Scrall has no for_each action

Scrall’s basis on data flow and relational semantics eliminates the need for an explicit “for each” action.

In the following example, all Dogs past a certain age are classified as ‘old’ by setting a status attribute.

Dog( Born < ^date).Status = _old

Consider this action language from the Elevator case study. Here a Bank Level instance (Floor in a Bank) is responding to an up/down floor call button by selecting the most suitable Cabin to respond to the call. Here is what it might look like in xtUML’s (BridgePoint) OAL (object action language):

shortest_delay = 0.0;  // seconds
first_cabin = true;
param.OUT_Shaft = "";

// Select one of the cabins with the shortest delay
select many bank_Cabins related by my_Bank->Shaft{R1}->Cabin{R2};

for each this_Cabin in bank_Cabins
	 select one its_Shaft related by this_Cabin->Shaft{R2};
	 if (its_Shaft.In_service)
		  cab_delay = this_Cabin.Estimate_travel_delay(
			  Floor:my_Floor.Name, Calling_dir:param.Dir
		  );
		  if ((cab_delay < shortest_delay) or (first_cabin))
			   shortest_delay = cab_delay;
			   param.OUT_Shaft = its_Shaft.ID;
		  end if;
	 end if; // in service
	 first_cabin = false;
end for;

if (param.OUT_Shaft != "")
	 return true;
else
	 return false;
end if;  // OUT_Shaft will be "" only if all Shafts are out of service

In Scrall, you could replace all that with two lines:

inservice cabins ..= /Bank/Shaft( In service )/R2/Cabin
=>> inservice cabins(1, ^-Estimate travel delay(
	Floor: /Floor.Name, Calling dir: in.Dir).Shaft

The first line selects all Cabin instances whose Shaft is inservice.

The second line invokes the Estimated travel delay method of each selected Cabin instance and chooses one with the least delay reporting its Shaft attribute value (which happens to be a referential attribute to a Shaft ID. Done!

In fact, the whole thing could have been done with one line, but at some point you violate the principle of making the activity easy to read. In that vein, the two line solution could easily be expanded to three with the introduction of an additional variable (best cabin) to make it easier for humans to parse.

But the OAL solution returns an ID value because it is not possible output instance references. In Scrall, we would probably just return the selected Cabin instance references which, could be more than one since two Cabins might have the same estimated delay.

=>> /Bank/Shaft( In service )/R2/Cabin( 
    ^-Estimate travel delay( Floor : /Floor.Name, Calling dir: ^dir ) 
)

Now the calling action can be:

servicing shaft .= Choose shaft( 1, ^dir )