Intersection types

[waywardmonkeys]

I found a concrete use case today where I could use intersection types (type-intersection).

Assume that:

• I have a class <file-descriptor> which wraps an integer file handle.
• I have bindings for read, recv, recvmsg, etc. I want them to operate on a readable file descriptor.
• I have bindings for write, send, sendmsg, etc. I want them to operate on a writable file descriptor.
• Most file descriptors will be both readable and writable.
• I model the readable and writable states as mixin classes: define class <readable-mixin> (<object>) and define class <writable-mixin> (<object>).
• These can not inherit from <file-descriptor> as it would create an invalid inheritance hierarchy.
• I don't want read / write and friends to only take the mixin class as an argument, because if I do that, then getting the raw file handle will be a generic dispatch.

It seems like being use an intersection type here would be useful:

define function recv (fd :: type-intersection(<file-descriptor>, <readable-mixin>), ...)
  ...
end;

It is valid to get file descriptors that you know are read-only, so it would be nice to be able to signify that at the type system level. But I also wanted to be able to specify readable / writable for a more complicated reason within the posix-sockets binding which then led to thinking about all of this.