Prolog also has constraints! You need to import a library called CLP(FD) / CLP(Z) (depending on the Prolog system) and instead of =, <, =<, etc. you need to use

Prolog also has constraints! You need to import a library called CLP(FD) / CLP(Z) (depending on the Prolog system) and instead of =, <, =<, etc. you need to use #=, #<, #=<, etc. :- use_module(library(clpfd)). min(Min, Y, Min) :- Min #=< Y, !. min(X, Min, Min) :- Min #=< X. find_min_and_subtract_([], [], Min, Min). find_min_and_subtract_([H|T], [R|Rs], TempMin, Min) :- min(TempMin, H, NewTempMin), R #= H - Min, find_min_and_subtract_(T, Rs, NewTempMin, Min). find_min_and_subtract([H|Hs], Rs, Min) :- find_min_and_subtract_([H|Hs], Rs, H, Min). With that (and an additional variable for the temporary minimum) it works for me: ?- find_min_and_subtract([9,2,4,12], Rs, Min). Min = 2, Rs = [7, 0, 2, 10] https://swish.swi-prolog.org/p/qgchfMfB.pl https://swish.swi-prolog.org/p/qgchfMfB.pl