#### react

## Primitive and basics

`type 'a t = 'a event`

The type for events with occurrences of type `'a`

.

`val never : 'a event`

A never occuring event. For all t, [`never`

]_{t} `= None`

.

`create ()`

is a primitive event `e`

and a `send`

function. The function `send`

is such that:

`send v`

generates an occurrence`v`

of`e`

at the time it is called and triggers an update step.`send ~step v`

generates an occurence`v`

of`e`

on the step`step`

when`step`

is executed.`send ~step v`

raises`Invalid_argument`

if it was previously called with a step and this step has not executed yet or if the given`step`

was already executed.

**Warning.** `send`

must not be executed inside an update step.

`val retain : 'a event -> ( unit -> unit ) -> [ `R of unit -> unit ]`

`retain e c`

keeps a reference to the closure `c`

in `e`

and returns the previously retained value. `c`

will *never* be invoked.

**Raises.** `Invalid_argument`

on `E.never`

.

`val stop : ?strong:bool -> 'a event -> unit`

`stop e`

stops `e`

from occuring. It conceptually becomes `never`

and cannot be restarted. Allows to disable effectful events.

The `strong`

argument should only be used on platforms where weak arrays have a strong semantics (i.e. JavaScript). See details.

**Note.** If executed in an update step the event may still occur in the step.

`equal e e'`

is `true`

iff `e`

and `e'`

are equal. If both events are different from `never`

, physical equality is used.

`trace iff tr e`

is `e`

except `tr`

is invoked with e's occurence when `iff`

is `true`

(defaults to `S.const true`

). For all t where [`e`

]_{t} `= Some v`

and [`iff`

]_{t} = `true`

, `tr`

is invoked with `v`

.

## Transforming and filtering

`once e`

is `e`

with only its next occurence.

- [
`once e`

]_{t}`= Some v`

if [`e`

]_{t}`= Some v`

and [`e`

]_{<t}`= None`

. - [
`once e`

]_{t}`= None`

otherwise.

`drop_once e`

is `e`

without its next occurrence.

- [
`drop_once e`

]_{t}`= Some v`

if [`e`

]_{t}`= Some v`

and [`e`

]_{<t}`= Some _`

. - [
`drop_once e`

]_{t}`= None`

otherwise.

`app ef e`

occurs when both `ef`

and `e`

occur simultaneously. The value is `ef`

's occurence applied to `e`

's one.

- [
`app ef e`

]_{t}`= Some v'`

if [`ef`

]_{t}`= Some f`

and [`e`

]_{t}`= Some v`

and`f v = v'`

. - [
`app ef e`

]_{t}`= None`

otherwise.

`map f e`

applies `f`

to `e`

's occurrences.

- [
`map f e`

]_{t}`= Some (f v)`

if [`e`

]_{t}`= Some v`

. - [
`map f e`

]_{t}`= None`

otherwise.

`filter p e`

are `e`

's occurrences that satisfy `p`

.

- [
`filter p e`

]_{t}`= Some v`

if [`e`

]_{t}`= Some v`

and`p v = true`

- [
`filter p e`

]_{t}`= None`

otherwise.

`fmap fm e`

are `e`

's occurrences filtered and mapped by `fm`

.

- [
`fmap fm e`

]_{t}`= Some v`

if`fm`

[`e`

]_{t}`= Some v`

- [
`fmap fm e`

]_{t}`= None`

otherwise.

`diff f e`

occurs whenever `e`

occurs except on the next occurence. Occurences are `f v v'`

where `v`

is `e`

's current occurrence and `v'`

the previous one.

- [
`diff f e`

]_{t}`= Some r`

if [`e`

]_{t}`= Some v`

, [`e`

]_{<t}`= Some v'`

and`f v v' = r`

. - [
`diff f e`

]_{t}`= None`

otherwise.

`changes eq e`

is `e`

's occurrences with occurences equal to the previous one dropped. Equality is tested with `eq`

(defaults to structural equality).

