An uninhabited type.
This is useful when interfaces require that a type be specified, but the implementer knows this type will not be used in their implementation of the interface.
Async.Std.Rpc.Pipe_rpc.t is parameterized by an error type, but a user
may want to define a Pipe RPC that can't fail.
[@@deriving enumerate] may seem strange due to the fact that generated
val all : t list is the empty list, so it seems like it could be of no use.
This may be true if you always expect your type to be
enumerate] can be useful if you have a type which you expect to change over time.
For example, you may have a program which has to interact with multiple servers which
are possibly at different versions. It may be useful in this program to have a
variant type which enumerates the ways in which the servers may differ. When all the
servers are at the same version, you can change this type to
Nothing.t and code
which uses an enumeration of the type will continue to work correctly.
This is a similar issue to the identifiability of
Nothing.t. As discussed below,
another case where
[@deriving enumerate] could be useful is when this type is part
of some larger type.
Because there are no values of type
Nothing.t, a piece of code that has a value of
Nothing.t must be unreachable. In such an unreachable piece of code, one can
unreachable_code to give the code whatever type one needs. For example:
let f (r : (int, Nothing.t) Result.t) : int = match r with | Ok i -> i | Error n -> Nothing.unreachable_code n ;;