Is this a good pure way to deal with side-effects?

I was looking for a way to manage side-effects but hopefully without using non-logical predicates like assert and retractall. Basically, I’m trying to mimic this Python code:

x = 3
x = 'something-else'
x = 5

I’ve came up with this code for Prolog:

% get_value(Value, History)
% History tracks values set for variable
% uses an arbitrary term t/1 just to allow adding a regular logical variable to the List
get_value(Val, List) :-
	nonvar(List), % just so that fails when nothing added yet.
	nextto(t(Val), Var, List),

available_spot(X, History) :-
	once((member(X, History), var(X))).

add_value(Val, History) :-
  available_spot(X, History),
  t(Val) = X.

Some example outputs:

% add one value for variable and read it back:
?- add_value(abc, History), get_value(Current, History).
History = [t(abc), _5110|_5112],
Current = abc.

% works for multiple "re-assignments" to same variable:
?- add_value(3, History), get_value(FirstWas, History), add_value('my-symbol', History), get_value(SecondWas, History), add_value(2, History), add_value(AVar, History), get_value(LatestIs, History), LatestIs = xyz.
History = [t(3), t('my-symbol'), t(2), t(xyz), _1946|_1948],
FirstWas = 3,
SecondWas = 'my-symbol',
AVar = LatestIs, LatestIs = xyz.

% fails when no value was previously set (as intended)
?- get_value(X, History).

So, it seems to be doing the trick! We can even go back to previous states if needed. But I don’t recall seeing anything like this from experts! So I’d like your advice:

  1. Does this idea make any sense?
  2. How do you manage state if you absolutely want to maintain logical purity?
  3. How would you improve this? (e.g maybe member/2 isn’t the fastest)

I’ll keep this gist updated in the future: Pure way for tracking state of variables in Prolog · GitHub

Perhaps you have something like this in mind:

For compling something like this nowadays, the code would be typically transformed into “static single assignment (SSA) form”, so it’d become internally

x1 = 3
x2 = 'something-else'
x3 = 5

(There’s also a φ operator for joining if-then-else’s and loops)

As a matter of good engineering practice, I’ve found that with imperative languages, keeping as close as possible to SSA form is beneficial (in fact, I recently fixed a subtle bug in some Python code that was due to reuse of a variable).

Without a clear purpose for the code you showed:

it is impossible to give useful advice.

The knee-jerk response to this is, “don’t do it”. But you must have a reason to do it; so what is it? The code you show looks a bit like a stack?

What kind of side-effect do you have in mind? Setting a variable to a value is not strictly a side effect, unless a change in the value triggers a side effect of some kind?

Could it be that you would like to keep some state?

Have you tried using backtrackable assignment using setarg/3 for your purpose?

Thanks Daniel. I’m not familiar with Logtalk but it seems to do the equivalent, right? Is that approach better than using member/2 ?

Thanks, Peter.

I’ve found that with imperative languages, keeping as close as possible to SSA form is beneficial

I only meant that as an example of side-effects from the user’s point of view.


Without a clear purpose for the code you showed

Yeah I should have been more specific.
What I had in mind is like implementing a memory simulator where you’d need to change variables values often (or something like implementing a virtual machine, ignoring memory efficiency).
I was hoping to find a pure logical way to handle such cases.

Have you tried using backtrackable assignment using setarg/3

I wasn’t aware of that. It says it’s extra-logical but supports backtracking. I’d like to follow the conventional wisdom of maintaining declarative and pure Prolog code.I will look into that today.


I once rewrote the logtalk code to swi prolog – by essentially turning the object into a module – its equivalent.

re: member/2

Not sure how you mean to use member – but, i guess, the variable manages the history in a list, so its likely similar.


Are those “variable values” locations in (addressable) memory? Or are they registers? Or a stack?

Yeah like registers and memory but please treat my example as a hypothetical.

:slight_smile: the issue with hypothetical questions is that you can’t really get concrete answers. Some people react by writing ad-hoc monographs, but I am not too fast at typing so that is too difficult for me.

I am not completely sure what you mean by “pure logical way”. Some would claim that using a data structure like the AVL-tree implementation in library(assoc) is purely logical. It is a key-value store, and you change a value at a key without too much typing. Take a look at it.

For an “array” which is addressed by an offset (not by an arbitrary term) I would prefer a flat term, and setarg/3 or even nb_setarg/3 or even nb_linkarg/3.

But this is all just guessing.

Or library(rbtrees). Or dicts. nb_setarg/3 is non-logical and is not undone on backtracking; setarg/3 is undone by backtracing. b_setval/2 is a global version.

Rbtrees have worked fine for me (e.g., a symtab that I pass around as a “hidden” argument, using EDCGs) - the performance issues haven’t been in updating or lookup (so far). I’d avoid global variables, as a matter of good engineering practice.

