I am trying to implement a verb-attached
preposition phrases in prolog.
So I've got:
VP -> Vbar
Vbar -> v, PP
PP -> Pbar
Pbar -> prep,np
the problem is when I try to phrase something like "Mary saw Tom with a telescope". There would be an error.
Can someone please give me some idea how to fix it?
Also can anyone please give me an idea how to implement the semantic of the Verb-attach prepositional phrase?
I do not know what the error is so I cannot say for sure, but the code you posted has a number of issues.
Prolog predicates must end in a period.
Prolog predicates begin with a lowercase letter.
You probably want to use -->, not ->. I am not sure what -> does, if anything at all.
I do not believe using strings would unify with any of your given predicates. You probably want to be using symbols, with ''.
You are using a bit of a weird syntax by including -->. You can find some information on it here. Because of this, if you want the symbol v to unify in your predicates you should write [v].
I am not sure if this code has the functionality you desire, but it does run:
vp --> vbar.
vbar --> [v], pp.
pp --> pbar.
pbar --> [prep,np].
You can call it like this:
vp([here, is, some, statement], []).
Related
I'm writing a code generator that converts definite clause grammars to other grammar notations. To do this, I need to expand a grammar rule:
:- initialization(main).
main :-
-->(example,A),writeln(A).
% this should print ([a],example1), but this is a runtime error
example --> [a],example1.
example1 --> [b].
But -->(example, A) doesn't expand the rule, even though -->/2 appears to be defined here. Is there another way to access the definitions of DCG grammar rules?
This is a guess of what your are expecting and why you are having a problem. It just bugs me because I know you are smart and should be able to connect the dots from the comments. (Comments were deleted when this was posted, but the OP did see them.)
This is very specific to SWI-Prolog.
When Prolog code is loaded it automatically goes through term expansion as noted in expand.pl.
Any clause with --> will get expanded based on the rules of dcg_translate_rule/2. So when you use listing/1 on the code after it is loaded, the clauses with --> have already been expanded. So AFAIK you can not see ([a],example1) which is the code before loading then term expansion, but example([a|A], B) :- example(A, B) which is the code after loading and term expansion.
The only way to get the code as you want would be to turn off the term expansion during loading, but then the code that should have been expanded will not and the code will not run.
You could also try and find the source for the loaded code but I also think that is not what you want to do.
Based on this I'm writing a code generator that converts definite clause grammars to other grammar notations. perhaps you need to replace the code for dcg_translate_rule/2 or some how intercept the code on loading and before the term expansion.
HTH
As for the error related to -->(example,A),writeln(A). that is because that is not a valid DCG clause.
As you wrote on the comments, if you want to convert DCGs into CHRs, you need to apply the conversion before the default expansion of DCGs into clauses. For example, assuming your code is saved to a grammars.pl file:
?- assertz(term_expansion((H --> B), '--->'(H,B))).
true.
?- assertz(goal_expansion((H --> B), '--->'(H,B))).
true.
?- [grammars].
[a],example1
true.
I have begun on a short journey of trying to understand how Prolog works, I need some help trying to figure out how to approach a problem and what my code is actually doing. I want to build a sentence from prolog and just as a super basic example that doesn't make much sense I want to compose a sentence like 'you are' and 'you art'.
These are the predicates I have:
line(you,[first, type]).
line(thee,[first,old]).
line(thou,[first, new]).
line(are, [second, word]).
line(art, [second, word]).
line(aurt, [second, place]).
Then I created a rule (which I know is wrong but I don't know why):
line(A, [composed, type]):-
line(B, [first, type]),
line(C, [second, word]),
append([B,C],A).
Typing into commandline:
?- line(A, [composed, type]).
false.
But what is not intuitive to me is that typing something like:
?- line(A, [first,type]).
A = you ;
false.
?- line(A, [second,word]).
A = are ;
A = art ;
false.
Give me the words I want. Can someone please help me better understand the way I should compose a rule such that my expected result is something like:
findall(X, line(X, [composed, type]),Y).
Y= you are;
Y= you art;
false.
I hope this makes a little sense and I didn't completely mess up the Prolog syntax in that last block of code. Thank you in advance.
I'm going to show you the proper way to approach this problem, which is with DCGs:
sentence --> noun, verb.
noun --> [you].
verb --> [are].
verb --> [art].
Now you can generate sentences simply by using the DCG driver phrase/2:
?- phrase(sentence, X).
X = [you, are] ;
X = [you, art].
What you have up there, with [first, type] and [second, word], I am having some trouble understanding. But DCGs are a great way to do light NLP tasks like this.
I'm trying to write a DCG for a command interface. The idea is to read a string of input, split it on spaces, and hand the resulting list of tokens to a DCG to parse it into a command and arguments. The result of parsing should be a list of terms which I can use with =.. to construct a goal to call. However, I've become really confused by the string type situation in SWI-Prolog (ver. 7.2.3). SWI-Prolog includes a library of basic DCG functionality, including a goal integer//1 which is supposed to parse an integer. It fails due to a type error, but the bigger problem is that I can't figure out how to make a DCG work nicely in SWI-Prolog with "lists of tokens".
Here's what I'm trying to do:
:- use_module(library(dcg/basics)).
% integer//1 is from the dcg/basics lib
amount(X) --> integer(X), { X > 0 }.
cmd([show,all]) --> ["show"],["all"].
cmd([show,false]) --> ["show"].
cmd([skip,X]) --> ["skip"], amount(X).
% now in the interpreter:
?- phrase(cmd(L), ["show","all"]).
L = [show, all].
% what is the problem with this next query?
?- phrase(cmd(L), ["skip", "50"]).
