On Tuesday 10 July 2007, Andrew Coppin wrote:
Stefan O'Rear wrote:
On Mon, Jul 09, 2007 at 09:57:14PM +0100, Andrew Coppin wrote:
(BTW... How in the hell do you get symbols like that in plain ASCII??)
You can't, but the most commonly used replacement for ASCII (Unicode-UTF8) supports them just fine.
Wait... I thought Unicode was still an experimental prototype? Since when does it work in the real world??
Are you serious? Unicode has been a (more-or-less) working reality on Linux for several years now. . .
Consider the ST monad, which lets you use update-in-place, but is escapable (unlike IO). ST actions have the form:
ST s α
Meaning that they return a value of type α, and execute in "thread" s. All reference types are tagged with the thread, so that actions can only affect references in their own "thread".
...so *that* is what that thing does...! (I thought it did something quite different.)
Now, the type of the function used to escape ST is:
runST :: ∀ α. (∀ s. ST s α) → α
The action you pass must be universal in s, so inside your action you don't know what thread, thus you cannot access any other threads, thus runST is pure. This is very useful, since it allows you to implement externally pure things like in-place quicksort, and present them as pure functions ∀ e. Ord e ⇒ Array e → Array e; without using any unsafe functions.
...so the 's' doesn't really "exist", it's just random hackery of the type system to implement uniqueness?
Exactly.
But that type of runST is illegal in Haskell-98, because it needs a universal quantifier *inside* the function-arrow! In the jargon, that type has rank 2; haskell 98 types may have rank at most 1.
...kinda wishing I hadn't asked... o_O
Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs