Reif cookbook #3100

dougransom · 2025-09-24T23:10:43Z

A few examples on using reif.

learn/reif_examples.dj

hurufu · 2025-09-25T16:54:47Z

learn/reif_examples.dj

+## if_/3
+
+if_1 calls predicate If_1,  adding on the last argument the predicate expects, which is a boolean value, T.
+if T then calls the predicate Then_0 
+otherwise, calls the predicate Else 0.  


I'm not an expert, but IMO the point of if_/3 is "indexing" it doesn't do any magic you can do everything it does while using dif/2, but it cleverly reduces amount of choice points – which is a real deal.

I am less of an expert. I understand these predicates are also likely to be more monotonic or pure and therefore harder to write incorrect programs as well as some efficiency considerations. You want to add some language?

FWIW I don’t think “indexing” is explained adequately either.

My goal at this point is to provide enough examples that make it easier to get going using reif for the basics.

I might (or any other contributor might) add more examples with an explanation of benefits such as efficiency or monotonicity.

My goal at this point is to provide enough examples that make it easier to get going using reif for the basics.

Totally. That's why I don't think it helps to explain in terms of jargon like "indexing". I think you're right that if_ should be understand in terms of what it gets YOU, the programmer, in terms of correctness.

…prolog into reif_cookbook

UWN

I recommend to read the cited article first. Not conforming to the naming convention, starting with an incorrect example is not helpful for a tutorial. Instead the major focus should be on using the existing conditions.

learn/reif_examples.dj

rotu · 2025-10-01T00:09:05Z

You keep using the term "reify" in many different senses, which complicates understanding. Are we reifying a predicate? A goal? A condition? A truth value? A definition? A concept?

I would stick to only talking about "reified predicates" or predicates "reified by a variable".

In reif the further conventions hold (and I think this is what you mean by the phrase "reif predicates"):

There are two truth values, corresponding to the atoms true and false. This is not trivial - the SICStus manual defines a reified constraint as having truth value 0 or 1.
The predicates in reif are all reified to their last argument.

…prolog into reif_cookbook

dougransom · 2025-10-06T00:03:33Z

You keep using the term "reify" in many different senses, which complicates understanding. Are we reifying a predicate? A goal? A condition? A truth value? A definition? A concept?

I would stick to only talking about "reified predicates" or predicates "reified by a variable".

In reif the further conventions hold (and I think this is what you mean by the phrase "reif predicates"):

There are two truth values, corresponding to the atoms true and false. This is not trivial - the SICStus manual defines a reified constraint as having truth value 0 or 1.

The predicates in reif are all reified to their last argument.

I think I have made this more clear.

dougransom · 2025-10-06T00:04:30Z

Ouptput from doclog, so you don't have to run it yourself:

reif_examples.html

dougransom · 2025-10-08T19:42:10Z

Is this ok now for a first cut? I plan to add to it in another version one i understand how to use (,) and (;). I’d like to get this published first.

triska · 2025-10-08T20:04:00Z

Is there any need to mention (->)/2 in this text, especially so close to the beginning? Or (is)/2, which is mentioned even earlier? These constructs are very hard to understand, and (->)/2 easily leads to elementary mistakes in programs.

dougransom · 2025-10-09T18:31:53Z

Is there any need to mention (->)/2 in this text, especially so close to the beginning? Or (is)/2, which is mentioned even earlier? These constructs are very hard to understand, and (->)/2 easily leads to elementary mistakes in programs.

Probably not. Honestly i never tried (->) before writing this tutorial. I’ll address that. I may reintroduce-> and (is) in a later version to illustrate the problems with them.

UWN

Here is a good example of defining reifying membership: https://stackoverflow.com/q/79275439/772868

rotu · 2025-10-30T05:08:01Z

Would FizzBuzz be a good example of how to use reification meaningfully?

e.g.

?- use_module(library(reif)).
?- use_module(library(clpz)).
?- N=3,
(0 is N mod 3 -> Fizz = true; Fizz = false),
(0 is N mod 5 -> Buzz = true; Buzz = false),
( Fizz,Buzz -> Answer = "FizzBuzz"
; Fizz -> Answer = "Fizz"
; Buzz -> Answer = "Buzz"
; number_chars(N, Answer)).

Versus a version using reif, which has the advantage that the clause order doesn't matter and you can backtrack through different solutions of N with the semicolon key:

?- use_module(library(reif)).
?- use_module(library(clpz)).
?- (N=2;N=3;N=6;N=15),
#=(0, N mod 3, Fizz),
#=(0, N mod 5, Buzz),
(
 Fizz=true,Buzz=true,Answer="FizzBuzz"
;Fizz=true,Buzz=false,Answer="Fizz"
;Fizz=false,Buzz=true,Answer="Buzz"
;Fizz=false,Buzz=false,number_chars(N, Answer)
).

