Tips for golfing in Prolog on codegolf.stackexchange.com

Use feature(argv,A) or '$argv'(A) instead of prolog_flag(argv,A).
'$argv'(A) has extra arguments at the beginning, you can filter them by index:

:-feature(argv,A),nth0(I,A,X),code(X).
:-'$argv'(A),nth0(I,A,X),I>3,code(X).

unix(argv(A)) is equivalent to '$argv'(A), but arguments that are syntactically valid numbers are passed as a number. You can filter them using number(X) or atom(X).

Because integers are less than atoms in the standard ordering of terms, X@<'' (or maybe X@<a if you're really lucky) works as a shorter alternative to number(X).

re_split with /n flag converts output atoms to numbers if possible:

:- S='2 3 4', re_split(' '/n,S,L), L=[2,' ',3,' ',4].

Instead of using maplist over an array, you can use member with careful backtracking. You can also use nth0 or nth1 to get the index of the element as well:

maplist(+,A). +A:-code.
member(V,A),code,0=1.
nth0(I,A,V),code,0=1.
nth1(I,A,V),code,0=1.

format can accept an arithmetic expression for numeric conversions.

:-X is 1+2,write(X).
:-format('~d',1+2).

:-X is 1+2,writeln(X).
:-format('~d
',1+2).

:-member(X,[2,3]),write(X),0=1.
:-member(X,[2,3]),\+write(X).

Use backticks to get a list of character codes:

:- `0{𞉀` = [48,123,123456].

Big numbers can be written using <radix>'<number> syntax or 0'<character> syntax:

:- 36'PROLOG = 1558151872.
:- 0'𞉀 = 123456.

Lists of one integer or one-character atom (e.g., [97], `a`, [a]), and strings of length 1 (e.g., "a") in arithmetic expression are interpreted as integers:

:-X is 100.
:-X is 0'd.
:-X is"d".

Predicates can be written using operators (CGSE for more details):

2+2. 1+b.
:-X+Y,write(X),\+write(Y). % prints '221b'

You can add predicates dynamically using assert:

:-X=2,Y=3,assert(X+Y),2+3,write('success'),2+5,write('unreachable').

Some predicates generate solutions if the arguments are unbound:

nth0(X,_,_).  % between(X,0,inf)
nth1(X,_,_).  % between(X,1,inf)

You don't need rdiv if at least one argument is rational:

:- X is 1/2r3. % X = 3r2

A rational number literal, rdiv result, or the value of an arithmetic expression (if it's a rational number) is converted to an integer if possible:

:- X is 4r2,      Y is 1/X. % X = 2 (integer), Y = 0.5 (float)
:- X is 4 rdiv 2, Y is 1/X. % X = 2 (integer), Y = 0.5 (float)
:- X is 4r7/2r7,  Y is 1/X. % X = 2 (integer), Y = 0.5 (float)
:- X =  4r7/2r7,  Y is 1/X. % X = 4r7/2r7 (expression), Y = 1r2 (rational)