14. Things to enter, travel in and push around

...the need to navigate a newly added river prompted the invention of vehicles (specifically, a boat).

...P. David Lebling, Marc Blank and Timothy Anderson

Some objects in a game are enterable, which means that a player can get inside or onto them. Usually, "inside" means that the player is only half-in, as with a car or a psychiatrist's couch. (If it's more like a prison cell, then it should be a separate place.) In practice one often wants to make an enterable thing also a container, or, as in the altar from 'Ruins', a supporter:

Object -> stone_table "slab altar"
  with name "stone" "table" "slab" "altar" "great",
       initial "A great stone slab of a table, or altar, dominates the Shrine.",
  has  enterable supporter;

If the player gets into a container and then closes it, the effect is like being in a different location. (Unless the container has the transparent attribute and is therefore see-through.) The interior may be dark, but if there's light to see by, the player will want to see some kind of room description. In any case, many enterable objects ought to look different from inside or on top. Inside a vehicle, a player might be able to see a steering wheel and a dashboard, for instance. On top of a cupboard, it might be possible to see through a skylight window.

For this purpose, any enterable object can provide an inside_description, which can be a string or a routine to print one, as usual. If the exterior location is still visible, then the "inside description'' is added to the normal room description, and otherwise it becomes that description. As an extreme example, suppose that the player gets into a huge cupboard, closes the door behind her and then gets into a plastic cabinet inside that. The resulting room description might read like so:

The huge cupboard (in the plastic cabinet)
It's a snug little cupboard in here, almost a room in itself.

In the huge cupboard you can see a pile of clothes.

The plastic walls of the cabinet distort the view.

All the classic games have vehicles (like boats, or fork lift trucks, or hot air balloons) which the player can journey in, so Inform makes this easy. Here is a simple case:

Object car "little red car" cave
  with name "little" "red" "car",
       description "Large enough to sit inside.  Among the controls is a
           prominent on/off switch.  The numberplate is KAR 1.",
       when_on  "The red car sits here, its engine still running.",
       when_off "A little red car is parked here.",
       before
       [; Go: if (car has on) "Brmm!  Brmm!";
              print "(The ignition is off at the moment.)^";
       ],
  has  switchable enterable static container open;

0 to disallow the movement, printing a refusal;
1 to allow the movement, moving vehicle and player;
2 to disallow but print and do nothing; or
3 to allow but print and do nothing.

Because you might want to drive the car "out'' of a garage, the "out'' verb does not make the player get out of the car. Usually the player has to type something like "get out'' to make this happen, though of course the rules can be changed.