> One's intuition would suggest that you could be safely
> implement mkAtom
> without wrapping it in a IO monad. After all, at least at a
> abstract level,
> an atom table is referentially transparent. The library
> documentation says
> that lack of side effects and environmental independance is
> sufficent to
> order for uses of unsafePerformIO to be safe. Is there a
> exact (or at least
> better) criterion for safety?
An exact criterion would require a formal semantics for Haskell, which
we don't have. But informally, if a function which is implemented using
unsafePerformIO is pure - that is, its result depends only on the values
of its arguments and possibly its free variables - then that is a "safe"
use of unsafePerformIO.
> "unsafePerformIO" is used in the implementation of
> mkFastString, so how is it's side effects "safe".
It is safe because the side effects aren't visible outside the
implementation of mkFastString.
HTH,
Simon
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