On Wed, 21 Dec 2005, Daniel Carrera wrote:
Creighton Hogg wrote:
x is a String, getLine has type IO String. That's what I was getting at in one of my last e-mails.
Hmm... let's see if I understand:
* getLine() has type IO String. * The <- will "convert" an IO String to a plain String * So if I do x <- getLine() then x has the type String.
So, the <- effectively ammounts to an IO a -> a conversion.
Yeah, pretty much. You're saying that "x is the result of action getLine", and in terms of types it means you're getting the a out of m a.
In another email John Hughes said that one could think of "IO a" as a set of instructins for obtaining a. I guess that means that IO is a sort of imperative layer that helps the purely functional code interact with the outside world.
So I can have an IO bit (e.g. a do-block) that calls functions (which are purely functional code) but I can't have a function that executes any IO.
For example, it is not possible to write a function "my_read_file" that could work like this:
my_data = my_read_file("my_file.txt")
Correct? Otherwise this would be a function that is not referentially transparent.
Assuming I understand correctly, then you are right. "=" implies definition. my_data is the symbol assigned to the result of a computation, not a defined constant or function. That's at least how I think of referential transparency.