist.foreach(txData => {
  response = RS_TxnNested(blabla) +: response
})

where consumertxnlist is a Seq[something] .

Another way is

var response = consumerTxnList._2.map(txData => RS_TxnNested(blabla))

Can someone help me in clarifying which one is better and why?

Answer 1

As you've noticed already, the map function in the Seq trait returns a value. Its signature in fact is

def map[B](f: (A) ⇒ B): Seq[B]

It is designed to apply a function to every element of a collection extending the Seq trait and return a new collection.

On the other hand, foreach, in the same trait, has the following signature:

def foreach(f: (A) ⇒ Unit): Unit

This means that it's designed to execute functions with side-effects.

In your example, you're treating the operation of assigning the value to "response" as a side effect. If you're actual goal is to return a value (like it seems you want to do), then use map.

Answer 2

The second version is more explicit in what it does.

map conveys a meaning of transforming something, which is exactly what you're doing.

foreach instead performs an operation on something outside its scope, which is not as clear.

In other words, when I read foreach I have to look at the whole scope, because I know you'll be performing a side effect (since foreach returns Unit)

When I read map I can confidently assume the only expression involved is the one I'm currently looking at.

map lowers the burden on the reader, and makes the code easier to reason about.