[Haskell-cafe] pure programs

Jason Dusek jason.dusek at gmail.com
Wed Nov 5 14:20:49 EST 2008

Jules Bean <jules at jellybean.co.uk> wrote:
> Jason Dusek wrote:
> > Though that seems reasonable, it is not, in general, true.
> > For example,    System.Info.os    is generally treated as
> > pure, though it is not. It's not clear to me how to
> > disambiguate  these "born again" values from really pure
> > values.
> System.Info is broken. "os" has the wrong type.
> Sorry about that. There is quite a lot of brokenness in the
> standard libs which stops pure functions being pure. It's a
> shame IMO.

  I've thought about this a little bit, and it may be okay. What
  if our rule for program purity is a program, once compiled,
  may be moved from environment to environment, and will either
  execute consistently or simply fail to execute. Consider this

import System.Info

main                         =  putStrLn os

  If I compile it on Linux and then run the executable on
  FreeBSD, it will either fail to run, because FreeBSD Linux
  compatibility got worse since I tried it last, or run and
  print    linux    as it would on Linux. Does this seem
  reasonable? As long as the programs are statically linked,
  through and through, they should be pure. Admittedly, this
  approach does not address "source level purity", which is
  impossible due to `unsafePerformIO` and, more generally,
  conditional compilation.


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