20.5.2 Value trailing

The simplest form of trailing is value trailing. This allows you to trail updates to memory and have the Mercury runtime engine automatically undo them on backtracking.

• MR_trail_value()

Prototype:

void MR_trail_value(MR_Word *address, MR_Word value);

Ensures that if future execution backtracks to the current choice point, then value will be placed in address.

• MR_trail_current_value()

Prototype:

void MR_trail_current_value(MR_Word *address);

Ensures that if future execution backtracks to the current choice point, the value currently in address will be restored.

‘MR_trail_current_value(address)’ is equivalent to ‘MR_trail_value(address, *address)’.

Note that address must be word aligned for both MR_trail_current_value() and MR_trail_value(). (The address of Mercury data structures that have been passed to C via the foreign language interface are guaranteed to be appropriately aligned.)