Information technology - Programming languages - Prolog - Part 1:
General Core
WORKING DRAFT TECHNICAL CORRIGENDUM 3

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5.5.12 Options.

A processor may support one or more additional options such as stream-options (7.10.2.11), close-options (7.10.2.12), read-options (7.10.3), and write-options (7.10.4) as an implementation specific feature. An invalid option E shall be associated with only two error conditions: an instantiation error when there is an instance (3.95) of E that is a valid option, and a domain error for the domain optname_option when there is no instance of E that is a valid option. Further, an instantiation error may occur in place of a domain error if a component of E is a variable, and an instantiated component is required.

NOTE — A valid option may still imply further errors ; as in 8.11.5.3 l and m.

6.2.1 Prolog text

p text = [ layout text sequence (* 6.4.1 *) ] ;

7.1.6.3 Iterated-goal term

a) If T unifies with has the form ^(_, G) then ...

7.8.3.4 Examples

Table 21 and 22 show the execution stack before and
after executing the control construct call(G) with goal
obtained from G in step 7.8.3.1 f via 7.6.2.

Table 22

N + 1     ( (G goal, N – 1),     Σ    nil

7.10.3 Read-options list

variables(Vars) — After inputting a term, Vars shall be unified with a list of the variables in the term input, in left-to-right traversal order.

variable_names(VN_list) — After inputting a
term, VN_list shall be unified with a list of ele-
ments where: (1) each element is a term A = V, and
(2) V is a named variable of the term, and (3) A is an
atom whose name is the characters of V, and (4) the elements appear in the order of the first occurrence of their variables V in the term input, in left-to-right traversal order.

singletons(VN_list) — After inputting a term,
VN_list shall be unified with a list of elements
where: (1) each element is a term A = V, and (2) V is
a named variable which occurs only once in the term,
and (3) A is an atom whose name is the characters of V,
and (4) the elements appear in the order of the first occurrence of their variables V in the term input, in left-to-right traversal order.

7.10.4 Write-option list

variable_names(VN_list) — Each variable V is output as the sequence of characters defined by the syntax for the atom A iff a term A = V is an element of the list VN_list. If more than one element applies, the leftmost is used.

numbervars(Bool) — Iff Bool (7.1.4.2) is true a
term of the form '$VAR'(N), where N is an a non-negative integer, is
output as a variable name consisting of a capital letter
possibly followed by an integer.

NOTE 2 — Many Prolog processors provided a modified write option numbervars/1 to print arbitrary variable names. The write option variable_names/1 serves this purpose and avoids vulnerabilities.

7.10.5 Writing a term

a1) If Term is a variable and there is an effective write-option variable_names(VN_list) and there is an element A = Term of the list VN_list with A an atom, then A is output with effective write-option quoted(false).

a) If a2) Else if Term is a variable, a character sequence repre-
senting that variable is output. The sequence begins
with _ (underscore) and the remaining characters are
implementation dependent. During the execution of write_term/3, the
The same character sequence
is used for each occurrence of a particular variable in
Term. A and a different character sequence is used for each
distinct variable in Term.

e) e1) If Term has the form '$VAR'(N) for some pos-
itive non-negative integer N, and there is an effective write-option
numbervars(true), a variable name as defined in
subclause 7.10.4 is output,

g) e2) Else if Term has the form '.'(Head, Tail), and
there is an effective write-option ignore_ops(false),
then Term is output using list notation, that is:

1) [ (open list char) is output.

2) Head is output by recursively applying these
rules. Head is preceded by ( (open char)
and followed by ) (close char), if the term could not be
re-input correctly with the same set of current operators.

3) If Tail has the form '.'(H,T) then , (comma
char) is output, set Head:=H, Tail:=T, and goto (2).

4) If Tail is [] then a closing bracket ] (close list
char) is output,

5) Else a | (head tail separator char) is output,
Tail is output by recursively applying these rules,
Tail is preceded by ( (open char)
and followed by ) (close char), if the term could not be
re-input correctly with the same set of current operators,
and finally, ] (close list char) is output.

e3) Else if Term has the form '{}'(Arg), and there is an effective write-option ignore_ops(false), then Term is output as a curly bracketed term (6.3.6), that is:

1) { (open curly char) is output

2) Arg is output by recursively applying these rules

3) } (close curly char) is output

f) Else if Term has a principal functor which is not
a current operator, or if there is an effective write-
option ignore_ops(true), then the term is output in
canonical form functional notation (6.3.3), that is:

1) The atom of the principal functor is output.

