mithril-vndb/docs/prop.md

# prop()

- [API](#api)
- [Static members](#static-members)
	- [prop.combine](#prop-combine)
	- [prop.reject](#prop-reject)
	- [prop.merge](#prop-merge)
	- [prop.HALT](#prop-halt)
- [Instance members](#static-members)
	- [stream.run](#stream-run)
	- [stream.end](#stream-end)
	- [stream.error](#stream-error)
	- [stream.catch](#stream-catch)
	- [stream.of](#stream-of)
	- [stream.map](#stream-map)
	- [stream.ap](#stream-ap)
- [Basic usage](#basic-usage)
	- [Streams as variables](#streams-as-variables)
	- [Bidirectional bindings](#bidirectional-bindings)
	- [Computed properties](#computed-properties)
	- [Loading icons and error messages](#loading-icons-and-error-messages)
- [Streams vs promises](#streams-vs-promises)
- [Chaining streams](#chaining-streams)
- [Combining streams](#combining-streams)
- [Absorbing streams](#absorbing-streams)
- [Stream states](#stream-states)
- [Handling errors](#handling-errors)
- [Serializing streams](#serializing-streams)
- [Streams do not trigger rendering](#streams-do-not-trigger-rendering)

---

### API

Creates a stream

`stream = m.prop(value)`

Argument    | Type                 | Required | Description
----------- | -------------------- | -------- | ---
`value`     | `any`                | No       | If this argument is present, the value of the prop is set to it
**returns** | `Stream`             |          | Returns a stream

[How to read signatures](signatures.md)

#### Static members

##### prop.combine

Creates a computed stream that reactively updates if any of its upstreams are updated. See [combining streams](#combining-streams)

`stream = prop.combine(combiner, streams)`

Argument    | Type                        | Required | Description
----------- | --------------------------- | -------- | ---
`combiner`  | `(Stream..., Array) -> any` | Yes      | See [combiner](#combiner) argument
`streams`   | `Array<Stream>`             | Yes      | A list of streams to be combined
**returns** | `Stream`                    |          | Returns a stream

[How to read signatures](signatures.md)

###### combiner

Specifies how the value of a computed stream is generated. See [combining streams](#combining-streams)

`any = combiner(streams..., changed)`

Argument     | Type                 | Required | Description
------------ | -------------------- | -------- | ---
`streams...` | splat of `Stream`s   | No       | Splat of zero or more streams that correspond to the streams passed as the second argument to [`prop.combine`](#prop-combine.md)
`changed`    | `Array<Stream>`      | Yes      | List of streams that were affected by an update
**returns**  | `any`                |          | Returns a computed value

[How to read signatures](signatures.md)

##### prop.reject

Creates a stream in a error state. See [stream states](#stream-states)

`stream = m.prop.reject(value)`

Argument     | Type                 | Required | Description
------------ | -------------------- | -------- | ---
`value`      | `any`                | Yes      | The error value
**returns**  | `Stream`             |          | Returns a stream in an error state

[How to read signatures](signatures.md)

##### prop.merge

Creates a stream whose value is the array of values from an array of streams

`stream = m.prop.merge(streams)`

Argument     | Type                 | Required | Description
------------ | -------------------- | -------- | ---
`streams`    | `Array<Stream>`      | Yes      | A list of streams
**returns**  | `Stream`             |          | Returns a stream whose value is an array of input stream values 

[How to read signatures](signatures.md)

##### prop.HALT

A special value that can be returned to stream callbacks to halt execution of downstreams

#### Instance members

##### stream.run

Creates a dependent stream whose value is set to the result of the callback function. See [chaining streams](#chaining-streams)

If the callback returns a stream, it is absorbed, and `dependentStream` adopts its value and state.

`dependentStream = m.prop().run(callback)`

Argument     | Type                 | Required | Description
------------ | -------------------- | -------- | ---
`callback`   | `any -> any`         | Yes      | A callback whose return value becomes the value of the stream
**returns**  | `Stream`             |          | Returns a stream

[How to read signatures](signatures.md)

##### stream.end

A co-dependent stream that unregisters dependent streams when set to true. See [ended state](#ended-state).

`endStream = m.prop().end`

##### stream.error

A co-dependent stream that is set if the stream is in an errored state. See [handling errors](#handling-errors).

`errorStream = m.prop().error`

##### stream.catch

Returns an active stream whose value is equal to the return value of `catch`'s callback.

