ets-var-pinning-example.erl

% Setup
tab = ets:new(tab, [set, named_table, public]),
true = ets:insert(tab, {foo, 0}).

% Slow: O(N) operation
Key = foo,
Threshold = 100,
NewVal = 101,
MatchSpec = ets:fun2ms(fun({K, V})
        when K =:= Key
        andalso V < Threshold ->
    {K, NewVal}
end),
ets:select_replace(tab, MatchSpec).

% Fast: O(log(N)) operation
Key = foo,
Threshold = 100,
NewVal = 101,
MatchSpec = [
    {
        {Key, '$1'},
        [{'<', '$1', {const, Threshold}}],
        [{{{const, Key}, {const, NewVal}}}]
    }
],
ets:select_replace(tab, MatchSpec).

% In theory, variable pinning could allow generation of the "Fast" match spec:
Key = foo,
Threshold = 100,
NewVal = 101,
MatchSpec = ets:fun2ms(fun({^Key, V})
        when V < Threshold ->
    {Key, NewVal}
end),
ets:select_replace(tab, MatchSpec).

% Here is the O(N) match spec for comparison:
Key = foo,
Threshold = 100,
NewVal = 101,
MatchSpec = [
    {
        {'$1', '$2'},
        [
            {
                'andalso',
                {'=:=', '$1', {const, Key}},
                {'<', '$2', {const, Threshold}}
            }
        ],
        [{{{const, Key}, {const, NewVal}}}]
    }
].