-- integer x: 20

-- integer x: 20

The data type is mandatory while declaring a variable in fastn.

Type of a variable can be one of the built-in types, a record, or an or-type.

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)

-- integer x: 10

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

Cannot have mutable reference of immutable variable `x`

Cannot have mutable reference of immutable variable `x`

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)

-- integer $x: 10

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

A variable can be defined as referring to another variable using $ as a prefix of referenced variable, $<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 $<referenced variable>. 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)

-- integer $x: 10

-- integer y: $x

-- ftd.integer: $x

-- ftd.integer: $y

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

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)

-- integer x: 10

-- integer y: *$x

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

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

-- integer $x: 10

-- $x: 20

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

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

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

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

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

-- import: module/assets

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

-- import: module/assets

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

-- import: module/assets

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