Searching

The predicate Big_map.mem tests for membership in a given big map, given a purported key.

const my_map: big_map<int,string> =

Big_map.literal([[1,"one"],[2,"two"]]);

const contains_2: bool = Big_map.mem(2, my_map); // == true

In practice, however, we would like to get the value associated to the key we searched. This is achieved by means of Big_map.find_opt.

const v : option<string> = Big_map.find_opt(2, my_map);

Notice how the value we read is an optional value: this is to force the reader to account for a missing key in the big map. This requires pattern matching.

const force_access = (key, map) =>

$match(Big_map.find_opt (key, map), {

"Some": value => value,

"None": () => failwith("No value.")

});

In fact, the predefined function Big_map.find does exactly that, except that the exception raised by failwith carries the default string "MAP FIND".