`stream = m.prop().catch(callback)`

Argument     | Type                 | Required | Description
------------ | -------------------- | -------- | ---
`callback`   | `any -> any`         | Yes      | A callback whose return value becomes the value of the stream returned by `catch`
**returns**  | `Stream`             |          | Returns a stream

[How to read signatures](signatures.md)

##### stream.of

This method is functionally identical to `m.prop`. It exists to conform to [Fantasy Land's Applicative specification](https://github.com/fantasyland/fantasy-land)

`stream = m.prop().of(value)`

Argument    | Type                 | Required | Description
----------- | -------------------- | -------- | ---
`value`     | `any`                | No       | If this argument is present, the value of the prop is set to it
**returns** | `Stream`             |          | Returns a stream

##### stream.map

Creates a dependent stream whose value is set to the result of the callback function. See [chaining streams](#chaining-streams)

This method is almost functionally identical to [`stream.run()`](#stream-run), except that if the return value is a stream, the stream is not absorbed.

This method exists to conform to [Fantasy Land's Applicative specification](https://github.com/fantasyland/fantasy-land)

`dependentStream = m.prop().map(callback)`

Argument     | Type                 | Required | Description
------------ | -------------------- | -------- | ---
`callback`   | `any -> any`         | Yes      | A callback whose return value becomes the value of the stream
**returns**  | `Stream`             |          | Returns a stream

[How to read signatures](signatures.md)

##### stream.ap

The name of this method stands for `apply`. If a stream has a function as its value, calling `ap` will call the function with the value of the input stream as its argument, and it will return another stream whose value is the result of the function call. This method exists to conform to [Fantasy Land's Applicative specification](https://github.com/fantasyland/fantasy-land)

`errorStream = m.prop().ap(value)`

Argument    | Type                 | Required | Description
----------- | -------------------- | -------- | ---
`value`     | `Stream`             | Yes      | If this argument is present, the value of the prop is set to it
**returns** | `Stream`             |          | Returns a stream

---

### Basic usage

#### Streams as variables

`m.prop()` returns a stream. At its most basic level, a stream works similar to a variable or a getter-setter property: it can hold state, which can be modified.

```javascript
var username = m.prop("John")
console.log(username()) // logs "John"

username("John Doe")
console.log(username()) // logs "John Doe"
```

The main difference is that a stream is a function, and therefore can be composed into higher order functions.

```javascript
var users = m.prop()

// request users from a server using the fetch API
fetch("/api/users")
	.then(function(response) {return response.json()})
	.then(users)
```

In the example above, the `users` stream is populated with the response data when the request resolves.

#### Bidirectional bindings

Streams can also be populated from other higher order functions, such as [`m.withAttr`](withAttr.md)

```javascript
// a stream
var user = m.prop("")

// a bi-directional binding to the stream
m("input", {
	oninput: m.withAttr("value", user),
	value: user()
})
```

In the example above, when the user types in the input, the `user` stream is updated to the value of the input field.

#### Computed properties

Streams are useful for implementing computed properties:

```javascript
var title = m.prop("")
var slug = title.run(function(value) {
	return value.toLowerCase().replace(/\W/g, "-")
})

title("Hello world")
console.log(slug()) // logs "hello-world"
```

In the example above, the value of `slug` is computed when `title` is updated, not when `slug` is read.

It's of course also possible to compute properties based on multiple streams:

```javascript
var firstName = m.prop("John")
var lastName = m.prop("Doe")
var fullName = m.prop.combine(function(first, last) {
	return first() + " " + last()
}, [firstName, lastName])

firstName("Mary")

console.log(fullName()) // logs "Mary Doe"
```

Computed properties in Mithril are updated atomically: the callback to `combine` will never be called more than once per value update, no matter how complex the computed property's dependency graph is.

#### Loading icons and error messages

Here's an example using [`m.request`](request.md) that uses streams to implement a loading indicator and an error message for an AJAX call:

```javascript
var RobustExample = {
	oninit: function(vnode) {
		var req = m.request({
			method: "GET",
			url: "/api/items",
		})
		vnode.state.items = req.catch(function() {
			return []
		})
		vnode.state.error = req.error.run(this.errorView)
	},
	view: function(vnode) {
		return [
			vnode.state.items() ? vnode.state.items().map(function(item) {
				return m("div", item.name)
			}) : m(".loading-icon"),
			vnode.state.error(),
		]
	},
	errorView: function(e) {
		return e ? m(".error", "An error occurred") : null
	}
}

m.route(document.body, "/", {
	"/": RobustExample
})
```

When this component is initialized, `m.request` is called and its return value is assigned to `req`. Before the request completes, the `req` stream remains in a pending state, and therefore has a value of `undefined`. `req.error` is the error stream for the request. Since `req` is pending, the `req.error` stream also remain in a pending state, and likewise, `vnode.state.error` stays pending and does not call `this.errorView`.

