# package octez-libs

`Tree`

provides immutable, in-memory partial mirror of the context, with lazy reads and delayed writes. The trees are Merkle trees that carry the same hash as the part of the context they mirror.

Trees are immutable and non-persistent (they disappear if the host crash), held in memory for efficiency, where reads are done lazily and writes are done only when needed, e.g. on `Context.commit`

. If a key is modified twice, only the last value will be written to disk on commit.

`include VIEW with type t := tree and type tree := tree`

### Getters

`mem t k`

is an Lwt promise that resolves to `true`

iff `k`

is bound to a value in `t`

.

`find t k`

is an Lwt promise that resolves to `Some v`

if `k`

is bound to the value `v`

in `t`

and `None`

otherwise.

`list t key`

is the list of files and sub-nodes stored under `k`

in `t`

. The result order is not specified but is stable.

`offset`

and `length`

are used for pagination.

`length t key`

is an Lwt promise that resolves to the number of files and sub-nodes stored under `k`

in `t`

.

It is equivalent to `let+ l = list t k in List.length l`

but has a constant-time complexity.

### Setters

`add t k v`

is an Lwt promise that resolves to `c`

such that:

`k`

is bound to`v`

in`c`

;- and
`c`

is similar to`t`

otherwise.

If `k`

was already bound in `t`

to a value that is physically equal to `v`

, the result of the function is a promise that resolves to `t`

. Otherwise, the previous binding of `k`

in `t`

disappears.

`remove t k v`

is an Lwt promise that resolves to `c`

such that:

`k`

is unbound in`c`

;- and
`c`

is similar to`t`

otherwise.

### Folding

```
val fold :
?depth:depth ->
tree ->
key ->
order:[ `Sorted | `Undefined ] ->
init:'a ->
f:(key -> tree -> 'a -> 'a Lwt.t) ->
'a Lwt.t
```

`fold ?depth t root ~order ~init ~f`

recursively folds over the trees and values of `t`

. The `f`

callbacks are called with a key relative to `root`

. `f`

is never called with an empty key for values; i.e., folding over a value is a no-op.

The depth is 0-indexed. If `depth`

is set (by default it is not), then `f`

is only called when the conditions described by the parameter is true:

`Eq d`

folds over nodes and values of depth exactly`d`

.`Lt d`

folds over nodes and values of depth strictly less than`d`

.`Le d`

folds over nodes and values of depth less than or equal to`d`

.`Gt d`

folds over nodes and values of depth strictly more than`d`

.`Ge d`

folds over nodes and values of depth more than or equal to`d`

.

If `order`

is ``Sorted`

(the default), the elements are traversed in lexicographic order of their keys. For large nodes, it is memory-consuming, use ``Undefined`

for a more memory efficient `fold`

.

### Configuration

`val is_empty : tree -> bool`

`is_empty t`

is true iff `t`

is `empty _`

.

`to_value t`

is an Lwt promise that resolves to `Some v`

if `t`

is a leaf tree and `None`

otherwise. It is equivalent to ```
find t
[]
```

.

`of_value _ v`

is an Lwt promise that resolves to the leaf tree `v`

. Is is equivalent to `add (empty _) [] v`

.

`val hash : tree -> Tezos_base.TzPervasives.Context_hash.t`

`hash t`

is `t`

's Merkle hash.

### Caches

`val clear : ?depth:int -> tree -> unit`

`clear ?depth t`

clears all caches in the tree `t`

for subtrees with a depth higher than `depth`

. If `depth`

is not set, all of the subtrees are cleared.