[ItsMonkk]

Finding is a query. Deleting is destructive. Make sure that you are not nesting your destructions within a query.

You would first build a pure function that read the files and returned matching lines. You could then display the count of the matched lines, and if the user wants they can see those lines and what line number it is. This looks a lot like a dry-run.

You can then pass these results into an executor. You then build a system that combines all deletes of a file and passes that to your DeleteLinesInFile fuction that does so in a single transaction. Cycle through each file and voila.

[rwl]

My point is not that this is impossible; it's that separating the tasks of building a list of query results and then executing an operation on those results requires some kind of convention about how to refer to those results across the two phases. Developing such a convention is a harder problem than it looks like.

The example I was responding to was from the (Unix) shell, which as far as I am aware does not offer any such convention other than filenames. Real programming languages of course offer more options, but even there the problem is not solved: as you say, you must then build the system which understands how to take the results of the query and execute something on them. That means designing such a convention inside your program. The article is about using a type system to help you do that cleanly.