Futures and Read-Only Vars

• we want to expose to consumers a read-only view of the stream
• new concept of future

declare X Y=!!X

%% Y is a read-only view of X

Y=f(a)      %% suspends until X is instantiated
X=f(_)      %% succeeds and then X=f(a) above unblocks

• !!X where X is not free just returns X
• the trick to designing safe abstractions is to keep X inside the abstraction and only expose !!X to consumers