Why is foreach better than get for Scala Options?

问题:

Why using foreach, map, flatMap etc. are considered better than using get for Scala Options? If I useisEmpty I can call get safely.

回答1:

Well, it kind of comes back to "tell, don't ask". Consider these two lines:

if (opt.isDefined) println(opt.get)
// versus
opt foreach println

In the first case, you are looking inside opt and then reacting depending on what you see. In the second case you are just telling opt what you want done, and let it deal with it.

The first case knows too much about Option, replicates logic internal to it, is fragile and prone to errors (it can result in run-time errors, instead of compile-time errors, if written incorrectly).

Add to that, it is not composable. If you have three options, a single for comprehension takes care of them:

for {
  op1 <- opt1
  op2 <- opt2
  op3 <- opt3
} println(op1+op2+op3)

With if, things start to get messy fast.

回答2:

One nice reason to use foreach is parsing something with nested options. If you have something like

val nestedOption = Some(Some(Some(1)))
for {
  opt1 <- nestedOption
  opt2 <- opt1
  opt3 <- opt2
} println(opt3)

The console prints 1. If you extend this to a case where you have a class that optionally stores a reference to something, which in turn stores another reference, for comprehensions allow you to avoid a giant "pyramid" of None/Some checking.

回答3:

There are already excellent answers to the actual question, but for more Option-foo you should definitely check out Tony Morris' Option Cheat Sheet.

回答4:

The reason it's more useful to apply things like map, foreach, and flatMap directly to the Option instead of using get and then performing the function is that it works on either Some or None and you don't have to do special checks to make sure the value is there.

val x: Option[Int] = foo()
val y = x.map(_+1) // works fine for None
val z = x.get + 1  // doesn't work if x is None

The result for y here is an Option[Int], which is desirable since if x is optional, then y might be undetermined as well. Since get doesn't work on None, you'd have to do a bunch of extra work to make sure you didn't get any errors; extra work that is done for you by map.

回答5:

Put simply:

• If you need to do something (a procedure when you don't need to capture the return value of each invocation) only if the option is defined (i.e. is a Some): use foreach (if you care about the results of each invocation, use map)

• If you need to do something if the option defined and something else if it's not: use isDefined in an if statement

• If you need the value if the option is a Some, or a default value if it is a None: use getOrElse

回答6:

Trying to perform our Operations with get is more imperative style where u need to tel what to do and how to do . In other words , we are dictating things and digging more into the Options internals. Where as map,flatmap are more functional way of doing things where we are say what to do but not how to do.