Can the above be improved maybe? I don't think that 4-way fork can be simplified with use of if_ but maybe I'm wrong.

triska · 2025-10-30T05:35:16Z

Personally, I think a good program makes clear that it follows the specification. Therefore, if we have for example the following specification of FizzBuzz I found online:

Given an integer n, for every positive integer i <= n, the task is to print,

    "FizzBuzz" if i is divisible by 3 and 5,
    "Fizz" if i is divisible by 3,
    "Buzz" if i is divisible by 5
                   "i" as a string, if none of the conditions are true.

Then what about this program:

i_output(I, Os) :-
    if_((I mod 3 #= 0, I mod 5 #= 0),
        Os = "FizzBuzz",
        if_(I mod 3 #= 0,
            Os = "Fizz",
            if_(I mod 5 #= 0,
                Os = "Buzz",
                number_chars(I, Os)))).

Yielding for example:

?- N = 15, between(1, N, I), i_output(I, Os).
   N = 15, I = 1, Os = "1"
;  N = 15, I = 2, Os = "2"
;  N = 15, I = 3, Os = "Fizz"
;  N = 15, I = 4, Os = "4"
;  N = 15, I = 5, Os = "Buzz"
;  N = 15, I = 6, Os = "Fizz"
;  N = 15, I = 7, Os = "7"
;  N = 15, I = 8, Os = "8"
;  N = 15, I = 9, Os = "Fizz"
;  N = 15, I = 10, Os = "Buzz"
;  N = 15, I = 11, Os = "11"
;  N = 15, I = 12, Os = "Fizz"
;  N = 15, I = 13, Os = "13"
;  N = 15, I = 14, Os = "14"
;  N = 15, I = 15, Os = "FizzBuzz".

We can test it also in other ways, for instance: Which integers yield "Fizz"?

?- length(_, I), catch(i_output(I, "Fizz"), error(syntax_error(_),_), false).
   I = 3
;  I = 6
;  I = 9
;  I = 12
;  I = 18
;  I = 21
;  I = 24
;  I = 27
;  I = 33
;  I = 36
;  ... .

rotu · 2025-10-30T06:49:40Z

I think that's reasonable but I still don't love the if_s nested 3 deep. Is there a reasonable alternative?

The more legible but less logically pure alternative is the following, and I imagine there must be a better pure option:

i_output_bad(I, Os) :-
  (I mod 3 #= 0, I mod 5 #= 0) -> Os = "FizzBuzz"
; (I mod 3 #= 0) -> Os = "Fizz"
; (I mod 5 #= 0) -> Os = "Buzz"
; number_chars(N, Os).

bakaq · 2025-10-30T13:13:11Z

match/2 from my reified pattern matching library Qupak helps flatten this thing while remaining pure. It was primarily made to do patterns, but you can abuse guards to do cond style things like this. This code is basically equivalent to the 3 deep if_/3 ladder that @triska showed earlier:

number_canonchars(N, Cs) :-
    number_chars(N, Cs),
    number_chars(N, Cs1),
    Cs1 = Cs.

fizzbuzz(N, Fizzbuzz) :-
    match(N, [
        ((*) | #N mod 3 #= 0, #N mod 5 #= 0) ~> Fizzbuzz = "FizzBuzz",
        ((*) | #N mod 3 #= 0) ~> Fizzbuzz = "Fizz",
        ((*) | #N mod 5 #= 0) ~> Fizzbuzz = "Buzz",
        (*) ~> number_canonchars(N, Fizzbuzz)
    ]).

?- length(_,N), fizzbuzz(N, Fb).
   N = 0, Fb = "FizzBuzz"
;  N = 1, Fb = "1"
;  N = 2, Fb = "2"
;  N = 3, Fb = "Fizz"
;  N = 4, Fb = "4"
;  N = 5, Fb = "Buzz"
;  N = 6, Fb = "Fizz"
;  N = 7, Fb = "7"
;  N = 8, Fb = "8"
;  N = 9, Fb = "Fizz"
;  N = 10, Fb = "Buzz"
;  N = 11, Fb = "11"
;  N = 12, Fb = "Fizz"
;  N = 13, Fb = "13"
;  N = 14, Fb = "14"
;  N = 15, Fb = "FizzBuzz"
;  N = 16, Fb = "16"
;  ... .
?- length(_, I), catch(fizzbuzz(I, "Fizz"), error(syntax_error(_),_), false).
   I = 3
;  I = 6
;  I = 9
;  I = 12
;  I = 18
;  I = 21
;  I = 24
;  ... .

This also made me realize a bug on my handling of modules, which was kinda expected given the cursed things I'm doing with them here.

rotu · 2025-10-30T21:39:21Z

That Qupak example is cool, but it's bringing in a lot!

I wonder if it makes sense to think of reif as something like a monad. Is this cursed?

:- meta_predicate(lift_t(0, ?)).
lift_t(G, true) :- G.

:- meta_predicate(eval_reif(1)).
eval_reif(G_1) :- call(G_1, true).

:- meta_predicate('->'(1, 0, ?)).
'->'(If_1, Then_0, T) :- cond_t(If_1, Then_0, T).

fizzbuzz(I, Os) :- eval_reif((
    (I mod 3 #= 0, I mod 5 #= 0) -> Os = "FizzBuzz"
  ; (I mod 3 #= 0) -> Os = "Fizz"
  ; (I mod 5 #= 0) -> Os = "Buzz"
  ; lift_t(number_chars(I,Os))
)).

