Pipeline reference

# Eval step

An Eval step sets fields to the values of expressions, on every record it processes.

Each assignment sets one target field to the value of one expression. The [expression reference](/docs/code-reference) lists the operators and functions an expression can use.

## Example

The parser declares `price` as `f64` and `qty` as `i64`, so the expressions can calculate with them.

Step

  Saved JSON

```
{
  "id": "eval",
  "type": "eval",
  "assignments": [
    { "target": "total", "expr": "price * qty" },
    { "target": "large", "expr": "total > 100" },
    { "target": "debug", "expr": "null" }
  ]
}
```

Input

```
{
  "price": 12.5,
  "qty": 10,
  "debug": "trace-1"
}
```

→

Output

```
{
  "price": 12.5,
  "qty": 10,
  "total": 125.0,
  "large": true
}
```

`large` reads `total`, which the assignment above it wrote. The step sets `debug` to null, so it is gone from the output. Fields the step does not name remain.

## Settings

It also takes the settings [every step](/docs/steps#settings) takes.

`assignments`  list of assignments required

Each `target` field is set to its `expr`, in order. Dots in a target name a nested field.

Examples:

- `[{"target": "event.dataset","expr": "\"nginx.access\""}]`
- `[{"target": "severity","expr": "status >= 500 ? \"error\" : \"info\""}]`

In the editor, **\+ assignment** adds a row.

## How it works

The assignments run in order, top to bottom. A later assignment reads the value an earlier one wrote. Each one replaces its whole target. When two assignments name the same target, the later one's value is the one that remains. Assigning `null` removes the field from the output.

Each assignment is the Code statement `target = expression;`. When you need conditions, loops or temporary variables, use a [Code step](/docs/steps/code).

### Process selected records

When the filter is false or null, the record continues with its existing values.

Step

  Saved JSON

```
{
  "id": "eval",
  "type": "eval",
  "filter": "status >= 500",
  "assignments": [{ "target": "severity", "expr": "\"error\"" }]
}
```

Input

```
{
  "service": "checkout",
  "status": 503
}
```

```
{
  "service": "checkout",
  "status": 200
}
```

→

Output

```
{
  "service": "checkout",
  "status": 503,
  "severity": "error"
}
```

```
{
  "service": "checkout",
  "status": 200
}
```

### Types and errors

Each assignment follows the language's rules for [assignment](/docs/code-reference#statements), [types](/docs/code-reference#types), [null](/docs/code-reference#nulls) and [exceptions](/docs/code-reference#exceptions).

An assignment with a blank target or a blank expression is an error when you save. The editor shows each assignment's error on its own row.

## Troubleshooting

**Saving reports a type error on an assignment**

Check the type of the first value written to the target. Declare the field in the parser or [convert](/docs/code-reference#types) the value to that type.

## See also

-   [Pipeline steps](/docs/steps)
-   [Expression reference](/docs/code-reference)
-   [Code step](/docs/steps/code)