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Storage

Contract parameter

A contract parameter is a storage element whose value is set at deployment (origination) time.

Variable

A contract variable is a storage element whose value is set at declaration time. It is declared with the variable keyword followed by an identifier, a type and the initial value.

For example:

variable counter : nat = 0
variable admin_candidate : option<address> = none

The value of a storage variable may be modified by assignment instructions in entrypoints.

Asset

An asset collection is declared with the asset keyword followed by the asset collection identifier and the list of fields (identifier and type).

For example, the following declares the vehicle asset:

asset vehicle {
vin : string;
manufacturer : string;
release : date;
nbdoors : nat;
}

Asset key

By default, the first field serves as the asset key. Any comparable field of the asset may be declared as the asset key with the identified by modifier; hence the declaration above is equivalent to:

asset vehicle identified by vin {
vin : string;
/* ... */
}

Several fields may be declared to form the asset key; for example the liquidity asset is identified by two fields:

asset liquidity identified by token_id owner {
token_id : string;
owner : address;
amount : nat;
}

The key is then the tuple of fields' value, which is in Michelson the right comb of pairs of values; for example, the expression to retrieve the amount of token 0 of tz1VSUr8wwNhLAzempoch5d6hLRiTh8Cjcjb is:

var a = liquidity[(0,tz1VSUr8wwNhLAzempoch5d6hLRiTh8Cjcjb)].amount;

See access operator [] for more information.

Default field value

It is possible to define a default value for an asset field; for example:

asset vehicle {
vin : string;
manufacturer : string;
release : date = now;
nbdoors : nat = 5;
}

Fields with default value are not required when adding a new asset in collection; for example, the following instruction adds a new vehicle asset with release date equal to now and nbdoors equal to 5:

vehicle.add({ vin = "1G1AF1F57A7192174"; manufacturer = "Renault" })

See the add instruction for more information.

Big map

The to big_map modifier compiles the collection as a big_map.

For example:

asset vehicle to big_map {
vin : string;
/* ... */
}

Iterable big map

The to iterable_big_map modifier compiles the collection as an iterable_big_map.

For example:

asset vehicle to iterable_big_map {
vin : string;
/* ... */
}

Initial collection

It is possible to specify the initial value of the asset collection at origination with the initialized by modifier followed by the collection literal: a set of asset definitions.

For example, the following declaration initializes the vehicle asset with 2 assets:

asset vehicle {
vin : string;
manufacturer : string;
release : date;
nbdoors : nat;
} initialized by {
{ "1G1AF1F57A7192174"; "Renault"; 2021-06-28; 5 };
{ "JNKCV61E49M014581"; "Peugeot"; 2021-07-12; 3 }
}

The initial value of an asset collection is interpreted by the completium CLI's deploy command to set the initial storage Michelson value.

Michelson representation

The asset collection is compiled to Michelson as a map (or big_map) of records (right comb of pairs of annotated data).

An asset with only one field is compiled to a Michelson set.

For example, the declaration above is quasi equivalent to the following lower-level code:

record vehicle_data {
manufacturer : string;
release : date;
nbdoors : nat;
}

variable vehicle : map<string, vehicle_data> = [
("1G1AF1F57A7192174", { "Renault"; 2021-06-28; 5 });
("JNKCV61E49M014581", { "Peugeot"; 2021-07-12; 3 })
]