Futures make sets safe

Set notation is very convenient.

E.g.  

index_set = { 1, 2, 3}
array[ index_set ] = 0

rather than a loop

But a problem with set notation is that intermediate sets can be very large.

E.g.  

index_set = 1, 2, 3 ...
array[index_set] = random()
array = array[i] where i < 5 

rearranging

index_set = 1, 2, 3 ...
small_index_set =  i IN index_set, where i < 5
array[small_index_set] = random()

If the array[index_set] = assignment makes things real, this statement is impractical. Whereas array[small_index_set] = is finite sized.

Futures make this sort of manipulation feasible.