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
On 5 Dec 2008, at 17:46, Duncan Coutts wrote: 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