Std.Control

Classic monadic effects: Reader (ambient environment) and Writer (ambient append-only log), expressed as algebraic effects.

Effects

Reader

effect Reader r {
  fun ask : Unit -> r
}

Reader effect: an ambient read-only environment of type r. Use ask! to read the environment from anywhere in the call tree; install with run_reader env (fun () -> ...).

Tell

effect Tell w {
  fun tell : w -> Unit
}

Writer effect: an ambient append-only sink of type w. Use tell! to push a value from anywhere in the call tree; install with run_writer (fun () -> ...) to recover the pair (result, log).

Functions

run_reader

fun run_reader : (env: r) -> (f: Unit -> a needs {Reader r}) -> a

Run a Reader-effecting computation against the given environment.

run_writer

fun run_writer : (f: Unit -> a needs {Tell w}) -> (a, List w)

Run a Tell-effecting computation, returning its value paired with everything told, in chronological order.