Then the component renders. Both `vnode.state.items()` and `vnode.state.error()` return `undefined`, so the component returns `[m(".loading-icon"), undefined]`, which in turn creates a loading icon element in the DOM.

When the request to the server completes, `req` is populated with the response data, which is propagated to the `vnode.state.items` dependent stream. (Note that the function in `catch` is not called if there's no error). After the request completes, the component is re-rendered. `req.error` is set to an active state (but it still has a value of `undefined`), and `vnode.state.error()` is then set to `null`. The `view` function returns a list of vnodes containing item names, and therefore the loading icon is replaced by a list of `div` elements are created in the DOM.

If the request to the server fails, `catch` is called and `vnode.state.items()` is set to an empty array. Also, `req.error` is populated with the error, and `vnode.state.error` is populated with the vnode tree returned by `errorView`. Therefore, `view` returns `[[], m(".error", "An error occurred")]`, which replaces the loading icon with the error message in the DOM.

To clear the error message, simply set the value of the `vnode.state.error` stream to `undefined`.

---

### Streams vs promises

Mithril streams have some similarities to [ES6 promises](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise):

- streams can be [chained](#chaining-streams) (analogous to `promise.then(callback)`)
- streams can [absorb other streams](#absorbing-streams) (analogous to `promise.then(function() {return Promise.resolve(1)})`)
- streams have [composable error handling semantics](#handling-errors) (analogous to `promise.catch`)

These semantic similarities are designed to make it easy to migrate from promise-based asynchronous code to stream-based code.

For example, here's some sample promise-based code:

```javascript
fetch("/api/users", {method: "GET"}).then(function(response) {return response.json()})
	.then(function(users) {
		if (users.length === 0) return Promise.reject("No users found")
	})
	.catch(function(e) {
		console.log(e)
	})
```

And here's equivalent stream-based code:

```javascript
m.request({url: "/api/users", method: "GET"})
	.run(function(users) {
		if (users.length === 0) return m.prop.reject("No users found")
	})
	.catch(function(e) {
		console.log(e)
	})
```

Aside from the syntax differences between the [`fetch API`](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) and [`m.request()`](request.md) in the first line of each snippet above, the only other syntax difference is that streams use the method `.run()` to chain, instead of `.then()`.

#### Differences

In most use cases, streams can be used as replacements for promises without much effort, by simply renaming `.then()` to `.run()`.

One major difference that can affect a migration is that `.run()` only accepts one argument (whereas `.then()` accepts an error handler as the second argument. Misplacement of error handlers is a common source of bugs in promise-based code, and it's generally recommended that error handlers be attached using `.catch()` rather than passed as a second argument to `.then()`. To avoid those issues, error handlers in streams can only be defined using the `.catch()` method.

Another more obscure functional difference is that if a promise is passed as an argument to `Promise.resolve()` and `Promise.reject()`, the promise is absorbed, whereas absorption does not occur in their stream counterparts `m.prop()` and `m.prop.reject()`.

There are also a few important differences in semantics between promises and streams. A promise can only ever resolve to one value. Streams, on the other hand, are *reactive*: a stream's value can be changed freely, and it automatically updates the values of other streams that depend on it.

Promises are required by spec to resolve asynchronously, even if the resolution value is known in advance (e.g. `Promise.resolve("hello")`). Mithril streams are guaranteed to update synchronously and atomically.

Mithril streams are also more oriented towards functional programming. In addition to being usable for composing higher order functions, the stream API comply with [Fantasy Land's Applicative specification](https://github.com/fantasyland/fantasy-land), which enables interoperability with functional libraries like Ramda and Sanctuary.