% e.g.:
% N = 15, between(1, N, I), fizzbuzz(I, Os).
% length(_, I), catch(fizzbuzz(I, "Fizz"), error(syntax_error(_),_), false).

doclog.config.pl

dougransom · 2025-11-15T19:49:07Z

I think that's reasonable but I still don't love the if_s nested 3 deep. Is there a reasonable alternative?

The more legible but less logically pure alternative is the following, and I imagine there must be a better pure option:
i_output_bad(I, Os) :-
  (I mod 3 #= 0, I mod 5 #= 0) -> Os = "FizzBuzz"
; (I mod 3 #= 0) -> Os = "Fizz"
; (I mod 5 #= 0) -> Os = "Buzz"
; number_chars(N, Os).

Is that what cond_t is for? if without the else?

from mthom#3100 (comment)

dougransom · 2025-11-15T20:04:32Z

Is there any need to mention (->)/2 in this text, especially so close to the beginning? Or (is)/2, which is mentioned even earlier? These constructs are very hard to understand, and (->)/2 easily leads to elementary mistakes in programs.

removed the offending (->) and is.

dougransom · 2025-11-15T20:05:45Z

Personally, I think a good program makes clear that it follows the specification. Therefore, if we have for example the following specification of FizzBuzz I found online:

Given an integer n, for every positive integer i <= n, the task is to print,
"FizzBuzz" if i is divisible by 3 and 5,
"Fizz" if i is divisible by 3,
"Buzz" if i is divisible by 5
               "i" as a string, if none of the conditions are true.
Then what about this program:

i_output(I, Os) :-
if_((I mod 3 #= 0, I mod 5 #= 0),
Os = "FizzBuzz",
if_(I mod 3 #= 0,
Os = "Fizz",
if_(I mod 5 #= 0,
Os = "Buzz",
number_chars(I, Os)))).
Yielding for example:

?- N = 15, between(1, N, I), i_output(I, Os).
N = 15, I = 1, Os = "1"
; N = 15, I = 2, Os = "2"
; N = 15, I = 3, Os = "Fizz"
; N = 15, I = 4, Os = "4"
; N = 15, I = 5, Os = "Buzz"
; N = 15, I = 6, Os = "Fizz"
; N = 15, I = 7, Os = "7"
; N = 15, I = 8, Os = "8"
; N = 15, I = 9, Os = "Fizz"
; N = 15, I = 10, Os = "Buzz"
; N = 15, I = 11, Os = "11"
; N = 15, I = 12, Os = "Fizz"
; N = 15, I = 13, Os = "13"
; N = 15, I = 14, Os = "14"
; N = 15, I = 15, Os = "FizzBuzz".
We can test it also in other ways, for instance: Which integers yield "Fizz"?

?- length(, I), catch(i_output(I, "Fizz"), error(syntax_error(),_), false).
I = 3
; I = 6
; I = 9
; I = 12
; I = 18
; I = 21
; I = 24
; I = 27
; I = 33
; I = 36
; ... .

Added this example since it shows pretty well how to use if_.

dougransom · 2025-11-15T20:13:22Z

Can it be merged now, at least as the first version? Who is making the decision about which PRs are merged? I presume @mthom?

rotu

Many typos. I'm sure some things can be better explained but I think this is a good start and definitely better than leaving reif undocumented!

learn/reif_examples.dj

rotu · 2025-10-30T05:17:30Z

learn/reif_examples.dj

+What is happening is that reif has provided (=)/3, and that is what is being used in tfilter (not the usual (=)/2).   
+
+But (in my system at the time of writing) (#>)/2 exists, but there is no (#>)/3.  
+A workaround is to define a three-arity predicate to do the comparison (it could be (#>)/3, but I choose pound\_gt\_t):


pound_gt_t is just miserable. What's wrong with #>? It's already an operator as defined by clpz.

Otherwise, I'd use greater_t or gt_t.

Because (#>)/3 was not available on my system despite loading clpz and reif. That quite confused me and likely any other Prolog beginner.

I also think it is likely (#>)/3 will one day be available so I didn't want to define one.

Maybe even one day very soon... #3163

I didn't mean to cause any trouble :).

learn/reif_examples.dj

Co-authored-by: Dan Rose <DanOfTheRoses@gmail.com>

…prolog into reif_cookbook

Doug and others added 4 commits September 22, 2025 10:47

added examples.

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dougransom mentioned this pull request Sep 24, 2025

Document some predicates in reif #2981

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updated based on pr feedback

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Merge branch 'mthom:master' into reif_cookbook

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incorporation some feedback.

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Handle modules properly in match patterns bakaq/qupak#2

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Doug added 3 commits November 15, 2025 05:03

removed ->

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from mthom#3100 (comment)

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tweaked some language, added a missing module load in fizzbuss.

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called partial goals partial goals.

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Reif cookbook #3100

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Reif cookbook #3100

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