Joachim Breitner
Hi Cafe,
this is an idea that has been floating in my head for a while, and I’m wondering about its feasibility and, if feasible, complexity (in the range from „trivial“ over “blog post” over “paper” to “thesis”).
Application authors in Haskell often face the problem of run-time constants, e.g. values that are expected to be determined once during program start and then stay fixed for the remainder of the run. Good examples are user configuration values (--verbose, certain paths, debugging level, etc.).
There are two aspects to this, both of which have potential solutions that I’ve been thinking about on and off for a long time. The first is command line arguments. As far as I’m concerned they ought to be passed as a parameter to the whole programme. So instead of main being a value exported to the outside world and all importing of values being done through the IO monad, we would have main going out and argv::[String] as a global variable (which, of course would not change during any run of the programme). The alternative version of this, to make main::[String] -> IO ExitCode has a superficial cleanliness but doesn’t help with the general problem of having to pass these things about, and in fact makes no real difference to the referential transparancy of a programme. The second is configuration data in files. This seems to fall into two parts: the data that can be fixed at link time and the data that changes from run to run. For the former, a simple solution would be to have a facility to compile a module from non-haskell data. This can be done with template Haskell doing IO. So that leaves configuration data that changes from run to run. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2010-09-14)