# Variables

In RuLa, variables that bind to case instances — such as `p is Person` — are called [declarations](/rule-language/declarations.md). This page is about the other kind: **value variables**, which let you capture any intermediate result and give it a name inside the assignment part of a rule.

### What Are Value Variables?

A value variable holds the result of any expression: a number, string, list, dictionary, date, and so on. You define one with [`:=`](/rule-language/operators/special-operators/assign.md), giving it a plain name on the left and the expression on the right.

```
p is Person
---
full_name := CONCAT(p.first_name, ' ', p.last_name)
p.display_name := full_name
```

`full_name` is a value variable. It is computed once and then reused on the next line. Without it, you would have to write the `CONCAT(...)` expression twice.

{% hint style="info" %}
A value variable is local to the rule it is defined in. It is available on all lines that follow its definition, but it does not persist between rules.
{% endhint %}

### Syntax

```
left_side := right_side
```

**Left side** — determines what receives the value:

* A **plain name** (e.g. `net_worth`) creates a local variable available on subsequent lines in the same rule.
* A **field path** (e.g. `p.net_worth`) writes the value into that information field on the instance.

**Right side** — any expression that produces a value: a calculation, a string operation, a conditional expression, a list, a dictionary, and so on.

***

### Examples

#### Storing a calculated value to reuse it

```
p is Person
---
net_worth := p.total_assets - p.total_liabilities
p.net_worth := net_worth
p.risk_score := if net_worth < 0 then high_risk else low_risk end
```

`net_worth` is computed once and referenced twice — once to store the figure and once to derive the risk score from it.

***

#### Cleaning up a string before assignment

```
p is Person
---
normalised := UPPER(TRIM(p.first_name))
p.first_name_normalised := normalised
```

The raw value of `first_name` is trimmed of whitespace and uppercased. The cleaned result is stored in a variable and then written to a separate field.

***

#### Building a list incrementally

```
p is Person
---
base_docs := ['id_copy', 'proof_of_address']
full_docs := if p.nationalities contains us then APPEND(base_docs, 'w8_form') else base_docs end
p.required_documents := full_docs
```

`base_docs` holds the default document list. `full_docs` extends it conditionally. The final list is then assigned to the field.

***

#### Using a variable across multiple assignments

```
c is Contract
p is Person
---
annual_fee := c.aum * 0.0075
p.expected_fee := if annual_fee > 10000 then annual_fee else 10000 end
```

`annual_fee` is derived from the contract and then used in an `if-then-else` to enforce a fee floor before being written to the person.

***

#### Intermediate dictionary for a structured assignment

```
p is Person
---
address := {"street": p.street, "city": p.city, "country": p.domicile}
p.formatted_address := PRETTY_PRINT(address)
```

A dictionary is assembled from several fields into `address`, which is then formatted and stored.

***

### Naming Variables

Variable names:

* Must start with a letter.
* Can contain letters, digits, and underscores.
* Are case-sensitive (`NetWorth` and `net_worth` are two different variables).
* Should be short but descriptive — the goal is to make the rule easier to read, not harder.


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