Internal overview


To use OCurrent effectively, it can be useful to understand how it works internally. This page describes how OCurrent is implemented. There are three main features provided by a Current.t value:

Let’s look at these individually…

Incremental evaluation

The automatic updating is built on the current_incr library (in lib_incr in the source). This library is very small and has no dependencies, and can be useful even outside of OCurrent.

Let’s try it out. You can get an interactive environment with:

opam depext -i current utop
# #require "current";;
# open Current_incr;;

# let x = Current_incr.var 3;;
# let y = (fun x -> x * 2) (Current_incr.of_var x);;
# (Printf.printf "y is now %d\n") y;;
y is now 6

This is the incremental equivalent of:

# let x' = 3;;
# let y' = x' * 2;;
# Printf.printf "y' is now %d\n" y';;
y' is now 6

However, we can change x and recompute everything that depends on it:

# Current_incr.change x 21;;
# Current_incr.propagate ();;
y is now 42

If you run propagate again, it will not display y is now 42 again, as the value hasn’t changed.

For more information about this library, see the Current_incr API docs.

Errors and pending results

The lib_term library depends on and wraps current_incr. It adds static analysis (which we’ll look at in the next section) and error handling. Normally it is wrapped in turn by the main OCurrent library, but we can use it directly like this:

# #require "current.term";;
# module Term = Current_term.Make(Unit);;

The functor argument just provides a type for job metadata, which we don’t care about for this example. Here are some examples showing how errors are handled:

# let test x =
   |> (fun x -> x * 2)
   |> Current_incr.observe;;
# test (Term.return 21);;
- : int Current_term__.Output.t = Ok 42

# test ( "Crashed");;
- : int Current_term__.Output.t = Error (`Msg "Crashed")

# test ( `Running);;
- : int Current_term__.Output.t = Error (`Active `Running)

This is a pretty standard result monad, except that we have an extra “error” type to represent a calculation that couldn’t produce a result yet, but will in future.

You can use Term.catch to turn a failed value back into a usable result:

# "Crashed"
       |> Term.catch
       |> (function
          | Ok _ -> "good"
          | Error _ -> "bad")
       |> Current_incr.observe;;
- : string Current_term__.Output.t = Ok "bad"

This pipeline catches the error at the start and successfully returns “bad”.

Term.state is similar, but also gives access to active values. For example, when testing a PR on GitHub your pipeline may want to handle the “active” state to set the GitHub commit status to “pending”.

To make any real use of Term, you’ll need to provide some “primitive” operations that do something interesting. For example:

# module Term = Current_term.Make(Unit);;
# open Term.Syntax;;

# let approved = Current_incr.var false;;
# let await_approval x =
  Term.component "approve" |>
  let> x = x in
  Current_incr.of_cc begin (Current_incr.of_var approved) @@ function
    | true -> Current_incr.write (Ok x, None)
    | false -> Current_incr.write (Error (`Active (`Ready)), None)

await_approval x is a term that is pending until approved is true, then returns x. Term.component provides a label for the static analysis, let> x = x gets the current value of term x (and records the static dependency on x), and the body is an incremental value with the result. You can use it like this:

# let result = Term.return ~label:"build result" "image1" |> await_approval;;
# result |> Current_incr.observe;;
- : string Current_term__.Output.t = Error (`Active `Ready)
# Current_incr.change approved true;;
- : unit = ()
# Current_incr.propagate ();;
- : unit = ()
# result |> Current_incr.observe;;
- : string Current_term__.Output.t = Ok "image1"

Static analysis

As well as an incremental result value, as described above, a Term.t also includes a “static” component. Term.t is defined like this:

type 'a t = {
  id : Id.t;
  bind : bind_context;
  ty : metadata_ty;
  v : 'a Dyn.t Current_incr.t;

v is the dynamic (incremental) part. The rest is “static” information about the shape of the pipeline, used to generate the diagrams. Note however that the “static” part can contain incremental components too in some cases. For example, a bind operation generates new bits of pipeline at runtime depending on its dynamic input.

The combinators in Term build up new terms with the correct static and dynamic parts. For example, lib_term/ defines the function like this:

let map f x =
  let id = () in
  node ~id (Map (Term x)) @@ ( ~id f) x.v

map operations aren’t very interesting (they only appear on diagrams if they fail), but primitive operations and some other combinators are shown. We can render the result pipeline above like this:

utop # Format.printf "@[<h>%a@]@." Term.Analysis.pp result;;
build result >>= approve

You can also use pp_dot instead of pp, to generate a graphviz dot file.


The above sections describe the internal libraries that are used by the main OCurrent library (in lib). This extends Current_term with primitives that use Lwt to run jobs asynchronously, a database to persist results, support for collecting log files, etc.

Instead of using our Term module above, you use Current, and instead of writing low-level primitives like await_approval directly, you can use the pre-built Current.Monitor, Current.Var and Current_cache. And instead of running manually, will run an Lwt thread that calls propagate in a loop, triggered by Current.Engine.update.

Then, various other libraries provide extensions, such as a web interface, Cap’n Proto RPC, GitHub support, etc. These are in their own packages to keep the number of dependencies of the core current package small.