> Aztec.nr API reference, v6.0.0 (mainnet and testnet). Index: https://docs.aztec.network/aztec-nr-api/v6.0.0/llms.txt

# Function conditionally_check_membership

Module: `noir_aztec::protocol::merkle_tree::membership`

```rust
pub fn conditionally_check_membership<let TREE_HEIGHT: u32, LEAF_PREIMAGE, VALUE>(
    key: Field,
    exists: bool,
    leaf_preimage: LEAF_PREIMAGE,
    membership_witness: MembershipWitness<TREE_HEIGHT>,
    tree_root: Field,
) -> (bool, bool, bool)
where
    LEAF_PREIMAGE: IndexedTreeLeafPreimage<VALUE>
```

Check either membership or non-membership depending on the value of `exists`:

- If `exists` == true, `key` is in the tree. `leaf_preimage` is for the key itself.
- If `exists` == false, `key` is not in the tree. `leaf_preimage` is for the low leaf of the key. `membership_witness` is the hint to prove that the `leaf_preimage` exists in the tree with the given `tree_root`.

Returns:

- is_valid: `true` if the membership (`exists == true`) or non-membership (`exists == false`) check succeeds, `false` otherwise.
- is_expected_leaf: `true` if the leaf preimage is the expected matching leaf (`exists == true`) or low leaf (`exists == false`) for the key, `false` otherwise.
- is_leaf_in_tree: `true` if the leaf preimage exists in the tree, `false` otherwise.