Reading Prolog Programs
While you may write
a Prolog program
only once, and may even rewrite it a few times, you
and others will typically read it many, many, many times.
Consequently, it is important to know how we can best read
Prolog programs: What exactly do they mean?
There are several ways to
read pure Prolog programs, and we
explain some of them in this text.
Declaratively, Prolog programs state what holds. A Prolog
program consists of clauses, and each clause is either
a fact or a rule. Facts state what is
always true. Rules state what is true under
Declaratively, a rule of the form:
Head :- Body.
is read as: "If Body is
true, then Head is true." In logical terms, we say
that Body implies Head, written
as Body→Head, or equivalently as:
Head ← Body.
Note that :- is in fact meant to represent the arrow
←. Since Body defines the conditions under
which Head holds, it can also be regarded as
a constraint on the set of solutions.
This way to read Prolog programs is also called concluding reading.
A major advantage of this approach is that it is easy to explain,
understand and use. You state what holds under what conditions,
and the Prolog engine finds solutions for you. A disadvantage of
this approach is that it does not explain why logically equivalent
program variants may exhibit different performance or termination
of two lists:
list_list_together(, Bs, Bs).
list_list_together([A|As], Bs, [A|Cs]) :-
list_list_together(As, Bs, Cs).
Let us read this definition declaratively
, that is in
terms of relations
that hold between arguments, as
described by the two clauses:
- The concatenation of the empty
list  and any
list Bs is just that
- If the concatenation of As
and Bs is Cs, then the
concatenation of [A|As] and Bs
is [A|Cs], for any
Note how general
this reading is: It is applicable if any
arguments are instantiated
, and also
To complement the declarative approach of reading Prolog programs,
we can also read them procedurally. This means that we take
into account the actual computation strategy of the Prolog
Operationally, invocation of a Prolog predicate is similar to
a procedure or function call in other
languages. However, two critical differences remain: First,
Prolog variables are truly logical variables
and may be unbound or only partially instantiated.
This cannot happen with variables in most other languages. Second,
Prolog provides backtracking as a built-in feature, and
will exhaustively try alternatives.
For these reasons, understanding a Prolog
program procedurally is significantly harder than
understanding the control flow of many other programming
languages. In particular, when tracing Prolog, you need to take
The need to keep track of these complexities and their
interactions is a major drawback of this approach. An advantage of
the approach is its potential to explain
characteristics and termination
properties of program variants.
- instantiation of variables
- aliasing between variables
- alternatives found on backtracking.
Note also that a procedural reading almost invariably implies a
particular direction of use, and therefore typically
does not do justice to the full generality of logical relations.
Let us read list_list_together/3
for the query ?- list_list_together([x,y], [z], Cs).
- Does the first clause apply? No, because [x,y] does not unify with .
- Does the second clause apply? Note that due to the way
resolution works, we must
introduce fresh variables when considering a
clause. So, let us
use A', As', Bs', and Cs'
for the variables A, As, Bs
and Cs that appear in the clause head. The answer is:
Yes, the second clause applies with the
bindings A'=x, As'=[y], Bs'=[z], Cs=[A'|Cs'].
This is already rather cumbersome and error-prone, and it
gets even harder to keep track of all bindings as we proceed
Carrying on, we consider the
goal list_list_together([y], [z], Cs'), repeating the
same questions for this goal.
- Does the first clause apply? No, because [y] does not unify with .
- Does the second clause apply? We need to rename the
variables again. Let us
use A'', As'', Bs''
and Cs''. Yes, the clause applies
with A''=y, As''=, Bs''=[z]
- Now the whole ordeal once more, as we consider the
goal list_list_together(, [z], Cs'').
- Does the first clause apply? Yes, at last!
Note that we again need to introduce fresh variables
of course. Let us use Bs''' to denote the single
variable of the first clause at this step of the
computation. So the first clause applies
with Bs'''=[z] and Cs''=Bs'''. This means
that at last a solution is found and reported as
a binding for the original variable Cs which
is the only variable that appears in the query. If you have
carefully followed this trace (as I am sure you have, since
it is so enjoyable to read), you know that Cs
was unified with [A'|Cs']. Since
A' was unified with x, and
Cs' was unified with [A''|Cs''], and
further A'' was unified with y, this
makes Cs the same as [x,y|Cs'']. As we
just mentioned, Cs'' was unified
with Bs''', and Bs'''
is [z]. Thus, the solution we found
is Cs=[x,y,z], and this solution is reported
by the toplevel.
- We still need to consider the second clause
too though: No, it does not apply,
because  does not unify with [_|_].
And this was only one particular case
covering all possible modes of invocation with a procedural
reading is extremely complex, and typically would take an
extremely elaborate explanation. In general, you will not be
able to carry this approach through, because there are too many
cases to consider.
Program slicing is a simple and powerful technique that
uses very general properties of pure Prolog to study the effects
of generalizations and specializations of a program.
Examples of such properties are:
In a very precise sense, program slices are explanations
that answer why we observe certain phenomena.
- removing a goal can make the program at
most more general, never more specific
- removing a clause can make the program at
most more specific, never more general
- inserting false/0 between any two goals in a
rule lets us ignore the procedural effects of all
goals after that point.
We illustrate this with a simple example. Suppose a programmer has
written list_length/2, relating a list to its length as
list_length([_|Ls], N) :-
N #> 0,
N #= N0 + 1.
The predicate works exactly as intended if the list is sufficiently
instantiated. For example:
?- list_length(, L).
L = 0.
?- list_length([_,_,_], L).
L = 3.
However, the predicate does not generate a single answer
for the most general query:
?- list_length(Ls, L).
Program slicing helps us to see the reason.
is used for parts that are not relevant for the behaviour we
list_length([_|Ls], N) :-
The remaining fragment is by itself already responsible for
the nontermination. No change
N #> 0,
N #= N0 + 1.
list_length(, 0) :- false.
strikeout parts can prevent it.
Program slices can be generated automatically and are a
powerful way to locate the causes of
nontermination and other unintended properties in Prolog programs.
See for example Stefan Kral et al., Slicing zur
Fehlersuche in Logikprogrammen, WLP 2000.
More about Prolog