mangle/3
Module: builtins
mangle/3 — destructively modify a structure
FORMS
mangle(Nth, Structure, NewArg)
DESCRIPTION
mangle/3 destructively modifies an argument of a compound term in a spirit similar to Lisp’s rplaca and rplacd. Structure must be instantiated to a compound term with at least Nth arguments. The Nth argument of Structure will become NewArg. Lists are considered to be structures of arity two in the usual way.
Modifications made to a structure by mangle/3 will survive failure and backtracking.
Even though mangle/3 implements destructive assignment in Prolog, it is not necessarily more efficient than copying a term. This is due to the extensive cleanup operation which ensures that the effects of a mangle/3 persist across failure.
EXAMPLES
?- Victim = doNot(fold,staple,mutilate), mangle(2,Victim,spindle).
Victim = doNot(fold,spindle,mutilate)
yes.