Thanks Tom and Jeremy, this helps a lot. Actually, Tom I see you
spelled this out for me a couple weeks ago in an earlier post. Sorry,
I'm a bit slow.
So am I correct in saying that World is never reference in the
program, it's only used to emulate imperative actions?
-Tom
On 3/4/08, Thomas Shackell
I believe 'tackling the awkward squad' explains the World parameter.
http://research.microsoft.com/~simonpj/papers/marktoberdorf/
Indeed this is the 'World' parameter. It's quite easy to see why it's needed. Image the getChar function ...
getChar :: IO Char
if we imagine that the World parameter wasn't present, this would make the type of getChar.
getChar :: _E Char
Since _E is just a box this is basically the same as.
getChar :: Char
Since this doesn't take any arguments it's a CAF, i.e. a constant value. Since it's constant, every call to getChar would always return the same character!
When we add the World parameter we turn getChar into.
getChar :: World -> Char
Now getChar takes an argument, so its value is no longer constant (its value can vary depending on the argument). Haskell neither knows, nor cares, that getChar doesn't even look at the World argument. As far as Haskell is concerned getChar has an argument so Haskell can't treat it like a constant.
In other words, it's all some nasty trickery to make impure functions work correctly in Haskell ;)
Hope that explains it :)
Tom