Empty List singleton

[Jesper Öqvist]

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Empty lists cost memory - the cost could be reduced by using a singleton empty list object. All empty lists would point to the singleton empty list, and a new list is created first when the list needs to be modified.

[Jesper Öqvist]

Some problems with this idea and why it may not be feasible to implement:

• An empty list singleton would naturally have to occur in many places in the AST to be useful
• If the singleton is in many places of the AST then the parent pointer can be considered incorrect for that node
• Any inter-type declarations for the singleton should not be used to store information in the node, for example line number information. Such info becomes clobbered.