🚀 go-pugleaf

lagache@violet.berkeley.edu (Edouard Lagache) writes:

> [ ... ] what would be wrong with having a "fully" destructive
> append that would be used only in the situations without reversibility
> and backtracking.

In general, it isn't enough to have directionality and determinism.  You
have to make sure that the "old" value of the (sub)structure being
destructively assigned to isn't accessible from anywhere else.  In other
words, it isn't enough to combine mode and determinacy analysis with
a simple live variable analysis to figure out when a structure can
be reused: sharing patterns between structures have to be looked at as
well.  As an example, consider

	p(X,Y,Z,W) :- W = [a|X], append(X,Y,Z).

where p/4 has the mode <ground, ground, free, free> -- then, given
the usual implementation of unification for the literal "W = [a|X]",
append/3 cannot use destructive assignment even though its use in
this context is directional and deterministic.

Maurice Bruynooghe has done some work on static analyses to detect
situations where destructive assignment can be introduced as a compiler
optimization, see his paper in SLP87.
-- 
Saumya Debray		CS Department, University of Arizona, Tucson

     internet:   debray@arizona.edu
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