Improve IOFacilities In SSL

Stratego -- Strategies for Program Transformation


We need to improve the IO facilities in the SSL. Because the oldest strategies are to abstract, many different implementations have been created in the past. The structure is now very unclear. Especially internally (in the native part of the SSL) it is quite a mess. EelcoVisser started on improving the IO facilities a while ago by providing some basic POSIX functions in the SSL.


The most important problem is to find a suitable representation of an open file where you can write data to. This used to be a complete mess in Stratego:

  1. an internal table saves references from file names to ints. The ints are in fact FILE* , but ints are not a suitable representation pointers.
  2. You can pass the same Int representation of a FILE* around in Stratego.
  3. A file descriptor is also an Int ...
  4. All native primitives repeat the process of converting from terms to streams and back. Not all implementations handle all the cases.

Now this is not really a problem if you just write io-wrap applications, but when you are doing more serious IO stuff (for example in StrategoNetworking ) the current strategies turn out to be incomplete and clumsy.


We need to be able to use a stream in Stratego. In fact we need three different kinds of 'files':

  • file locator (char* in C)
  • file descriptor (int in C)
  • stream (FILE* in C)

Choosing a representation for the first two is easy: just invent some nifty constructors that describe the required data and your finished. The stream is a problem: the internal structure of a FILE shouldn't be used and a pointer is difficult to represent in an ATerm.

Representing a stream

First of all hiding the internal structure is a good idea: applications shouldn't need to worry what data is needed to describe a stream. Therefor the represention (whatever it will be) is packaged in Stream constructor. The only child of the Stream constructor is defined as implementation dependent and the internal structure of this term should never be used in your application or the SSL. This makes the SSL more or less back-end independent.

A FILE* can be described by a ATermBlob?. The size of the blob is the number of bytes required to represent a pointer to a FILE. This is approach is platform and processor independent.

File descriptor

A file descriptor is an Int on all POSIX platforms, so we can just define a file descriptor as a FileDescr constructor that takes an Int.

File paths

A string as a file location might look nice at first sight, but in fact we need more: stdin, stdout, stderr should be available on all levels. In C they are file descriptors (0, 1, 2) and a stream (stdin, stdout, stderr). In Stratego the stdin, stderr and stdout constructors are used for streams as well as file locations. Because of this all implementations that operate on streams have to handle these cases. I want the constructors stdin, stderr and stdout to just be a file location. The file descriptors and streams are available as a result of (POSIX) strategies. You can make a stream from any file location with open-stream.

File paths are represented with the Path constructor. A Path can be absolute or relative. Maybe we can make more variants with a separation of directories and filenames in the future.


      Stream    : ImplDep -> Stream
      FileDescr : Int -> FileDescr

      Path      : String -> FileLoc
      stdin     : FileLoc
      stdout    : FileLoc
      stderr    : FileLoc

Maybe the stdin, stdout and stderr constructs can even be replaced by overlays for a Stream. This will probably not break existing code, except for native code in the SSL that cannot Stream constructs.

    stdin  = Stream(<prim("SSL_stdin_stream")> ())
    stdout = Stream(<prim("SSL_stdout_stream")> ())
    stderr = Stream(<prim("SSL_stderr_stream")> ())

Existing code

Now we have a stream representation in Stratego, things get much more easy and powerful for both library users and library implementors. A complete switch to this style will lead to an enormous reduction of the amount of native code in the SSL part of the SRTS. I've rewritten some code and most native implementation are reduced by more than 50%.

I suggest to rewrite all existing code that operate on streams and file descriptors. These strategies are relatively new, are just used by Eelco and me and are not used in legacy code. Most of the existing code will not break because it doesn't matter a lot to client code how streams and file descriptors are represented.

For strategies that operate on file names, this is of course not acceptable. open-file is for example widely used. Some of these strategies could be implemented on top of the new IO facilities because the new facilities are more atomic. There is one big problem: the evil file table. The new IO solution with fds, streams and file locations requires you to actually use the result of open-file, which will be a Stream. Currently open-file just returns the filename. This encourages a style where you just open the file and just work with the filename later in the program. If we rewrite open-file, this will break existing (and very old) code.

So we should leave the real legacy strategies alone for now. Most strategy names are not in the way because the new facilities prefer the name "stream" over "file" and POSIX procedure names for operations on them. In the future we could deprecate the old IO strategies or define them in terms of the new facilities. The open file table should be removed in the future.


  • All strategies should specify whether they operate on:
    • file paths -- text based name
    • file descriptors -- int
    • streams -- platform dependent structure
  • If strategies accept different kinds of file representations, overloading should happen in Stratego, not in the native code.
  • Represent stream ( FILE* ) by ATermBlob? in constructor (Stream) instead of 'int' pointer.
  • Remove 'open file' table in io.c
  • Represent file descriptor in constructor
  • Make POSIX process and file procedures available in the SSL
  • Merge with IO facilities of XTC

-- MartinBravenboer - 24 Mar 2003