PersistentMaps

A PersistentMap is a data structure like State but is more flexible.

PersistentMap can contain multiple data entries and therefore is a more economical use of the ledger.

Declare a PersistentMap with PersistentMap<K, V> where K is a unique key and V is the value associated with the key.

function main()

    // create a PersistentMap
    // var pm : PersistentMap<String, String>(myPm : String); // error at '<', expected '=' or ';'
    // create a value
    // var val : String = "hello";
    // add the value to the PersistentMap at a specific key name
    // pm["0"] = val;

endfunction

To add a State to a PersistentMap, do the following:

Note

Coming soon: support for PersistentMap.

function main()

    // declare a State and set a value on it
    var myState = State<Int32>("xyz.balance", 0);
    myState.set(100);
    // declare a PersistentMap with the same name as the State
    // var myPm = PersistentMap<String, Int32>("xyz"); // error at ',', expected ';'
    // Failed to compile.
    // access a variable in the State using a handle to the PeristentMap
    // printLn(myPm["balance"]);

endfunction

In a PersistentMap the key must be a String or an Address type.

A PersistentMap can wrap around State types. For example, a common requirement for smart contracts is to facilitate transfers. Below, the from and to variables are Address types representing the two parties to a transaction along with their balances.

    var from_address = State<UInt64>(from, 0u64);
    var to_address = State<UInt64>(to, 0u64);

This is better represented by a PersistentMap where from_address and to_address represent keys, the values of which are set prior to building the data structure.

function main()

    // var transactPm : PersistentMap<Address, String>(tx : Int32); // error at '<', expected '=' or ';'
    // create transaction account details
    // var from_address = Address("AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7DF8+Pw==");
    // var to_address = Address("AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0+Pw==");
    // create values representing the balances of the accounts
    // var from_balance : String = 1000u64;
    // var to_balance : String = 0u64;
    // add the value to the PersistentMap at a specific key name
    // transactPm[from_address] = from_balance;
    // transactPm[to_address] = to_balance;

endfunction

A duplicate key overwrites the previously defined entry.