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DispatchedTuples are like immutable dictionaries (so, they're technically more like NamedTuples) except that the keys are instances of types. Also, because DispatchedTuples are backed by tuples, they are GPU-friendly.

There are two kinds of DispatchedTuples with different behavior:

┌────────────────────────────────────┬───────────────────────────┬────────────────────┐
│                       Return value │           DispatchedTuple │ DispatchedSet      │
│                                    │ (non-unique keys allowed) │ (unique keys only) │
├────────────────────────────────────┼───────────────────────────┼────────────────────┤
│                               Type │                     Tuple │              Value │
│ Unregistered key (without default) │                        () │              error │
│    Unregistered key (with default) │                (default,) │            default │
│                    Duplicative key │     all registered values │          one value │
└────────────────────────────────────┴───────────────────────────┴────────────────────┘

DispatchedTuple and DispatchedSets have three constructors:

1. A variable number of (vararg) Pairs + keyword default
2. A Tuple of Pairs + positional default
3. A Tuple of 2-element Tuples (the first element being the "key", and the second the "value") + positional default

The first field of the Pair (the "key") is an instance of the type you want to dispatch on. The second field of the Pair is the quantity (the "value", which can be anything) returned by dtup[key].

A default value, if passed to DispatchedTuple and DispatchedSet, is returned for any unrecognized keys as shown in the table above.

Here is an example in action

julia> using DispatchedTuples

julia> struct Foo end;

julia> struct Bar end;

julia> struct Baz end;

julia> dtup = DispatchedTuple((
                Pair(Foo(), 1),
                Pair(Foo(), 2),
                Pair(Bar(), 3),
              ))
DispatchedTuple with 3 entries:
  Foo() => 1
  Foo() => 2
  Bar() => 3
  default => ()


julia> dtup[Foo()]
(1, 2)

julia> dtup[Bar()]
(3,)

julia> dtup[Baz()]
()