alt Hacker NewsA language that revives capabilities, brings them up-to-date, and works in the modern environment is my #1 request from the programming language community right now. I don't need another language with sum types and higher-order functions and a functional focus. I need a language with capabilities. That language may have the other goodies as well, sure, no problem, but we all need capabilities.
I've done some stabby stabs at a design for it, using an AI as the rubber duck. My initial research indicates that the field of "static language that natively supports capabilities" is surprisingly uncovered and there may be a rich field there. E, the closest match, was tied at the hip to Java, which has some advantages but also comes with disadvantages for languages that are trying to do something as exotic as this. Other existing work was on dynamic languages, and hardly rose to the level of "practical for any use" let alone something that could solve our supply chain issues.
My issue is primarily that the reward for successfully designing a language and creating a community around it is that you're in charge of a language community... and, uh, my personality is not suited for that, that sounds more like something I'd pay to avoid then something I'd spend months and years of hard work to attain.
(My advice to anyone doing this is to spend some time with the AI researchers to find the existing work on the topic, not to just sit down and sketch out your initial ideas and run with them. Learn from the past. Expect this to be weeks and probably months of just thinking and noodling before you get to a design. Also, don't try to hook deeply to an existing language, as tempting as it is. This is way too large an impedance mismatch with existing languages. Any external code has to be treated like a nuclear bomb anyhow.)
Replies
What is a capability in terms of programming language design? It sounds more like the sort of thing that would belong at the standard library level, where builtin APIs are guarded by flags.
Deno has something vaguely built in with permissions flags, and old school Blackberry (at least in the J2ME days) had permissions settings for almost everything that an app could do, but again, those are all external to the language design itself.
> My initial research indicates that the field of "static language that natively supports capabilities" is surprisingly uncovered and there may be a rich field there.
You want to look for white papers that talk about object-capability systems. It's a fairly old and well-trod area of research. The E programming language[1] was all about that, and it was pretty late in the game on this stuff.
You emphasize natively, but the problem is that's not really well defined. For static capabilities, you're just essentially asking for a suffciently strong module system with parameterized abstract data types. It's literally a subset of the grammar and what it's designed to express. Mark Miller (one of the creators of E) demonstrated that[2].
The knock on effect of that quality is that anything which fulfills that requirement natively supports capabilities. It's part of the grammar. Doesn't even have to be object-oriented. A hackjob demonstration of an SML filesystem library with a brand/mint object capability pattern:
brand.sig:
signature BRAND =
sig
type token
end
signature MINT =
sig
include BRAND
val mint : unit -> token
end
functor MakeBrand () =
struct
type token = unit ref
fun mint () = ref ()
end
signature FILESYSTEM =
sig
type token
val readFile : token -> string -> string
val writeFile : token -> string -> string -> unit
end
functor FileSystem (B : BRAND) :> FILESYSTEM where type token = B.token =
struct
type token = B.token
fun readFile (_ : token) (path : string) : string =
"contents of " ^ path
fun writeFile (_ : token) (path : string) (_ : string) : unit =
()
end
local
structure FileAuthority :> MINT = MakeBrand ()
in
structure FS :> FILESYSTEM = FileSystem (FileAuthority)
val rootFileToken : FS.token = FileAuthority.mint ()
end
Library
signature FILESYSTEM
structure FS
val rootFileToken
is
brand.sig
mint.sig
makebrand.fun
filesystem.sig
filesystem.fun
trusted_fs_setup.sml
val doc = FS.readFile rootFileToken "/etc/motd" (* Works fine *)
fun helper (t : FS.token) = FS.readFile t "log.txt"
val log = helper rootFileToken
val fake : FS.token = ref () (* Trying to forge a token *)
val t = FileAuthority.mint () (* Trying to bypass the trusted kernel in trusted_fs_setup.sml by calling the mint *)
(* Trying to self-issue authority by making our own brand and mint *)
structure MyCap = MakeBrand ()
val t : FS.token = MyCap.mint () (* type mismatch *)
You can also do a lot of this with phantom types, and it'd be much more terse and easier to handle dynamic capabilities and stuff like a capability algebra, but it ends up way less auditable and is easy to have subtle errors which defeats the point. Also compiler errors will be much more opaque. IMO needing to manually make wrappers for composite capabilities, or to handle dynamic capabilities, is the lesser of two evils. With higher order modules, those problems go away entirely.
[1] - https://en.wikipedia.org/wiki/E_(programming_language)
[2] - https://homepages.ecs.vuw.ac.nz/~kjx/papers/ARND2018.pdf
Been working on something like that for years: https://www.firefly-lang.org/