BTW, there’s an implementation of fast fourier transforms on page 172 of these set of benchmarks (fft is by Richard O’Keefe, I think): (there might be a better version of the code elsewhere).

1 Like

Yes, there are quite a few different key-value store implementations available in SWI-Prolog.

Dicts are SWI-Prolog specific.

library(rbtrees) and library(nb_rbtrees) that goes with it are great and I use them. They are a bit difficult to discover by browsing the website; and the docs don’t have a polished feel.

Sounds a lot like fluents to me. A fluent is a fact that is true at some time or in some situation and not at others. So if we have an x fluent you could re-write your example as:

holds(x(3), line(1)).
holds(x(something_else), line(2)).
holds(x(5), line(3)).

Of course, you don’t want to be using lines to denote your time/situation and there’s other ways to write them, but I think this makes the idea clear. Usually you’d number discrete situations because dealing with continuous times is more complicated. Now you have 3 options for how you carry fluents around.

  • You can keep asserting them to the knowledge base, in which case you get fast look-ups and the history, but you need to track the current situation and you’ll either be asserting every fluent for every change of state or keeping a history term for look-ups into the past if the fluent isn’t asserted for the situation you want, slowing down the look-ups.

  • You can keep a collection of them in a data-structure, like the trees being discussed here, in which case you get reasonably fast look-ups but you loose the history if you’re not keeping all versions of the trees around. This is a popular choice because you’re mutation free and you can use this in a multi-threaded environment by passing the tree around.

  • Or you can keep the history of the events around that changed the state and query that with regards to what fluents hold (as per Golog in the next paragraph). This gives the slowest look-ups, but you get the history to query (if you need it).

For the logical side, people have been wanting to reason about changes in state and the events that cause these changes in AI for a long time. There are entire calculus’ dedicated to doing this, from Situation Calculus to Event Calculus and Fluent Calculus. None are perfect, each have their strengths. Raymond Reiter published a language called “Golog” which had a simple interpreter in Prolog that included a Situation Calculus reasoner. His book “Knowledge in Action” is invaluable if you want to get into this topic, “Commonsense Reasoning” is the book to get for Event Calculus. I’ve done a Situation Calculus reasoner in Logtalk, so you can use it with SWI (I usually do!). There’s RTEC which is a work-in-progress towards an Event Calculus reasoner in Prolog, and I think there’s a non-realtime Prolog one too that I’ve forgotten the name of.

The key additional benefit of using a calculus like these is that you also define how the state is updated (actions/events) and you get a language for querying the state. This mirrors what’s gone on in the database-world and industry with Event Sourcing and CQRS, but with a logical language rather than a collection of good ideas recognised through lessons learnt.

If you just want to have a set of ‘variables’ with their values changing, you can always use dicts:

State1 = state{ x: 1, y: "string" },
State2 = State1.put(y, "new string")

This lets you keep a state that is easy to update. To pass this state around implicitly you can always use a DCG:

eval(noop) --> [].

eval(Var += 1), [NewState] -->
  Value #= State.Var + 1,
  NewState = State.put(Var, Value).

You can also look at the pack EDCG that lets you write DCGs that have more than one threaded value, meaning you could use it to have multiple ‘updatable’ registers. Just be careful about backtracking

Looks like I need to read more about these. There are a lot to digest in this thread. Thanks all for your input.

What do you mean by that?

I doubt local variables “writes” are side effects?
Which other thread will see these “writes”?

The ISO core standard possibly defines side effect
differently. Here are some examples:

  • I/O, writing/reading to/from a file, if you restart the same
    process/thread the outcome need not anymore be same.
  • message passing between threads, depending what gets
    into your inbox as an actor, your threads outcome possibly differs.

On the other hand if you translate an imperative idiom
into something declarative, you get a function or a non-deterministic
function aka relation, that behaves always the same.

Picat is a logic programming languages that translates
a variety of imperative idioms like assignment, if-then-else
and loops. Here is a Picat example:

sum_list(L) = Sum =>
    foreach (X in L)

How does picat treat this …

Is this internally unrolled into two logic variables – a before and after – like DCG …

I assume that the body can be backtracked over …

The Art of Prolog, Shapiro, last chapter:


You just stop somewhere in the compilation pipeline, instead of generating some assembler, you generate some Prolog. Its explained in the Picat documentation how Picat does it, and it should be open source somewhere.

I see …

I didn’t know that Picat is simply a preprocessor for Prolog.

So, its in the category of languages like Logtalk – language that “compiles” to Prolog – i thought it would have its own optimized VM or compiler.

The rewriting is described here in this paper. It rewrites to pattern
matching rules. But underneath there is also B-Prolog somewhere.

Canonicalizing High-Level Constructs in Picat