> For the complete documentation index, see [llms.txt](https://docs.atfinity.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.atfinity.io/rule-language/operators/list-operators/flatten.md).

# FLATTEN

### Description

`FLATTEN` takes a list of dictionaries and extracts values from them. Given a list of dictionaries and a key (or a tuple of keys), it returns a flat list of the values at that key across all dictionaries.

If called with no second argument, it concatenates the elements of the list into a single value.

### Example with a single key

```
FLATTEN([{"key": "a", "name": "Alice"}, {"key": "b", "name": "Bob"}], "key")
```

This extracts the value at `"key"` from each dictionary, returning `["a", "b"]`.

### Example with multiple keys

```
FLATTEN([{"key": "a", "age": 25, "name": "Alice"}, {"key": "b", "age": 30, "name": "Bob"}], ("key", "name"))
```

This returns only the specified keys from each dictionary: `[{"key": "a", "name": "Alice"}, {"key": "b", "name": "Bob"}]`.

### Example without a key

```
FLATTEN([["a", "b"], ["c", "d"]])
```

This concatenates the sublists into a single list, returning `["a", "b", "c", "d"]`.


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