Koen Claessen writes: i find your idea very good. indeed for the library GetOpt, the argument of a program never change so it make sense to make this library without using IO monad, same for argv and for the enviroment. for openFile it seems harder, it would require to have a lock on the file to be sure that no one modify the file, same for a webpage.

Dear all,

At the moment, a discussion on haskell-cafe is going on about how to neatly program the fact that an entire program depends on a number of parameters that are read in once at the beginning of a program.

The suggestion that many people came up with was using unsafePerformIO in the beginning to read the file. Here is my version of that:

| data Configuration = ... -- config data | | getConfig :: Configuration | getConfig = unsafePerformIO $ | do ...read configuration from file... | return configuration | | main = | do doStuff

It is quite disturbing that there is no other easy way to do this than using unsafePerformIO (except for using implicit parameters perhaps, but there are other reasons for not using those).

I have been thinking a little bit more about this and here is what I found.

Remember the Gofer days, when Gofer had a "function":

openFile :: FilePath -> String

This was of course a cheap and dirty way of implementing things like the getConfig above, but it is impure. However, one could imagine a functional version of this function:

readFileOnce :: FilePath -> Maybe String

This function will read the contents of the file (and return Nothing if something went wrong), but it is memoized, so that the second time you use this function you get the same result.

So, it is a pure function. (Admittedly, it is somewhat unpredictable, but you will always get the same result for the same arguments.) It is no more strange than GHC's pure version of the getArgs function (I forgot what it was/is called).

How about space behavior, you say? Reading a file, and memoizing the result means storing the whole contents of the file in memory!

The point is that the use of this function will typically happen at the beginning of a program, when reading the configuration file(s). When all this has happened, the function readFileOnce, and its memo table, will be garabage collected. (Of course there is no guarantee that all calls to readFileOnce will be evaluated at the beginning of a program, and it is not required, but when you do, there are no space problems.)

There could of course be pure "-Once" versions of other IO operations. Here is a list of possibilities:

- reading a file - getting arguments - getting environment variables - downloading a webpage - ...

What do you think?

Regards, /Koen.

-- Koen Claessen http://www.cs.chalmers.se/~koen Chalmers University, Gothenburg, Sweden.

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