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First when I read up on different predicates on Prolog, like for example http_server, it's written like this: http_server(:Goal, +Options) what does : and + mean here? Sometimes ? is also written.
Secondly sometimes I see variables declared with an underscore before them like _Request, even though there isn't any another Request, why is that?
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symbol represents the cut . You can read more about cut here. Also, an example in prolog can be found here.
Because of the problems of negation-as-failure, negation in Prolog is represented in modern Prolog interpreters using the symbol \+ , which is supposed to be a mnemonic for not provable with the \ standing for not and the + for provable.
functor, functor In Prolog, the word functor is used to refer to the atom at the start of a structure, along with its arity, that is, the number of arguments it takes. For example, in likes(mary, pizza) , likes/2 is the functor.
Prolog provides a predicate that performs this function. It is called the cut, represented by an exclamation point (!). The cut effectively tells Prolog to freeze all the decisions made so far in this predicate. That is, if required to backtrack, it will automatically fail without trying other alternatives.
Introduction : Prolog is a logic programming language. It has important role in artificial intelligence. Unlike many other programming languages, Prolog is intended primarily as a declarative programming language. In prolog, logic is expressed as relations (called as Facts and Rules). Core heart of prolog lies at the logic being applied.
However, other languages like Prolog, have declarative and also imperative properties. This may also include procedural statements like “To solve the problem H, perform B1, B2 and B3”. ALF (algebraic logic functional programming language).
The syntax and semantics of the Prolog programming language are the set of rules that defines how a Prolog program is written and how it is interpreted. The rules are laid out in ISO standard ISO/IEC 13211 although there are differences in the Prolog implementations. 1 Data types.
It has a single data type, the term, which has several subtypes: atoms, numbers, variables and compound terms . An atom is a general-purpose name with no inherent meaning. It is composed of a sequence of characters that is parsed by the Prolog reader as a single unit. Atoms are usually bare words in Prolog code,...
The +, -, : etc. sigils are called mode declarations. They describe the expected instantiation of predicate arguments, i.e., whether you are expected to call the predicate with an unbound variable, an instantiated term, etc. These are not completely standardized; here is a description of the conventions for SWI-Prolog: http://www.swi-prolog.org/pldoc/man?section=modes
As a first approximation, a + argument is an input to the predicate, you are supposed to provide a ground term. A - argument is an output of the predicate, the predicate will try to unify it with a term. A ? term may be partially instantiated at the call, and the predicate may instantiate it further. A : argument is a meta-argument, i.e., it is a goal to be called by the predicate (as in setof/3, for example).
In the example of http_server(:Goal, +Options), you are supposed to call this predicate with the first argument bound to a goal, probably a predicate name. The second argument must be instantiated, presumably to a list whose format is further described in the documentation. If you do not call this predicate like this, for example, if you pass an unbound variable as the second argument, you might get unexpected behavior or an instantiation error.
As for your second question (which would better have been separate), a variable that begins with an underscore is called an anonymous variable. Every such variable may only occur once per clause, except _ itself, which may occur several times and refers to separate variables at each occurrence.
Prolog systems usually emit a "singleton variable" warning for non-anonymous variables that occur only once, because those might be typos or a sign the programmer forgot something. You use anonymous variables to express the notion that "there must be something here (e.g., a predicate argument), but I don't care what it is". In your example, presumably you call a predicate that has a "request" argument, but in your particular use case you don't care about the request.
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