I'm using text files as a database for saving users' information for a game which i made using swi-prolog. The information is saved like this:user(Name,Password,Age,Points). What i want to do is to change a user's Points without having to rewrite the entire db. In other words, I am looking for something that will work like retractall(user(Name,_,_,_)), but with the text file. I know how to find the specific user using read/2, and how to assert a new fact using write/2, but i don't know how to delete one specific line in the text file.
Thank you for helping.
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1 Answers
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Take a look at SWI-Prolog's library(persistency). It removes a fact by adding a line that the fact is removed. If the file gets too big with add/remove lines, it provides db_sync/1 to write a clean file. OS file system operations do not allow to remove part of a file (except from truncating the end). The normal way to do this is to write a new file and, if successful, rename this to the existing one, so nothing is lost if you crash while writing the new file.
