Partial application of mutating method is not allowed

// Error
[1, 2, 3, 4].forEach(foo.add)

// Works
[1, 2, 3, 4].forEach { foo.add($0) }


  • In the error-case Swift tries to mutate foo directly by reassigning a copy of it with the given value. This is a problem since the closure scope does not hold a copy of foo and is therefore tied to the original foo.

    → The original struct falling out of scope would become an issue for the closure

    → Swift then seems to assume that the closure is escaping even though it is not

  • In the working-case Swift creates a copy of foo for the scope of the closure, mutates that copy and reassigns that copy to foo.

    → Since the closure holds a copy of the original struct, it falling out of scope is not an issue

Kudos to: