- Reference manual
- Built-in Predicates
- Notation of Predicate Descriptions
- Character representation
- Loading Prolog source files
- Editor Interface
- Verify Type of a Term
- Comparison and Unification of Terms
- Control Predicates
- Meta-Call Predicates
- Delimited continuations
- Exception handling
- Printing messages
- Handling signals
- DCG Grammar rules
- Declaring predicate properties
- Examining the program
- Input and output
- Status of streams
- Primitive character I/O
- Term reading and writing
- Analysing and Constructing Terms
- Analysing and Constructing Atoms
- Localization (locale) support
- Character properties
- Character Conversion
- Misc arithmetic support predicates
- Built-in list operations
- Finding all Solutions to a Goal
- Formatted Write
- Global variables
- Terminal Control
- Operating System Interaction
- File System Interaction
- User Top-level Manipulation
- Creating a Protocol of the User Interaction
- Debugging and Tracing Programs
- Debugging and declaring determinism
- Obtaining Runtime Statistics
- Execution profiling
- Memory Management
- Windows DDE interface
- Built-in Predicates
- Reference manual
4.7 Control Predicates
The predicates of this section implement control structures. Normally the constructs in this section, except for repeat/0, are translated by the compiler. Please note that complex goals passed as arguments to meta-predicates such as findall/3 below cause the goal to be compiled to a temporary location before execution. It is faster to define a sub-predicate (i.e., one_character_atoms/1 in the example below) and make a call to this simple predicate. See also the Prolog flag compile_meta_arguments.
one_character_atoms(As) :- findall(A, (current_atom(A), atom_length(A, 1)), As).
- Always fail. The predicate fail/0 is translated into a single virtual machine instruction.
- Same as fail, but the name has a more declarative connotation.
- Always succeed. The predicate true/0 is translated into a single virtual machine instruction.
- Always succeed, provide an infinite number of choice points.
- Cut. Discard all choice points created since entering the predicate in
which the cut appears. In other words, commit to the clause in
which the cut appears and discard choice points that have been
created by goals to the left of the cut in the current clause. Meta
calling is opaque to the cut. This implies that cuts that appear in a
term that is subject to meta-calling (call/1)
only affect choice points created by the meta-called term. The following
control structures are transparent to the cut: ;/2, ->/2
*->/2. Cuts appearing
in the condition part of
->/2 and *->/2
are opaque to the cut. The table below explains the scope of the cut
with examples. Prunes here means “prunes X
choice point created by X” .
t0 :- (a, !, b).
% prunes a/0 and t0/0
t1 :- (a, !, fail ; b).
% prunes a/0 and t1/0
t2 :- (a -> b, ! ; c).
% prunes b/0 and t2/0
t3 :- (a, !, b -> c ; d).
% prunes a/0
t4 :- call((a, !, fail ; b)).
% prunes a/0
\+(a, !, fail).
% prunes a/0
- [ISO]:Goal1 , :Goal2
- Conjunction (and). True if both Goal1 and Goal2 are true.
- [ISO]:Goal1 ; :Goal2
- Disjunction (or). True if either Goal1 or Goal2
succeeds. Note that the semantics change if Goal1 contains
->/2 or *->/2. ;/2
is transparent to cuts. See !/0
for details. For example:
?- (between(1,2,X) ; X = a). X = 1 ; X = 2 ; X = a.
It is strongly advised to always use parenthesis around disjunctions. Conjunctions inside a disjunction should not use parenthesis. Traditionally the
is placed at the start of the line rather than at the end because a
at the end of a line is easily overlooked. Below is an example of the preferred style used in SWI-Prolog.72Some users prefer a newline after the
p :- a, ( b, c ; d ).
Although ;/2 is a control structure that is normally handled by the compiler, SWI-Prolog implements ;/2 as a true predicate to support call/2 and friends as well as to allow for querying predicate properties, for example to support code analysis.
- :Goal1 | :Goal2
- Equivalent to ;/2. Retained for compatibility only. New code should use ;/2.
- [ISO]:Condition -> :Action
- If-then and If-Then-Else. The ->/2
construct commits to the choices made at its left-hand side, destroying
choice points created inside the clause (by ;/2),
or by goals called by this clause. Unlike !/0,
the choice point of the predicate as a whole (due to multiple clauses)
is not destroyed. Disregarding the interaction with !/0,
;/2 and ->/2
acts as if defined as:
If -> Then; _Else :- If, !, Then. If -> _Then; Else :- !, Else. If -> Then :- If, !, Then.
Please note that (If
->Then) acts as (If
->Then ; fail), making the construct fail if the condition fails. This unusual semantics is part of the ISO and all de-facto Prolog standards.
Please note that (if
->then;else) is read as ((if
->then);else) and that the combined semantics of this syntactic construct as defined above is different from the simple nesting of the two individual constructs, i.e., the semantics of ->/2 changes when embedded in ;/2. See also once/1.
As with ;/2, this construct is always nested in parenthesis. Here is an example of the preferred layout for SWI-Prolog.
p :- a, ( b, c -> d, e ; f -> g ; h ).
- :Condition *-> :Action ; :Else
- This construct implements the so-called‘soft-cut'. The control is
defined as follows: If Condition succeeds at least once, the
semantics is the same as (
call(Condition), Action).73Note that the Condition is wrapped in call/1, limiting the scope of the cut (!/0 If Condition does not succeed, the semantics is that of (
\+Condition, Else). In other words, if Condition succeeds at least once, simply behave as the conjunction of
call(Condition)and Action, otherwise execute Else. The construct is known under the name if/3 in some other Prolog implementations.
The construct A
*->B, i.e., without an Else branch, the semantics is the same as (
This construct is rarely used. An example use case is the implementation of optional in sparql. The optional construct should preserve all solutions if the argument succeeds as least once but still succeed otherwise. This is implemented as below.
optional(Goal) :- ( Goal *-> true ; true ).
Now calling e.g.,
optional(member(X, [a,b]))has the solutions X=a and X=b, while
optional(member(X,))succeeds without binding X.
- [ISO]\+ :Goal
- True if‘Goal' cannot be proven (mnemonic:
refers to provable and the backslash (
) is normally used to indicate negation in Prolog).
Many Prolog implementations (including SWI-Prolog) provide not/1. The not/1 alternative is deprecated due to its strong link to logical negation.