#### Interoperability with promises

An increasing number of third party APIs return promises, and it's often desirable to transfer their resolved values to Mithril streams. This can be accomplished by simply chaining the stream itself to the promise chain:

```javascript
var promise = Promise.resolve(123)
var stream = m.prop()

// set the stream to listen to the promise resolution event
promise.then(stream)
```

To track promise rejections as well as resolutions, pass the error stream as a rejection callback:

```javascript
promise.then(stream, stream.error)
```

To use a stream value to resolve a promise, simply pass the stream value to it:

```javascript
var stream = m.prop("hello")
var promise = Promise.resolve(stream())
// promise resolves to "hello"
```

---

### Chaining streams

Streams can be chained using the `run` method. A chained stream is also known as a *dependent stream*.

```javascript
// parent stream
var stream = m.prop(1)

// dependent stream
var doubled = stream.run(function(value) {
	return value * 2
})

console.log(doubled()) // logs 2
```

Dependent streams are *reactive*: their values are updated any time the value of their parent stream is updated. This happens regardless of whether the dependent stream was created before or after the value of the parent stream was set.

You can prevent dependent streams from being updated by returning the special value `m.prop.HALT`

```javascript
var halted = m.prop(1).run(function(value) {
	return m.prop.HALT
})

halted.run(function() {
	// never runs
})
```

---

### Combining streams

Streams can depend on more than one parent stream. These kinds of streams can be created via `m.prop.combine()`

```javascript
var a = m.prop(5)
var b = m.prop(7)

var added = m.prop.combine(function(a, b) {
	return a() + b()
}, [a, b])

console.log(added()) // logs 12
```

A stream can depend on any number of streams and it's guaranteed to update atomically. For example, if a stream A has two dependent streams B and C, and a fourth stream D is dependent on both B and C, the stream D will only update once if the value of A changes. This guarantees that the callback for stream D is never called with unstable values such as when B has a new value but C has the old value. Atomicity also bring the performance benefits of not recomputing downstreams unnecessarily.

You can prevent dependent streams from being updated by returning the special value `m.prop.HALT`

```javascript
var halted = m.prop.combine(function(stream) {
	return m.prop.HALT
}, [m.prop(1)])

halted.run(function() {
	// never runs
})
```

---

### Absorbing streams

Similar to promises, stream can absorb other streams. Returning a stream from the callback to `.run()` or `.catch()` will cause the wrapper stream to *absorb* the inner stream and adopt its value and [state](#stream-states):

```javascript
var stream = m.prop()
var mapped = m.prop(1).run(function(value) {
	return stream(value * 2)
})

console.log(mapped()) // logs 2

stream(4)

console.log(mapped()) // logs 4
```

```javascript
var mapped = m.prop.reject(new Error("error")).catch(function(e) {
	return m.prop(2)
})

console.log(mapped()) // logs 2
```

Stream absorption does not occur in fantasy-land methods (i.e. `.map()`, `.ap()`, `.of()`)

---

### Stream states

At any given time, a stream can be in one of four states: *pending*, *active*, *errored* and *ended*.

#### Pending state

Pending streams can be created by calling `m.prop()` with no parameters.

```javascript
var pending = m.prop()
```

If a stream is dependent on more than one stream, any of its parent streams is in a pending state, the dependent streams is also in a pending state, and does not update its value.

```javascript
var a = m.prop(5)
var b = m.prop() // pending stream

var added = m.prop.combine(function(a, b) {
	return a() + b()
}, [a, b])

console.log(added()) // logs undefined
```

In the example above, `added` is a pending stream, because its parent `b` is also pending.

This also applies to dependent streams created via `stream.run`:

```javascript
var stream = m.prop()
var doubled = stream.run(function(value) {return value * 2})

console.log(doubled()) // logs undefined because `doubled` is pending
```

#### Active state

When a stream receives a value, it becomes active (unless the stream is ended).

```javascript
var stream1 = m.prop("hello") // stream1 is active

var stream2 = m.prop() // stream2 starts off pending
stream2("world") // then becomes active
```

A dependent stream with multiple parents becomes active if all of its parents are active.

