Question

I would like to implement a “tail -f” like behavior in Racket. That is, I would like to read from a file, and when I hit the end, be able making something like a “blocking” (read-line file), that shall return when some other process appends a line to file.
I tried synchronizing with (read-line-evt file) but, if I am at the end of file, instead of blocking until other data is available, it returns immediately.
Is there a way to do it?

Answer 1

I don’t think that you have any way to avoid polling the file.

Note that all of Racket’s input functions consider eof a value that should be returned when it reaches the end of the input stream — so all of the events immediately return that when the end is reached. At least I don’t see anything that looks like a “wait until some input is ready, not eof“.

In any case, you also have the ffi, if you know about some system call that triggers a callback instead of polling the file. AFAICT, the linux source code for tail uses inotify, so you might be able to use an old package that interfaces that from racket called mzfam. (But it’s pretty old and might need some update work.)