The difficulty here lies in trying to avoid any step-by-step recurring details while still keeping in mind the procedural behaviour the predicate should exhibit.
When we try to figure out the declarative meaning of a predicate, we try to describe solutions for which the predicate holds. Viewed declaratively, append(L1,L2,元) holds when the list 元 is the result of appending lists L1 and L2. Recursive (continuing) clause: Contains any required logic including a call to itself, continuing recursion.Īs an example we shall define the well-known predicate append/3. It non-recursively describes the base of the recursive process. When writing such predicates in Prolog, a standard recursive pattern always has at least two parts:īase (non-recursive) clause: Typically the base-case rule(s) will represent the smallest possible example(s) of the problem that you are trying to solve - a list with no members, or just one member, or if you're working with a tree structure, it might deal with an empty tree, or a tree with just one node in it, etc. Recursion appears when a predicate contains a goal that refers to itself. The questions tagged with SWI-Prolog will give you examples of SWI-Prolog-specific code.Prolog doesn't have iteration, but all iteration can be rewritten using recursion. Stackoverflow is actually a great source of code examples in Prolog. If you ask yourself why is it so difficult to do this, it is because Prolog is not meant to be used as a tool for simple text processing there are tools that are far better at it (grep, sed, awk, and of course perl). There are plenty of examples of using DCG here on stackoverflow. It would be a proper "Prolog" way of doing it. Now this is one way to do it, another way would be to use a DCG. Again, you need to know the Prolog basics. Study the sections in the manual containing read_line_to_codes, atom_codes, and open, they have a lot of useful predicates. Read_line_to_codes(File, Codes), Codes \= end_of_file,įile is a stream object, to get it you can use: open(FileName, read, File).
You can define the following predicate to read lines from a file to a list, stopping at a line consisting of nothing but "stop": read_until_stop(File, ) :.
0 kommentar(er)
