History

History access maintains a list of tuples, which can be subscripted by using the input port name. For example:

 composite Main {
   graph stream<int32 id, int32 diff, rstring name> In = Beacon() {}
         stream<int32 id, int32 diff, rstring name> Out = Functor(In) {
          param  filter : name != In[1].name;
          output Out    : diff = id - In[1].id;
         }
 }

The expression In[1].name refers to a historical tuple on port In, one tuple in the past, and extracts the name attribute from it. The current tuple is at index 0, so a bare stream name like In is synonymous with In[0]. For example, the filter parameter might equivalently be written as In[0].name != In[1].name. The subscript must be a compile-time constant. History access does not support slicing.

The tuples in the history list correspond to the last n tuples received on the stream. Primitive operator parameters like partitionBy do not affect the history.