- [
`changes eq e`

]_{t}`= Some v`

if [`e`

]_{t}`= Some v`

and either [`e`

]_{<t}`= None`

or [`e`

]_{<t}`= Some v'`

and`eq v v' = false`

. - [
`changes eq e`

]_{t}`= None`

otherwise.

`on c e`

is the occurrences of `e`

when `c`

is `true`

.

- [
`on c e`

]_{t}`= Some v`

if [`c`

]_{t}`= true`

and [`e`

]_{t}`= Some v`

. - [
`on c e`

]_{t}`= None`

otherwise.

`dismiss c e`

is the occurences of `e`

except the ones when `c`

occurs.

- [
`dimiss c e`

]_{t}`= Some v`

if [`c`

]_{t}`= None`

and [`e`

]_{t}`= Some v`

. - [
`dimiss c e`

]_{t}`= None`

otherwise.

`until c e`

is `e`

's occurences until `c`

occurs.

- [
`until c e`

]_{t}`= Some v`

if [`e`

]_{t}`= Some v`

and [`c`

]_{<=t}`= None`

- [
`until c e`

]_{t}`= None`

otherwise.

## Accumulating

`accum ef i`

accumulates a value, starting with `i`

, using `e`

's functional occurrences.

- [
`accum ef i`

]_{t}`= Some (f i)`

if [`ef`

]_{t}`= Some f`

and [`ef`

]_{<t}`= None`

. - [
`accum ef i`

]_{t}`= Some (f acc)`

if [`ef`

]_{t}`= Some f`

and [`accum ef i`

]_{<t}`= Some acc`

. - [
`accum ef i`

]`= None`

otherwise.

`fold f i e`

accumulates `e`

's occurrences with `f`

starting with `i`

.

- [
`fold f i e`

]_{t}`= Some (f i v)`

if [`e`

]_{t}`= Some v`

and [`e`

]_{<t}`= None`

. - [
`fold f i e`

]_{t}`= Some (f acc v)`

if [`e`

]_{t}`= Some v`

and [`fold f i e`

]_{<t}`= Some acc`

. - [
`fold f i e`

]_{t}`= None`

otherwise.

## Combining

`select el`

is the occurrences of every event in `el`

. If more than one event occurs simultaneously the leftmost is taken and the others are lost.

- [
`select el`

]_{t}`=`

[`List.find (fun e ->`

[`e`

]_{t}`<> None) el`

]_{t}. - [
`select el`

]_{t}`= None`

otherwise.

`merge f a el`

merges the simultaneous occurrences of every event in `el`

using `f`

and the accumulator `a`

.

[`merge f a el`

]_{t} ```
= List.fold_left f a (List.filter (fun o -> o <> None)
(List.map
```

[]_{t}` el))`

.

`switch e ee`

is `e`

's occurrences until there is an occurrence `e'`

on `ee`

, the occurrences of `e'`

are then used until there is a new occurrence on `ee`

, etc..

- [
`switch e ee`

]_{t}`=`

[`e`

]_{t}if [`ee`

]_{<=t}`= None`

. - [
`switch e ee`

]_{t}`=`

[`e'`

]_{t}if [`ee`

]_{<=t}`= Some e'`

.

`fix ef`

allows to refer to the value an event had an infinitesimal amount of time before.

In `fix ef`

, `ef`

is called with an event `e`

that represents the event returned by `ef`

delayed by an infinitesimal amount of time. If `e', r = ef e`

then `r`

is returned by `fix`

and `e`

is such that :

- [
`e`

]_{t}`=`

`None`

if t = 0 - [
`e`

]_{t}`=`

[`e'`

]_{t-dt}otherwise

**Raises.** `Invalid_argument`

if `e'`

is directly a delayed event (i.e. an event given to a fixing function).

## Lifting

Lifting combinators. For a given `n`

the semantics is:

- [
`ln f e1 ... en`

]_{t}`= Some (f v1 ... vn)`

if for all i : [`ei`

]_{t}`= Some vi`

. - [
`ln f e1 ... en`

]_{t}`= None`

otherwise.

## Pervasives support

`module Option : sig ... end`

Events with option occurences.