On Fri, 2008-12-05 at 18:50 +0100, Thomas Davie wrote:
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.
We don't have a (denotational) semantics for Haskell, either. But, in principle, one could be produced if desired. The pure subset is easy; IO only requires a semantics for the underlying OS and --- in the presence of the FFI --- the C language. Unless you add unsafePerformIO or evaluate to the language. I think that, if someone was sufficiently motivated to work out the details, that you could prove that there is *no* denotational semantics for IO which admits either function without changing the definition of one or more pure Haskell types. jcc