When to opCall instead of opIndex and opSlice?

[Per Nordlöw]

On Monday, 2 October 2017 at 18:14:24 UTC, Jacob Carlborg wrote:
> On 2017-10-02 17:57, Nordlöw wrote:
>> Is implementing opCall(size_t) for structures such as array 
>> containers that already define opIndex and opSlice deprecated?
>> 
>> I can't find any documentation on the subject on when opCall 
>> should be defined to enable foreach (isIterable).
>
> opCall is not related to foreach. It's used to overload the 
> call operator, i.e. ().
>
> struct Foo
> {
>     void opCall() {};
> }
>
> Foo foo;
> foo();
>
> Are you thinking of opApply [1]?
>
> [1] 
> https://dlang.org/spec/statement.html#foreach_over_struct_and_classes

Ahh, yes of course.

It seems like defining opIndex anf opSlice is enough in the array 
container case. Why do we have opApply as well? Non-random access?