The Reactive structure
Author: John Reppy
Keywords: Standard ML, SML, SML/NJ Library

The SML of NJ Library
The `Reactive` structure

The `Reactive` structure provides types and operations to build and run reactive systems. The inputs and outputs of a reactive system are sets of signals, which can either be present (i.e., `true`) or absent (i.e., `false`). A reactive system runs in discrete timesteps.

## Synopsis

    signature REACTIVE
    structure Reactive : REACTIVE

## Interface

    type machine
    type instruction
    type signal = Atom.atom
    type config
    type in_signal
    type out_signal

    val machine : {
    	inputs : signal list,
    	outputs : signal list,
    	body : instruction
          } -> machine

    val run : machine -> bool
    val reset : machine -> unit

    val inputsOf : machine -> in_signal list
    val outputsOf : machine -> out_signal list

    val inputSignal : in_signal -> signal
    val outputSignal : out_signal -> signal

    val setInSignal  : (in_signal * bool) -> unit
    val getInSignal  : in_signal -> bool
    val getOutSignal : out_signal -> bool

    val || : (instruction * instruction) -> instruction
    val &  : (instruction * instruction) -> instruction

    val nothing : instruction
    val stop : instruction
    val suspend : instruction

    val action : (machine -> unit) -> instruction
    val exec   : (machine -> {stop : unit -> unit, done : unit -> bool})
    	      -> instruction

    val ifThenElse : ((machine -> bool) * instruction * instruction) -> instruction
    val repeat     : (int * instruction) -> instruction
    val loop       : instruction -> instruction
    val close      : instruction -> instruction

    val signal   : (signal * instruction) -> instruction
    val rebind   : (signal * signal * instruction) -> instruction
    val when     : (config * instruction * instruction) -> instruction
    val trap     : (config * instruction) -> instruction
    val trapWith : (config * instruction * instruction) -> instruction
    val emit     : signal -> instruction
    val await    : config -> instruction

    val posConfig : signal -> config
    val negConfig : signal -> config
    val orConfig  : (config * config) -> config
    val andConfig : (config * config) -> config

## Description

The description of the interface is organized into sections.

### Types

`type machine`

  The type of a reactive system.

`type instruction`

  The abstract representation of a reactive program.

`type signal = Atom.atom`

  The name of a signal.

`type config`

  A signal configuration is a logical combination of signals.

`type in_signal`

  An input signal for a reactive system.

`type out_signal`

  An output signal for a reactive system.

### Machines

`val machine : { …​ } -> machine`

  `machine {inputs, outputs, body}` creates a new reactive system (or machine) from a list of input signal names, a list of output signal names, and a reactive program.

`val run : machine -> bool`

  `run m` will run the reactive system `m` one instant (or activation). It returns `true` if, and only if, the machine ends in a terminal state (_i.e., by executing the `stop` instruction).

`val reset : machine -> unit`

  `reset m` resets the state of `m` to its initial state.

`val inputsOf : machine -> in_signal list`

  `inputsOf m` returns a list of the input signals in the machine.

`val outputsOf : machine -> out_signal list`

  `outputsOf m` returns a list of the output signals in the machine.

`val nameOfInput : in_signal -> signal`

  `inputSignal inSig` returns the name of the input signal.

`val nameOfOutput : out_signal -> signal`

  `inputSignal outSig` returns the name of the output signal.

`val setInSignal : (in_signal * bool) -> unit`

  `setInSignal (inSig, b)` sets the value of the input signal to `b`.

`val getInSignal : in_signal -> bool`

  `getInSignal inSig` gets the current value of the input signal.

`val getOutSignal : out_signal -> bool`

  `getOutSignal inSig` gets the current value of the output signal.

### Instructions

`val || : (instruction * instruction) -> instruction`

  `|| (i1, i2)` forms the parallel composition of the two programs. Activation of the resulting program will interleave the two programs until either one of them suspends (see the `suspend` instruction) or both programs terminate.

`val & : (instruction * instruction) -> instruction`

  `& (i1, i2)` forms the sequential composition of the two programs.

`val nothing : instruction`

  The program that does nothing.

`val stop : instruction`

  The program that stops; i.e., reaches the terminal state for the current and all future activations.

`val suspend : instruction`

  The program that suspends the current activation.

`val action : (machine -> unit) -> instruction`

  something

`val exec : (machine -> {stop : unit -> unit, done : unit -> bool}) -> instruction`

  `exec f` returns a program that encapsulates the SML computation defined by the function `f`.

`val ifThenElse : ((machine -> bool) * instruction * instruction) -> instruction`

  something

`val repeat : (int * instruction) -> instruction`

  something

`val loop : instruction -> instruction`

  something

`val close : instruction -> instruction`

  something

`val signal : (signal * instruction) -> instruction`

  something

`val rebind : (signal * signal * instruction) -> instruction`

  something

`val when : (config * instruction * instruction) -> instruction`

  something

`val trapWith : (config * instruction * instruction) -> instruction`

  `trapWith (cfg, i1, i2)` returns the program that …​

`val trap : (config * instruction) -> instruction`

  `trap (cfg, i)` This expression is equivalent to

      trapWith (cfg, i, nothing)

`val emit : signal -> instruction`

  `emit sigId` returns the program that emits the signal with the given name (i.e., the signal is present).

`val await : config -> instruction`

  `await cfg` returns the program that waits for the configuration to hold.

### Signal configurations

`val posConfig : signal -> config`

  `posConfig sigId` defines a configuration that holds if, and only if, the signal named `sigId` is present.

`val negConfig : signal -> config`

  `negConfig sigId` defines a configuration that holds if, and only if, the signal named `sigId` is not present.

`val orConfig : (config * config) -> config`

  `orConfig (cfg1, cfg2)` defines a configuration that holds if either `cfg1` or `cfg2` (inclusive) holds.

`val andConfig : (config * config) -> config`

  `andConfig (cfg1, cfg2)` defines a configuration that holds if both `cfg1` and `cfg2` hold.

### Deprecated functions

The following functions are part of the interface, but have been deprecated.

`val inputSignal : in_signal → signal`

  use `nameOfInput` instead.

`val outputSignal : out_signal → signal`

  use `nameOfOutput` instead.

## See Also

The Reactive Library

SML/NJ Version 110.99.9 (November 4, 2025)
Last updated 2025-11-04 15:48:25 UTC