In the example above, setting `b(7)` would cause `b` to become active, and therefore `added` would also become active, and be updated to have the value `12`

#### Errored state

Errored streams can be created by calling `m.prop.reject()`

```javascript
var erroredStream = m.prop.reject(new Error("Server is offline"))
```

A stream can also become errored if it's a dependent stream and its [`combiner`](#combiner) or [`run`](#stream-run) function throws an error

```javascript
var errored1 = m.prop(1).run(function(value) {
	if (typeof value !== "string") {
		throw new Error("Not a string")
	}
	return value
})
// errored1 is in an errored state
```

```javascript
var stream = m.prop(1)
var errored2 = m.prop.combine(function(stream) {
	if (typeof stream() !== "string") {
		throw new Error("Not a string")
	}
	return value
}, [stream])
// errored2 is in an errored state
```

When a stream is in a errored state, its value is set to `undefined` and its `error` method is set to the error value

```javascript
var errored = m.prop.reject("Server is offline")

console.log(errored()) // logs undefined
console.log(errored.error()) // logs "Server is offline"
```

#### Ended state

A stream can stop affecting its dependent streams by calling `stream.end(true)`. This effectively removes the connection between a stream and its dependent streams.

```javascript
var stream = m.prop()
var doubled = stream.run(function(value) {return value * 2})

stream.end(true) // set to ended state

stream(5)

console.log(doubled())
// logs undefined because `doubled` no longer depends on `stream`
```

Ended streams still have state container semantics, i.e. you can still use them as getter-setters, even after they are ended.

```javascript
var stream = m.prop(1)
stream.end(true) // set to ended state

console.log(stream(1)) // logs 1

stream(2)
console.log(stream()) // logs 2
```

Ending a stream can be useful in cases where a stream has a limited lifetime (for example, reacting to `mousemove` events only while a DOM element is being dragged, but not after it's been dropped).

---

### Handling errors

When a stream is in a errored state, its value is set to `undefined`, and its `error` method returns the error value.

```javascript
var erroredStream = m.prop.reject("Server is offline")

console.log(erroredStream()) // logs undefined
console.log(erroredStream.error()) // logs "Server is offline"
```

Errors can be set in various ways:

```javascript
// via m.prop.reject
var errored1 = m.prop.reject("Server is offline")
console.log(errored1.error()) // logs "Server is offline"

// via `.error`
var errored2 = m.prop("hello")
errored.error("Server is offline")
console.log(errored2.error()) // logs "Server is offline"

// by throwing an error in a chain
var errored3 = m.prop("hello").run(function() {
	throw "Server is offline"
})
console.log(errored3.error()) // logs "Server is offline"

var errored4 = m.prop.combine(function() {
	throw "Server is offline"
}, [m.prop("hello")])
console.log(errored4.error()) // logs "Server is offline"

// by returning an errored stream in a chain
var errored5 = m.prop("hello").run(function() {
	return m.prop.reject("Server is offline")
})
console.log(errored5.error()) // logs "Server is offline"

var errored6 = m.prop.combine(function() {
	return m.prop.reject("Server is offline")
}, [m.prop("hello")])
console.log(errored6.error()) // logs "Server is offline"
```


Errors in stream chains propagate: if a stream is in an errored state, all of its dependent streams will have the same errored state, unless the error is handled via a `catch` method.

```javascript
var dependentStream = erroredStream.run(function(value) {return value})
console.log(dependentStream()) // logs undefined
console.log(dependentStream.error()) // logs "Server is offline"

var recoveredStream = dependentStream.catch(function() {return "hello"})
console.log(recoveredStream()) // logs "hello"
console.log(recoveredStream.error()) // logs undefined
```

Like in ES6 promises, the `catch` callback is only called if there is an error. If there isn't an error, it adopts the same value as its parent stream:

```javascript
erroredStream("hi")

console.log(dependentStream()) // logs "hi"
console.log(dependentStream.error()) // logs undefined

console.log(recoveredStream()) // logs "hi"
console.log(recoveredStream.error()) // logs undefined
```

---

### Serializing streams

Streams implement a `.toJSON()` method. When a stream is passed as the argument to `JSON.stringify()`, the value of the stream is serialized.

```javascript
var stream = m.prop(123)
var serialized = JSON.stringify(stream)
console.log(serialized) // logs 123
```

Streams also implement a `valueOf` method that returns the value of the stream.

```javascript
var stream = m.prop(123)
console.log("test " + stream) // logs "test 123"
```

---

### Streams do not trigger rendering

Unlike libraries like Knockout, Mithril streams do not trigger re-rendering of templates. Redrawing happens in response to event handlers defined in Mithril component views, route changes, or after [`m.request`](request.md) calls resolve.

If redrawing is desired in response to other asynchronous events (e.g. `setTimeout`/`setInterval`, websocket subscription, 3rd party library event handler, etc), you should manually call [`m.redraw()`](redraw.md)