Problem with if->then & cut

[infradig]

This is ok...

$ scryer-prolog
?- [user]. main :- (true;true), writeln(X), (true->!;!), fail. main:-writeln(oops). end_of_file.
% Warning: singleton variables X at line 0 of user
?- main.
_16515
   false.

~/trealla (devel) $ tpl
?- [user]. main :- (true;true), writeln(X), (true->!;!), fail. main:-writeln(oops). end_of_file.
   true.
?- main.
_0
   false.
?- 

but...

$ scryer-prolog
?- [user]. a(1). a(2). main :- a(X), writeln(X), (true->!;!), fail. main:-writeln(oops). end_of_file.
?- main.
1
   false.
?- 
~/trealla (devel) $ tpl
?- [user]. a(1). a(2). main :- a(X), writeln(X), (true->!;!), fail. main:-writeln(oops). end_of_file.
   true.
?- main.
1
oops
   true.

[flexoron]

Variant of 'This is ok...' from above:

$ scryer-prolog -f
?- [user]. main :- (true;true), write(X), (!->true;!), fail. main:-write(oops). end_of_file.
% Warning: singleton variables X at line 0 of user
?- main.
_78   false.
?- halt.

% This seems not ok...
$ tpl
?- [user]. main :- (true;true), write(X), (!->true;!), fail. main:-write(oops). end_of_file.
   true.
?- main.
_0_0oops   true.
?- 

As you say the problem is within 'if' and 'then' but not with the 'else' cut

?- [user]. main :- (true;true), write(X), ((\+!)->true;!), fail. main:-write(oops). end_of_file.
Warning: overwriting 'main'/0
   true.
?- main.
_0   false.

[infradig]

Scryer is wrong here...

$ scryer-prolog -f
?- [user]. main :- (true;true), write(X), (!->true;!), fail. main:-write(oops). end_of_file.
% Warning: singleton variables X at line 0 of user
?- main.
_78   false.
?- halt.

and Trealla right. The if is cut-proof.

[flexoron]

Learning never stops.
Another dissent on the issue:

$ scryer-prolog -f
?- [user]. a(1). a(2). main :- (a(X);b(X)), write(X), nl, (true -> !;!), fail. main:-write(oops). b(3).  end_of_file.
?- main.
1
   false.
?- halt.

$ tpl
?- [user]. a(1). a(2). main :- (a(X);b(X)), write(X), nl, (true -> !;!), fail. main:-write(oops). b(3).  end_of_file.
   true.
?- main.
1
3 % Scryer does not print b(X).
   false.
?- halt. 

$ tpl
?- [user]. a(1). a(2). main :- (a(X);b(X)), write(X), nl, (true -> !;!), fail. main:-write(oops). b(3). b(4).  end_of_file.
   true.
?- main.
1
3
oops   true. % a second b/1 restores the issue
?-