2) ( (open char) is output.

3) Each argument of the term is output by recursively
applying these rules. The argument is preceded by ( (open char) and followed by ) (close char), if the term could not be re-input correctly with the same set of current operators.

4) , (comma char) is output between each successive
pair of arguments.

5) ) (close char) is output.

h) If Else if Term has a principal functor which is an
operator, and there is an effective write-option
ignore_ops(false), then the term is output in operator
form, that is:

1) The atom of the principal functor is output
in front of its argument (prefix operator), between
its arguments (infix operator), or after its argument
(postfix operator). In all cases, a space is output
to separate an operator from its argument(s) if any
ambiguity could otherwise arise.

Operators ',' and '|' are output as , (comma char) and | (bar char) respectively.

2) Each argument of the term is output by recursively
applying these rules. When an An argument is itself
to be output in operator form, it is preceded by (
(open char) and followed by ) (close char) if: (i) the
principal functor is an operator whose priority is so
high that the term could not be re-input correctly with
same set of current operators, or (ii) the argument is
an atom which is a current operator, or (iii) the principal functor is output as a prefix operator - and the argument is a non-negative number, or (iv) the principal functor is output as a prefix operator - and the argument is output in infix or postfix operator form.

7.12.2 Error classification

8.1.3 Errors

7 When a built-in predicate has an argument whose type is a list of options as input, that argument will be an +optname_options_list. That argument will always be associated with

1. an instantiation error, when the argument is a partial list, or an element is not a valid option but an instance of the element is a valid option.
2. a type error of the form type_error(list, Options), when the argument is neither a partial list nor a list.
3. a domain error of the form domain_error(optname_option, E)), when an element is neither a valid option nor any instance is a valid option.
4. additionally, an instantation error may occur in place of the domain error, if a component is a variable and an instantiated component is required.

Errors caused by a valid option are reported using different errors.

8.5.1.4 Examples

current_prolog_flag(max_arity, A),
      X is A + 1,
      functor(T, foo, X).

   If the Prolog flag max_arity has the value unbounded
      type_error(evaluable, unbounded/0)
   else
      representation_error(max_arity).

8.9.2.1 Description (assertz/1)

e) Constructs the clause with head H and body B G

8.11.4.1

b) the goal succeeds.

c/b) Options is a partial list or a list with an element
E which is a variable or which has a component
which is a variable, and an instantiated component is required.
— instantiation_error.

i/e) An element E of a list prefix of the Options list is neither a
variable nor a stream-option and there is no instance of E that is a stream-option.
— domain_error(stream_option, E).

8.14.1.1 k

k) Parses C_Seq as a read-term (6.4) (6.2.2) T.,

8.14.2.4 Examples

write_canonical([1,2,3]).
   Succeeds, outputting the characters
.(1,.(2,.(3,[ ] '.'(1,'.'(2,'.'(3,[])))
    to the current output stream.

write_term(1,[quoted(non_boolean)]).
   domain_error(write_option,quoted(non_boolean)),
write_term(1,[quoted(B)]).
   instantiation_error.
B = true, write_term(1,[quoted(B)]).
   Succeeds, unifying B with true, and outputting
   the character 1.

8.17.1.4 Examples

set_prolog_flag(date, 'July 1988').
   domain_error(flag prolog_flag, date).

9.3.1.3 c

c) VX is negative and Y VY is not.

9.3.10 (^)/2 – integer power

9.3.10.3 Errors

e) VX and VY are integers and VX is not equal to 1 and VY is less than -1 VY is negative and VX is not equal to 1, 0, or -1.
— type_error(float, VX).

9.3.10.4 Examples

2^(-1).
   evaluation_error(undefined). type_error(float, 2).

Annex B
(informative)
Editorial notes

Unusual characters

All error subclauses (X.Y.Z.3)

Mdash: —, Theta: θ, Ndash: –, Times: ×, Rightwards arrow: →, Union: ∪, Floor: ⌊ ⌋, Inequality: ≠, Logical and: ∧, Element of, not in: ∈, ∉, Pi: π Less or equal: ≤

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