ERROR: code_type/2: Type error: `character' expected, found `"50"' (a string)
I have read Section 5.2 of the SWI manual, but it didn't quite answer my questions:
What type is expected by integer//1 in the dcg/basics library? The error message says "character", but I can't find any useful reference as to what exactly this means and how to provide it with "proper" input.
How do I pass a list of strings (tokens) to phrase/2 such that I can use integer//1 to parse a token as an integer?
If there's no way to use the integer//1 primitive to parse a string of digits into an integer, how should I accomplish this?
I did quite a bit of expermenting with using different values for the double_quote flag in SWI-Prolog, plus different input formats, such as using a list of atoms, using a single string as the input, i.e. "skip 50" rather than ["skip", "50"], and so on, but I feel like there are assumptions about how DCGs work that I don't understand.
I have studied these three pages as well, which have lots of examples but none quite address my issues (some links omitted since I don't have enough reputation to post all of them):
The tutorial "Using Definite Clause Grammars in SWI-Prolog" by Anne Ogborn
A tutorial from Amzi! Prolog about writing command interfaces as DCGs.
Section 7.3 of J. R. Fisher's Prolog tutorial
A third, more broad question is how to generate an error message if an integer is expected but cannot be parsed as one, something like this:
% the user types:
> skip 50x
I didn't understand that number.
One approach is to set the variable X in the DCG above to some kind of error value and then check for that later (like in the hypothetical skip/1 goal that is supposed to get called by the command), but perhaps there's a better/more idiomatic way? Most of my experience in writing parsers comes from using Haskell's Parsec and Attoparsec libraries, which are fairly declarative but work somewhat differently, especially as regards error handling.
Prolog doesn't have strings. The traditional representation of a double quoted character sequence is a list of codes (integers). For efficiency reasons, SWI-Prolog ver. >= 7 introduced strings as new atomic data type:
?- atomic("a string").
true.
and backquoted literals have now the role previously held by strings:
?- X=`123`.
X = [49, 50, 51].
Needless to say, this caused some confusion, also given the weakly typed nature of Prolog...
Anyway, a DCG still works on (difference) lists of character codes, just the translator has been extended to accept strings as terminals. Your code could be
cmd([show,all]) --> whites,"show",whites,"all",blanks_to_nl.
cmd([show,false]) --> whites,"show",blanks_to_nl.
cmd([skip,X]) --> whites,"skip",whites,amount(X),blanks_to_nl.
and can be called like
?- phrase(cmd(C), ` skip 2300 `).
C = [skip, 2300].
edit
how to generate an error message if an integer is expected
I would try:
...
cmd([skip,X]) --> whites,"skip",whites,amount(X).
% integer//1 is from the dcg/basics lib
amount(X) --> integer(X), { X > 0 }, blanks_to_nl, !.
amount(unknown) --> string(S), eos, {print_message(error, invalid_int_arg(S))}.
prolog:message(invalid_int_arg(_)) --> ['I didn\'t understand that number.'].
test:
?- phrase(cmd(C), ` skip 2300x `).
ERROR: I didn't understand that number.
C = [skip, unknown] ;
false.
Our textbook gave us this example of a structurer for a math equation in Prolog:
math(Result) --> number(Number1), operator(Operator), number(Number2), { Result = [Number1, Operator, Number2] }.
operator('+') --> ['+'].
number('number') --> ['NUMBER'].
I'm quite new to Prolog, however, and I have no idea how to use this example to get the output. I'm under the impression it restructures the input using Result and outputs it for use.
The only input I've tried that doesn't cause an error is math('number', '+', 'number'). but it always outputs false and I don't know why. Furthermore shouldn't it restructure it and give me the result in Result as well?
What should I be inputting here?
This example is a DCG. You should use the phrase/2 interface predicate to access DCGs.
To find out what the DCG describes, start with the most general query, relating the nonterminal math(R) to a list Ls that is described by the first argument:
?- phrase(math(R), Ls).
From the answer you get (very easy exercise!), you will notice that R is probably not what you meant it to be. Hint: Look up (=..)/2.
Notice in particular that you need not be "inputting" anything here: A DCG describes a list. The list can be specified, but need not be given: A variable will do too! Think in terms of relations between arbitrary terms.
i'm new to prolog and I try to program a answering machine. At first I like to figure out for what was asked, and to check correct syntax.
question(P) --> [where],[is], article(G,K,N), location(P,G,K,N).
location(P,G,K,N) --> [P], {member(P, [bakery, mcdonalds, kfc, embassy]),noun(G,K,N)}.
article(m, akk, sg) --> [a].
article(f, akk, sg) --> [an].
noun(m, akk, sg) --> [bakery]|[mcdonalds]|[kfc].
noun(f, akk, sg) --> [embassy].
but i got this error:
question(What, [where,is,a,bakery],[]).
ERROR: location/6: Undefined procedure: noun/3
ERROR: However, there are definitions for:
ERROR: noun/5
however, i found that the last two arguments of dcg variables are some kind of lists, but i really found nothing for that topic... do you have any tipps or solution for me?
PS: I tried to translate the example from german grammar, so don't be confused ;)
In the location rule, you put noun inside {} so it's treated like an ordinary Prolog rule. Take it outside the {} and your grammar "works" (well, the parse fails, but it doesn't throw an error).
location(P,G,K,N) --> [P], {member(P, [bakery, mcdonalds, kfc, embassy])},
noun(G,K,N).
a 'DCG predicate' has two 'hidden' arguments; noun(G,K,N) would be noun(G,K,N,L,R) where L would be the input list and R what is left after a noun has be recognised.
swi-prolog related man page
note that it's better to use the predicates phrase/[2,3] instead of using the equivalent predicate (the implementation could change).
as larsmans said, in this code noun should be outside {}