OPENLAMBDA macros

[pamoroso]

What are the typical use cases for OPENLAMBDA macros in Interlisp?

IRM says this which I find unclear as I don't know what the frequency of evaluation is:

When the compiler processes an OPENLAMBDA, it attempts to substitute the actual arguments for the formals wherever this preserves the frequency and order of evaluation that would have resulted from a LAMBDA expression, and produces a LAMBDA binding only for those that require it.

The Medley source provides several examples of OPENLAMBDA macros but the code alone doesn't hint at why OPENLAMBDA is used instead of other types of macros.

[nbriggs]

The arguments of a lambda expression are evaluated once (frequency) and from left to right (order).

If your OPENLAMBDA uses the same formal more than once (e.g. (OPENLAMBDA (X Y) (PLUS X X Y))) it will still evaluate X only once (it'll create a LAMBDA binding for it) but Y will be substituted directly into the expansion. If you wrote (OPENLAMBDA (X Y) (PLUS Y X)) I would expect it to bind both X and Y to preserve the order rather than directly substitute the arguments into the call to PLUS.

[pamoroso]

Now it's much clearer, thanks.

[MattHeffron]

It appears that OPENLAMBDA is sort of an optimization of a LAMBDA macro, on the assumption that the LAMBDA binding has a cost that one wishes to avoid. (E.g., used in a tight loop, or in implementing low-level, frequently-used, functions.)
If that's the reason for OPENLAMBDA, then with the performance now available, it might be of less utility than in the past.

This is all my *guess, and I well could be completely wrong about this.

[nbriggs]

The processor speed is greatly increased but the stack capacity and basic frame size is the same as it always was. I think that saving a frame is one reason you might want it.