Array Argument of Service Functions

How would you set the key field of an array argument in a service function? When there's an array field, you can create an array, but are limited to the indexes of the array.

Passing Hello, there, Foo, Bar would result in an array of:
[0] => "Hello";
[1] => "there";
[2] => "Foo";
[3] => "Bar";

But what if you want:
["Hello"] => "there";
["Foo"] => "Bar";