`fastn` has support for [rich data modelling](/ftd/data-modelling/), and it supports declaring variables. A variable is a named storage that programs can manipulate.

-- integer x: 20

Lang:

ftd

Variables have `type`s.

Types

The data type is mandatory while declaring a variable in `fastn`. Type of a variable can be one of the [built-in types](ftd/built-in-types/), a [record](ftd/record/), or an [or-type](ftd/or-type/).

Immutable

By default, variables are immutable − read only in `fastn`. In other words, the variable's value cannot be changed once a value is bound to a variable name.

-- integer x: 10

-- ftd.integer: $x
$on-click$: $ftd.increment($a = $x)

Lang:

ftd

The output will be as shown below:

Cannot have mutable reference of immutable variable `x`

Lang:

txt

The error message indicates the cause of the error - Cannot have mutable reference of immutable variable `x`. This is one of the many ways `fastn` allows programmers to write code and takes advantage of the safety.

Mutable

Variables are immutable by default. Prefix the variable name with `$` to make it mutable. The value of a mutable variable can be changed. The syntax for declaring a mutable variable is as shown below −

-- integer $x: 10

-- ftd.integer: $x
$on-click$: $ftd.increment($a = $x)

Lang:

ftd

The output will be as shown below: Click on `10`.

Output

Referring To Another Variable

A variable can be defined as referring to another variable using `$` as a prefix of referenced variable, `$`. This means if the referenced variable get changed the referer will change too. When you define a variable in `fastn`, you can make it refer to another variable by adding a `$` sign before the referenced variable's name, like `$`. This basically means that if the referenced variable gets updated or changed, the referring variable will also change accordingly. For example, let's say you have an integer variable called `x` with a value of `10`, and you define another integer variable called `y` with `$x` as its value. Now, `y` is the referring variable and `x` is the referenced variable. So, if you update or change the value of `x`, the value of `y` will also change.

-- integer $x: 10

-- integer y: $x

-- ftd.integer: $x

-- ftd.integer: $y

-- ftd.text: I change x
$on-click$: $ftd.increment($a = $x)

Lang:

ftd

Give it a try and click on "I change x" to see how `x` and `y` change together!

Output

I change x :)

Clone the value of a Variable

A value of the variable can be cloned by de-referencing the variable reference. This means that cloning creates a duplicate value and if the cloned value changes, the object, that clones it, will not change.

-- integer x: 10

-- integer y: *$x

-- ftd.text: I change x :)
$on-click$: $ftd.increment($a = $x)

Lang:

ftd

Here, if x changes, y doesn't changes. The output will be as shown below: Click on `I change x :)` to see the result.

Output

I change x :)

Updating a Variable

Once a `mutable` variable has been defined it can be updated too. Any variable can be made `mutable` by prefixing it with `$`. Note: By default, `fastn variables` are `immutable` (can't be changed once initialized).

-- integer $x: 10

-- $x: 20

Lang:

ftd

The type of the variable can not be updated.

`$processor$`: dynamic variables

`fastn` documents are processed in the context of a "platform", and platform can provide access to dynamic variables. Say platform has provided a dynamic variable `os`, which is the operating system on which this document is getting rendered, you can use that like this:

-- string name-of-os:
$processor$: os

Lang:

ftd

`type` is mandatory when using `$processor$`. Available processors would be documented as part of platform documentation. Processors can also look at data passed, so its possible to create a processor:

-- string greeting: hello, world
$processor$: uppercase

Lang:

ftd

Say the platform has provided a processor `uppercase`, which takes the current value, `hello, world` and returns its upper case value. In this case the variable greeting will hold the value: `HELLO, WORLD`.

Foreign variables

Like `$processor$`, the platform provides foreign variables against a module. The `fastn` stuck itself to fetch value of foreign value, which is, then, provided by platform. The most common foreign variable is `assets`.

-- import: module/assets

-- ftd.text: Check bar content
link: $assets.files.some-folder.bar.ftd

Lang:

ftd

The `files` field in `assets` variable gives url to files present in package. So, `$assets.files.some-folder.bar.ftd` returns url of `/some-folder/bar.ftd` as value. It's not so for image file.

-- import: module/assets

-- ftd.image:
src: $assets.files.images.my-image.png

Lang:

ftd

`$assets.files.images.my-image.png` gives url of `/images/my-image.png` as value (for `light` field). If an image with the same name but with `-dark` suffix exists in the package, i.e. `/images/my-image-dark.png`, then `dark` field gets the url of this file. Checkout [`src`](ftd/image/#src-ftd-image-src) property and [`ftd.image-src`](ftd/built-in-types/#ftd-image-src) type for more details.

Support `fastn`!

Enjoying `fastn`? Please consider giving us a star ⭐️ on [GitHub](https://github.com/fastn-stack/fastn) to show your support!

[⭐️](https://github.com/fastn-stack/fastn)

Getting Help

Have a question or need help? Visit our [GitHub Q&A discussion](https://github.com/fastn-stack/fastn/discussions/categories/q-a) to get answers and subscribe to it to stay tuned. Join our [Discord](https://discord.gg/a7eBUeutWD) channel and share your thoughts, suggestion, question etc. Connect with our [community](/community/)!

[💻️](/community/)

Found an issue?

If you find some issue, please visit our [GitHub issues](https://github.com/fastn-stack/fastn/issues) to tell us about it.

Quick links:

- [Install `fastn`](install/) - [Create `fastn` package](create-fastn-package/) - [Expander Crash Course](expander/) - [Syntax Highlighting in Sublime Text](/sublime/)

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