swift - Result of call to [myFunction] is unused

Question

In Obj-C, a common practice was to use convenience functions to perform common operations, like configuring auto layout for views:

func makeConstraint(withAnotherView : UIView) -> NSLayoutConstraint
{
   // Make some constraint 
   // ...

   // Return the created constraint
   return NSLayoutConstraint()
}

If you just needed to set the constraint and forget about it, you could call:

[view1 makeConstraint: view2]

If you wanted to store the constraint later so that you could remove/modify it, you would do something like:

NSLayoutConstraint * c;
c = [view1 makeConstraint: view2]

I want to do this in swift, but if I call the above function and do not capture the returned constraint, I get the warning:

Result of call to 'makeConstraint(withAnotherView:)' is unused

VERY annoying. Is there some way to let Swift know that I don't always want to capture the return value?

NOTE: I know about this. It is ugly and not what I'm looking for:

_ = view1.makeConstraint(withAnotherView: view2)

Answer 1

This is behaviour that has been introduced in Swift 3. Instead of having to explicitly annotate functions with @warn_unused_result in order to tell the compiler that the result should be used by the caller, this is now the default behaviour.

You can use the @discardableResult attribute on your function in order to inform the compiler that the return value doesn't have to be 'consumed' by the caller.

@discardableResult
func makeConstraint(withAnotherView : UIView) -> NSLayoutConstraint {

   ... // do things that have side effects

   return NSLayoutConstraint()
}

view1.makeConstraint(view2) // No warning

let constraint = view1.makeConstraint(view2) // Works as expected

You can read about this change in more detail on the evolution proposal.