Q&A Session for Programming Languages Lecture 7
Session Number: 1206497878
Date: 2020-9-25
Starting time: 14:23
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ANON – 14:48
Q: For question 6, do we count duplicate answers?
Priority: N/A
Ana L. Milanova – 14:48
A: You should count what Prolog would count.
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ANON – 14:49
Q: How is class going to proceed today with career fair?
Priority: N/A
Ana L. Milanova – 14:50
A: As usual with recording. We expect sparser Q&A but if you
have questions, bring them back on Tuesday.
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ANON – 15:04
Q: do you have advice on how to become more used to Proglog syntax?
Priority: N/A
Konstantin Kuzmin – 15:06
A: I would say just by trying solve as many exercises (from
simple to harder) as you can. It’s like swimming, you just need to
dive in. 😉
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ANON – 15:08
Q: Can you show the pl file side by side with the terminal?
Priority: N/A
Konstantin Kuzmin – 15:16
A: We can make the pl file available, so that you can replay
the recording of this Q&A session and have the source code side-by-
side.
Ana L. Milanova – 16:26
A: I didn’t get your request in real time while presenting,
but yes, you can split the window and show them next to each other. I
usually switch between windows when coding in Python or Prolgog to
have a larger screen but it’s just preference.
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ANON – 15:12
Q: What does “[debug].” do in Prolog?
Priority: N/A
Steven Haussmann – 15:13
A: It disables some optimizations that can cause it to lose
information about errors (amongst other things, probably!). It
shouldn’t affect what your queries do.
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ANON – 15:14
Q: Just to be clear, I streamed all lecture videos and also attended
lecture events this week. My checkin is complete correct, no need to
visit office hours?
Priority: N/A
Konstantin Kuzmin – 15:16
A: Correct!
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ANON – 15:25
Q: could we overcome the failure of snowy(troy) by adding a new
variable Y, s.t. snowy(X) :- rainy(Y), !, cold(Y), Y=X. ?
Priority: N/A
Ana L. Milanova – 15:29
A: No, I don’t think this will work. The ! succeeds, which
will prevent the interpreter from trying the other choice at the
parent, the other choice being snowy(troy).
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ANON – 15:29
Q: If I attended live lecture, do I need to send in an email saying I
attended? or will you just look at the automated return emails from
webex?
Priority: N/A
Konstantin Kuzmin – 15:30
A: No need to send an email in this case. We get logs of
students who attended live lectures directly from WebEx.
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ANON – 15:31
Q: if we add snowy(troy) to the end of example 4 or 5, both of them
cannot output C = troy right?
Priority: N/A
Ana L. Milanova – 15:34
A: Correct.
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ANON – 15:31
Q: for this last example would there be no other answers printed
(assuming there are other possible answers) since it’s commited at the
end now?
Priority: N/A
Ana L. Milanova – 15:34
A: Correct again.
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ANON – 15:32
Q: Is there a certain amount of time you have to present for the live
lecture to count? I had to join a few minutes late because of the
career fair but will have attended the majority of lecture.
Priority: N/A
Konstantin Kuzmin – 15:36
A: WebEx registers every time you join or leave, even if it is
multiple times during the Event. For the purposes of attendance
credit, it is sufficient to have at least one live lecture attendance
record on WebEx, even if it is just one minute in duration.
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ANON – 15:35
Q: so essencially, once the program backtracks to an “!” the program
terminates?
Priority: N/A
Ana L. Milanova – 15:36
A: No, not necessarily. It just commits the interpreter to the
bindings that the program made since the unification of the parent of
the cut (!).
Ana L. Milanova – 15:38
A: We may have a tree above the goal that has the Cut and
Prolog will continue to search there, even as it has committed to
those bindings due to the cut.
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ANON – 15:49
Q: Can I attend office hours after this lecture to get attendance?
Priority: N/A
Ana L. Milanova – 15:51
A: You are getting attendance by attending the lecture.
Attendence is an OR: you can do either one of lecture attendance,
streaming videos on Mediasite, or checking in in office hours.
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ANON – 15:52
Q: what is meant by negation is not the same in prolog as in logic?
Isn’t prolog logical by design?
Priority: N/A
Ana L. Milanova – 15:53
A: It does not implement full first order logic, because if it
did inference would be undecidable. So it has restrictions and one of
the restrictions is that it does not allow us to make negative
assiertions.
Ana L. Milanova – 15:53
A: For example, we cannot assert in the database of facts
something like not(food(jane)).
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ANON – 15:53
Q: is “_” considered a variable?
Priority: N/A
Konstantin Kuzmin – 15:53
A: In Prolog, “a single underscore ( _ ) denotes an anonymous
variable and means “any term”” (from Wikipedia).
Steven Haussmann – 16:03
A: If you have a variable that’s never actually used, Prolog
will warn you about it being a “singleton” — i.e., it only shows up
once. That’s where you want to use _; it takes up space, but has no
meaning.
Ana L. Milanova – 16:05
A: Yes. For example, if you write member(A,[A|B]). Prolog will
warn you about B being a singleton variable. To avoid that, you can
write member(A,[A|_]). meaning that we don’t care what the tail of the
list is. The way we had it in n-Queens.
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ANON – 16:12
Q: can you show the liast slide on the parser again?
Priority: N/A
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ANON – 16:15
Q: So the closest thing to normal programming Prolog Functions are is
the “Recursive functions only challange?”
Priority: N/A
Steven Haussmann – 16:16
A: That does get you closer to Prolog’s style, yes — so those
familiar with functional languages will probably find it more
comfortable
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ANON – 16:23
Q: Can you explain a little more about invertibility?
Priority: N/A
Ana L. Milanova – 16:30
A: Yes, you can think of giving the result of the computation,
and prolog computing the arguments. For example, if you write
append(l1,l2) in other languages, it works in the forward way, i.e.,
it expects l1 and l2 to be fully bound values, and comutes
Ana L. Milanova – 16:30
A: the result of appending l2 onto l1. But we cannot give
append the result, and have it compute l1 and l2. In Prolog we can do
just that!
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ANON – 16:24
Q: Can I simply think of it as having variables on the rhs, which is
not allowed since variables can’t self bind?
Priority: N/A
Ana L. Milanova – 16:32
A: I think yes, you can think of having this result (or out)
argument/variable, which we can’t give on the rhs, but must give as
argument.
Ana L. Milanova – 16:33
A: And prolog computes the arguments based off of a fully
isntantiated result variable. (Of course, not all predicates are
invertible!)