[zem]

the "with-" pattern (originally from lisp, i believe, but ruby did a lot to bring it to the masses), where something like a filehandle manages its own lifecycle, and calls your closure in between. so rather than the C-like

    let f = open-file-for-writing(filename);  
    for line in array {  
      write-line-to-file(f, line);
    }
    close-file(f);

you can do

    with-open-file-for-writing(filename) {|f|
      for line in array {
        write-line-to-file(f, line);
      }
    }

where the definition of with-open-file-for-writing() would look like

    def with-open-file-for-writing(filename, closure) {
      let f = open-file-for-writing(filename);
      call-closure(closure, f);
      close-file(f);
    }

the benefit of having this be a closure rather than just a function pointer can be seen in the write array to file example above, where the "array" variable is in the scope of the calling function, but when with-open-file-for-writing calls your closure it can make full use of its own local variables.

[dllthomas]

Of course, you can build your own closure:

    void do_stuff_with_file(struct relevant_data *, FILE *);

    ...

    {
        struct relevant_data data = { ... }
        with_open_file_for_writing(do_stuff_with_file, data, filename);

    }

IMO, the biggest downside there being how far it typically pushes the definition of that function from the call site. Small functions - a good practice anyway - ameliorates that a bit.

[zem]

you can, but it's sufficiently clunky that it simply doesn't feel like a natural thing to do in the language. good language design is a lot more about the things it makes easy and natural than the things it makes possible.

[dllthomas]

"you can, but it's sufficiently clunky that it simply doesn't feel like a natural thing to do in the language."

It does to me, but I've done enough functional programming that I easily reach for concepts from that space.

"good language design is a lot more about the things it makes easy and natural than the things it makes possible."

Of course. I don't know where you got the idea I was saying closures aren't a good thing to have language support for. I said precisely the opposite.

[edwintorok]

You can also wrap the call-closure with an exception handler to make sure that 'f' is always closed when you leave with-open-file-for-writing.

[zem]

right. and the beautiful thing is that once you realise that you only need to do it once, not everywhere you open, write to, and close a file.