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20.5 Trailing

In certain compilation grades (see the “Compilation model options” section of the Mercury User’s Guide), the Melbourne Mercury implementation supports trailing. Trailing is a means of having side-effects, such as destructive updates to data structures, undone on backtracking. The basic idea is that during forward execution, whenever you perform a destructive modification to a data structure that may still be live on backtracking, you should record whatever information is necessary to restore it on a stack-like data structure called the “trail”. Then, if a computation fails, and execution backtracks to before those updates were performed, the Mercury runtime engine will traverse the trail back to the most recent choice point, undoing all those updates.

The interface used is a set of C functions (which are actually implemented as macros) and types. Typically these will be called from C code within ‘pragma foreign_proc’ or ‘pragma foreign_code’ declarations in Mercury code.

For an example of the use of this interface, see the module extras/trailed_update/tr_array.m in the Mercury extras distribution.