Extra-logical Predicates

Predicates with side effects

Predicates that produce side effects leave the realm of pure logic. These are for example:

writeq/1
read/1
format/2

Side effects are phenomena that cannot be reasoned about within the program. For example, deletion of a file or output on the system terminal.

Meta-logical predicates

Predicates that reason about instantiations are called meta-logical. Examples are:

var/1
ground/1
integer/1

These predicates are outside the realm of pure monotonic logic programs, because they break properties like commutativity of conjunction.

Other predicates that are meta-logical include:

arg/3
functor/3
(=..)/2

These predicates could in principle be modeled within first-order logic, but require an infinite number of clauses.

All-solutions predicates

Predicates that reason about all solutions are extra-logical. These are for example:

setof/3
findall/3
bagof/3

!/0 and related predicates

Predicates that impede or prohibit a declarative reading of Prolog programs are extra-logical. Examples of such predicates are:

!/0
(->)/2 and if-then-else
(\+)/1

These predicates can only be understood procedurally, by taking into account the actual control flow of the interpreter, and as such are beyond the realm of pure logic.