Gregg Reynolds wrote:
Donn Cave wrote:
Quoth Gregg Reynolds wrote:
Look again at the sentence you trimmed off the end:
Of course, the point is that this result is an *IO action* of
type IO Int, it's not the Int you would get "when executing
this action".
I believe that more or less points to the key to this discussion.
If it didn't make sense to you, or didn't seem relevant to the
question of pure functions, then it would be worth while to think
more about it.
Ok, let's parse it out. "…it's not the int you would get 'when
executing this action". Close, but no cooky: it's not any kind of
int at all (try doing arithmetic with it). "IO Int" is a piece of
rhetoric for the mental convenience of the user; Haskell does not and
cannot know what the result of an IO action is, because it's outside
the scope of the language (and computation). (The "Int" part of "IO
Int" refers to the input, not the output; it's just a sort of type
annotation.) It's not even a computation, unless you want to take a
broad view and include oracles, interaction, etc. in your definition
of computation.
Why would IO Int be something special or mysterious? It's an ordinary
value like everything else; it's on the same footing as [Char], Maybe
Int, Int -> String, Bool, and so on. I see no difference between the
list [1,2,3] :: [Int] and the action "pick a random number between 1
and 6" :: IO Int .
Best regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
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