13.20.4 Changes Related to Standard Conformance

• throw/1 now throws its argument as is. raise_exception/1 however transforms SICStus error terms into ISO (error/2) exceptions. Unless a forged SICStus error term is thrown by throw/1, the net behavior is unchanged.
• In cases where the ISO and SICStus error term previously had different principal functors, the SICStus error term has been changed to correspond to the ISO one.
• subsumes_term/2 is a new standard built-in predicate, aliased by terms:subsumeschk/2, and is now correct in case the arguments share variables.
• term_variables/2 is a new standard built-in predicate, aliased by terms:term_variables_bag/2, and performs conformant error handling. terms:term_variables/2 has been renamed to terms:term_variables_set/2.
• acyclic_term/2, previously in library(terms), is now a standard built-in predicate.
• The built-in predicates callable/1, ground/1, retractall/1, call/n, false/0 are now standard.
• The built-in predicates number_codes/2, number_chars/2, atom_codes/2, and atom_chars/2 now perform conformant error handling. The syntax for numbers has changed (plus sign is no longer allowed¹). Full ISO syntax allowed in Codes/Chars, including comments and whatnot.
• The built-in predicates length/2, keysort/2, sort/2, compare/3, and open/[3,4] now perform conformant error handling.
• The built-in predicates call/1, assert/1 and similar predicates now perform conformant error handling when encountering an invalid goal.
• X div Y is a new standard arithmetic expression, whose value is the integer quotient of X and Y, rounded downwards to the nearest integer.
• xor(X,Y) is the standard syntax for X \ Y.
• X ^ Y is a new standard arithmetic expression, whose value is X raised to the power of Y, represented as a float if any of X and Y is a float; otherwise, as an integer.
• pi is a new standard arithmetic expression, whose value is approximately 3.14159.
• + is a new standard prefix operator declared as :-op(200, fy, [+]).
• ‘|’ can now be declared as an operator. The default, when it has not been declared as an operator, is to treat it like before, i.e. as a synonym for infix ‘;’ when used as an operator. If ‘|’ is declared as an operator, you can no longer use ‘|’ to denote disjunction in clause bodies and grammar rules.
• Arithmetic error handling is now conformant. Previously, in many cases a domain error would be thrown where an evaluation error was required.
• Quoted atoms are no longer written with the non-conformant escape sequences ‘\e’ and ‘\d’. They are still allowed when reading, though.
• Operators as immediate argument of an operator are now bracketed when written, as required by the standard. E.g. writeq(^(*,\+)) outputs ‘(*)^(\+)’.
• clause/[2,3] and retract/1 return a close equivalent to the asserted clause, as required by the standard. This also affects listing/[0,1].
• The read_term/[2,3] option singletons/1 is now conformant. It now includes all named variables that occur only once in the read term, as required by the standard. It used to exclude some variables for which singleton warnings should not be emitted, e.g. ‘_A’. Any such filtering must now be performed by the caller instead.
• The read_term/[2,3] options variable_names/1 and singletons/1 now order their entries by the order the variable occurred in the input term, i.e. in the same order as the variables/1 option. In previous releases the order was undefined, and unpredictable. The new ordering is permitted, but not (yet) required, by the standard.
• The Prolog parser, read_term/[2,3], now correctly parses some valid input that used to give syntax error.
• The write_term/[2,3] option numbervars(true) (also used by writeq/[1,2] and others), is now standard compliant and only affects terms $VAR(N) when N is a non-negative integer. In particular it no longer does special processing when N is an atom.

  To supply names for variables when writing a term you can use the new option variable_names/1 that takes a list of Name=Variable pairs, exactly like the list produced by the read_term/[2,3] option variable_names/1.

  To get the legacy behavior, where '$VAR'(Name) is written as Name when Name is an atom, you can use the new option legacy_numbervars(true) which behaves like numbervars(true) did in earlier releases.

• A negative zero floating point value is now written as ‘0.0’, rather than ‘-0.0’. There is no difference in behavior between these two zero values so the distinction should no longer be detectable by Prolog code.