作者在为《Logic for Programmers》新增答案集编程与约束逻辑编程内容时，回顾并列出自己对 Prolog 的一些痛点与不满。
For the next release of Logic for Programmers, I'm finally adding the sections on Answer Set Programming and Constraint Logic Programming that I TODOd back in version 0.9. And this is making me re-experience some of my pain points with Prolog, which I will gripe about now. If you want to know more about why Prolog is cool instead, go here or here or here or here.
No standardized strings
ISO "strings" are just atoms or lists of single-character atoms (or lists of integer character codes). The various implementations of Prolog add custom string operators but they are not cross compatible, so code written with strings in SWI-Prolog will not work in Scryer Prolog.
No functions
Code logic is expressed entirely in rules, predicates which return true or false for certain values. For example if you wanted to get the length of a Prolog list, you write this:
?- length([a, b, c], Len).
Len = 3.
Now this is pretty cool in that it allows bidirectionality, or running predicates "in reverse". To generate lists of length 3, you can write length(L, 3). But it also means that if you want to get the length a list plus one, you can't do that in one expression, you have to write length(List, Out), X is Out+1.
For a while I thought no functions was necessary evil for bidirectionality, but then I discovered Picat has functions and works just fine. That by itself is a reason for me to prefer Picat for my LP needs.
(Bidirectionality is a killer feature of Prolog, so it's a shame I so rarely run into situations that use it.)
No standardized collection types besides lists
Aside from atoms (abc) and numbers, there are two data types:
- Linked lists like
[a,b,c,d]. - Compound terms like
dog(rex, poodle), which seem like record types but are actually tuples. You can even convert compound terms to linked lists with=..:
?- L =.. [a, b, c].
L = a(b, c).
?- a(b, c(c)) =.. L.
L = [a, b, c(c)].
There's no proper key-value maps or even struct types. Again, this is something that individual distributions can fix (without cross compatibility), but these never feel integrated with the rest of the language.
No boolean values
true and false aren't values, they're control flow statements. true is a noop and false says that the current search path is a dead end, so backtrack and start again. You can't explicitly store true and false as values, you have to implicitly have them in facts (passed(test) instead of test.passed? == true).
This hasn't made any tasks impossible, and I can usually find a workaround to whatever I want to do. But I do think it makes things more inconvenient! Sometimes I want to do something dumb like "get all atoms that don't pass at least three of these rules", and that'd be a lot easier if I could shove intermediate results into a sack of booleans.
(This is called "Negation as Failure". I think this might be necessary to make Prolog a Turing complete general programming language. Picat fixes a lot of Prolog's gripes and still has negation as failure. ASP has regular negation but it's not Turing complete.)
Cuts are confusing
Prolog finds solutions through depth first search, and a "cut" (!) symbol prevents backtracking past a certain point. This is necessary for optimization but can lead to invalid programs.
You're not supposed to use cuts if you can avoid it, so I pretended cuts didn't exist. Which is why I was surprised to find that conditionals are implemented with cuts. Because cuts are spooky dark magic conditionals sometimes conditionals work as I expect them to and sometimes leave out valid solutions and I have no idea how to tell which it'll be. Usually I find it safer to just avoid conditionals entirely, which means my code gets a lot longer and messier.
Non-cuts are confusing
The original example in the last section was this:
foo(A, B) :-
\+ (A = B),
A = 1,
B = 2.
foo(1, 2) returns true, so you'd expect f(A, B) to return A=1, B=2. But it returns false. Whereas this works as expected.
bar(A, B) :-
A = 1,
B = 2,
\+ (A = B).
I thought this was because \+ was implemented with cuts, and the Clocksin book suggests it's call(P), !, fail, so this was my prime example about how cuts are confusing. But then I tried this:
?- member(A, [1,2,3]), \+ (A = 2), A = 3.
A = 3. % wtf?
