lisp file pointers in classes

Question

I'm running up against a problem in understanding the CLOS way of handling file access within a class. In c++ I would be able to do this:

class Foo {
   Foo (string filename);  // opens the file (my_file) requested by the filename
   ~Foo ();  // close the file

   FILE * my_file;  // a persistent file-handle
   DataStruct my_data;  // some data

   void ParseData ();  // will perform some function on the file and populate my_data
   DataStruct * GetData () { return &my_data; }  // accessor to the data
};

What I'd like to point out is that PraseData() will be called multiple times, and each time a new block of data will be parsed from the file and my_data will be altered.

I'm trying to perform the same trick in CLOS - create all the generic methods to parse the data, load the file, read headers, etc. as well as the class definition which I have as:

(defclass data-file ()
  ((filename :initarg :filename :accessor filename)
   (file :accessor file)
   (frame :accessor frame)))

In the "constructor" (i.e. initialize-instance) I open the file just as my c++ idiom. Then I have access to the data and I can parse the data as before. However, I'm told that using a "destructor" or (finalize) method to close the file is not idiomatic CLOS for handling this type of situation where I need the file to be around so I can access it outside of my data-file methods.

I'm going to define a function that loads a data-file, and then performs a series of analyses with its data, and then hopefully close it. What's a way to go about doing this? (I'm assuming a macro or some type of closure would work in here, but I'm not familiar enough with the lisp way to decide what is needed or how to implement it).

Answer 1

One option is to have the stream as a slot instead of the filename, and then scope it with WITH-OPEN-FILE:

(with-open-file (stream file)
  (let ((foo (make-instance 'foo :stream stream)))
    (frob foo)
    (...other processing of foo...)))

Then your stream will be closed automatically.

Answer 2

I think I would lean towards making classes only to store complete authoritative data (what you call DataStruct?).

You don't really need a special class for "loading + storage of another class". Plus, that way has the unspoken invariant that my_data holds the data of my_file up to the current seek position, which seems a bit strange to my eye.

Put another way: what does Foo do? Given a filename, it loads data, and gives you a DataStruct. That sounds like a function to me. If you need to be able to run it in a thread, or fire events between loading records, a class is the natural way to do it in C++, but you don't need a class for those things in Lisp.

Also, remember that you don't need to use DEFCLASS in order to use generic methods in CLOS.

I don't know what the structure of your data is, but in similar situations I've made a parse-one-chunk function that takes a stream and returns one record, and then create a complete Foo instance inside a loop in a with-open-file. If the stream is never needed outside the scope of a with-open-file expansion, you never need to worry about closing it.