# Why no if-but-not-else construct? **URL:** \ **Category:** Predicate\ **Tags:** discussion\ **Created:** [July 9, 2021, 2:52pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149 "2021-07-09T14:52:25Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![dmchurch](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/dmchurch/32/3427_2.png) [@dmchurch](https://swi-prolog.discourse.group/u/dmchurch)\ **Post date:** [July 9, 2021, 2:52pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/1 "2021-07-09T14:52:25Z")
I often find myself using the `(P -> Q; true)` formulation, when I need to do an extra check or take an extra action _but only_ when a certain precondition is true; when it isn’t, the clause can proceed as it would otherwise. Why isn’t there a built-in operator or predicate that does this? Perhaps something that binds tighter than `,/2`, like say: ```prolog :- op(800, xfy, ?). % Whenever P succeeds, then Q must succeed, but \+P is fine P ? Q :- P *-> Q; true. % Match *or* generate "(At)" in text, when At is an atom parenthesized_atom(At) --> `(`, {ground(At) ? atom_codes(At,AC)}, string(AC), `)`, {atom_codes(At,AC)}. ``` Note that this isn’t a “how can I do this, please” - I have a number of ways I _can_ implement this, and it’s not all that cumbersome (and more to the point, I want to make sure that I am writing _idiomatic_ Prolog). It’s more of a “why was Prolog designed this way?” combined with some amount of “are there hidden pitfalls to doing this kind of thing that I’m not aware of?” ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [July 9, 2021, 6:18pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/2 "2021-07-09T18:18:57Z")
I don’t know the reason (I have a couple of theories), but I do know that Logtalk’s linter complains about `(P->Q)` but doesn’t complain about `(P->Q;fail)`. Paulo Moura (author of Logtalk) doesn’t seem to be in this discussion group, but maybe somebody knows the reason? ---
**Author:** ![swi](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@swi](https://swi-prolog.discourse.group/u/swi)\ **Post date:** [July 9, 2021, 6:33pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/3 "2021-07-09T18:33:03Z")
I think the reason is that from a logical point of view, `P -> Q` is seen as (in logic) `P implies Q`. Now, when `P implies Q` (in logic), you are not saying anything at all about what happens if `not P` (you could still have `Q` even if `not P`), and therefore I think it would be sensible from this point of view to always specify what happens if `not P` when you are implementing a programming language. ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [July 10, 2021, 4:42pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/7 "2021-07-10T16:42:24Z")
Almost every time I use `(P->Q;true)`, `Q` is something “impure”, such as a print statement – I have more examples of `(P->true;Q)`. So, maybe it’s a Good Thing that there’s no short-cut for `(P->Q;true)` because it’s a “[code smell](https://en.wikipedia.org/wiki/Code_smell)”. ---
**Author:** ![swi](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@swi](https://swi-prolog.discourse.group/u/swi)\ **Post date:** [July 10, 2021, 8:48pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/8 "2021-07-10T20:48:47Z")
> [@peter.ludemann](#): > > maybe it’s a Good Thing that there’s no short-cut for `(P->Q;true)` because it’s a “[code smell](https://en.wikipedia.org/wiki/Code_smell)”. Yes, I don’t often need this construct. On the other hand `(P->true;Q)` is much more common because `P` unifies what needs to be unified. ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [July 13, 2021, 4:34am UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/10 "2021-07-13T04:34:34Z")
`(bad_input(X)->throw(error(bad_input(X), _));true)` … this is similar to must\_be/2 or domain\_error/2. Although there’s been a long discussion about failure vs throwing an exception. Another useful case: In a DCG: `({some_test(X)}->[X];[])`. ---
**Author:** ![dmchurch](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/dmchurch/32/3427_2.png) [@dmchurch](https://swi-prolog.discourse.group/u/dmchurch)\ **Post date:** [July 14, 2021, 3:06pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/13 "2021-07-14T15:06:54Z")
> [@anon95304481](#): > > Quizz, whats the result of these in terms of choice points. > Can you answer without running [SWI-Prolog](https://www.swi-prolog.org/pldoc/doc_for?object=manual)? Ooh fun, I’ll play! Let’s see. I’ll mark the `true`s that get executed with `%%%%` ```prolog ?- ((true->true),true;true). % %%%% %%%% %%%%^ Choice point here, this isn't a ->; construct ?- ((true->true);true). % %%%% %%%% No choice points, the -> cuts the ; choice ?- ((true->true);true;(true->true)). % %%%% %%%% ^ Choice point here, the second ; doesn't get cut ?- (((true->true);true);(true->true)). % %%%% %%%% ^ Choice point here, syntactically equivalent to the above ``` I _think_ these are correct. Not so sure about #3, but I’m pretty sure the precedence rules group on the left `;/2`, so only that one gets cut by the `->/2`. Well? How’d I do? 😄 ---
**Author:** ![dmchurch](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/dmchurch/32/3427_2.png) [@dmchurch](https://swi-prolog.discourse.group/u/dmchurch)\ **Post date:** [July 14, 2021, 3:16pm UTC](https://swi-prolog.discourse.group/t/why-no-if-but-not-else-construct/4149/15 "2021-07-14T15:16:35Z")
Ah! I had my precedence rules for `;/2` backward. I always have trouble with those… which is why `write_canonical/1` is my best friend 😄