There's no way to get that behavior with cuts! I don't think \+ uses cuts at all! And now I have to figure out why
foo(A, B) doesn't returns results. Is it floundering? Is it because \+ P only succeeds if P fails, and A = B always succeeds? A closed-world assumption? Something else?1
Straying outside of default queries is confusing
Say I have a program like this:
tree(n, n1).
tree(n, n2).
tree(n1, n11).
tree(n2, n21).
tree(n2, n22).
tree(n11, n111).
tree(n11, n112).
branch(N) :- % two children
tree(N, C1),
tree(N, C2),
C1 @< C2. % ordering
And I want to know all of the nodes that are parents of branches. The normal way to do this is with a query:
?- tree(A, N), branch(N).
A = n, N = n2; % show more...
A = n1, N = n11.
This is interactively making me query for every result. That's usually not what I want, I know the result of my query is finite and I want all of the results at once, so I can count or farble or whatever them. It took a while to figure out that the proper solution is bagof(Template, Goal, Bag), which will "Unify Bag with the alternatives of Template":
?- bagof(A, (tree(A, N), branch(N)), As).
As = [n1], N = n11;
As = [n], N = n2.
Wait crap that's still giving one result at a time, because N is a free variable in bagof so it backtracks over that. It surprises me but I guess it's good to have as an option. So how do I get all of the results at once?
?- bagof(A, N^(tree(A, N), branch(N)), As).
As = [n, n1]
The only difference is the N^Goal, which tells bagof to ignore and group the results of N. As far as I can tell, this is the only place the ISO standard uses ^ to mean anything besides exponentiation. Supposedly it's the existential quantifier? In general whenever I try to stray outside simpler use-cases, especially if I try to do things non-interactively, I run into trouble.
I have mixed feelings about symbol terms
It took me a long time to realize the reason bagof "works" is because infix symbols are mapped to prefix compound terms, so that a^b is ^(a, b), and then different predicates can decide to do different things with ^(a, b).
This is also why you can't just write A = B+1: that unifies A with the compound term +(B, 1). A+1 = B+2 is false, as 1 \= 2. You have to write A+1 is B+2, as is is the operator that converts +(B, 1) to a mathematical term.
(And that fails because is isn't fully bidirectional. The lhs must be a single variable. You have to import clpfd and write A + 1 #= B + 2.)
I don't like this, but I'm a hypocrite for saying that because I appreciate the idea and don't mind custom symbols in other languages. I guess what annoys me is there's no official definition of what ^(a, b) is, it's purely a convention. ISO Prolog uses -(a, b) (aka a-b) as a convention to mean "pairs", and the only way to realize that is to see that an awful lot of standard modules use that convention. But you can use -(a, b) to mean something else in your own code and nothing will warn you of the inconsistency.
Anyway I griped about pairs so I can gripe about sort.
go home sort, ur drunk
This one's just a blunder:
?- sort([3,1,2,1,3], Out).
Out = [1, 2, 3]. % wat
According to an expert online this is because sort is supposed to return a sorted set, not a sorted list. If you want to preserve duplicates you're supposed to lift all of the values into -($key, $value) compound terms, then use keysort, then extract the values. And, since there's no functions, this process takes at least three lines. This is also how you're supposed to sort by a custom predicate, like "the second value of a compound term".
(Most (but not all) distributions have a duplicate merge like msort. SWI-Prolog also has a sort by key but it removes duplicates.)
Please just let me end rules with a trailing comma instead of a period, I'm begging you
I don't care if it makes fact parsing ambiguous, I just don't want "reorder two lines" to be a syntax error anymore
I expect by this time tomorrow I'll have been Cunningham'd and there will be a 2000 word essay about how all of my gripes are either easily fixable by doing XYZ or how they are the best possible choice that Prolog could have made. I mean, even in writing this I found out some fixes to problems I had. Like I was going to gripe about how I can't run SWI-Prolog queries from the command line but, in doing do diligence finally finally figured it out:
swipl -t halt -g "bagof(X, Goal, Xs), print(Xs)" ./file.pl
It's pretty clunky but still better than the old process of having to enter an interactive session every time I wanted to validate a script change.
(Also, answer set programming is pretty darn cool. Excited to write about it in the book!)