On 5 Dec 2008, at 17:46, Duncan Coutts wrote:
On Fri, 2008-12-05 at 17:06 +0100, Thomas Davie wrote:
On 5 Dec 2008, at 17:00, Duncan Coutts wrote:
On Fri, 2008-12-05 at 16:50 +0100, Thomas Davie wrote:
Sure, and he could then use a fold instead of a map. Reading files
is
problematic, but as long as you're only doing it once (the most
common
situation) is entirely fine wrapped up in an unsafePerformIO.
Nooooo!
Please don't go telling people it's entirely fine to use
unsafePerformIO
like that (or at all really).
Exactly what isn't fine about it?
It's the antithesis of pure functional programming. It's so unsafe
that
we don't even have a semantics for it.
Yes, but we also don't have semantics for IO, so it's no real surprise
that we have none for something that runs an IO action.
One needs pretty special justification for using unsafePerformIO and
such cases should be hidden in libraries presenting pure interfaces,
not
used willy-nilly in general application code.
Note that I'm not claiming that it's necessarily going to do bad
things
in the specific case you're imagining using it in. However just
because
it happens not to do bad things in this case does not mean that it's
ok
to use it here or in general.
No, and I never said that it should be used more generally -- I was
very careful that in this case I was following the rules for making
sure that verifyItsSafeYourselfPerformIO was indeed safe.
Bob
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