-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Monday 12 August 2002 02:08 pm, Scott J. wrote:
----- Original Message ----- From: "Scott J."
What I meant was discussion about the state transformer ST s a itself. And how it works. What does mean the second inner forall loop and so on. I can't find explanations of this in the Haskell library.
Oh! If you look in the paper that's mentioned: _Lazy_Functional_State_Threads_, by John Launchbury and Simon Jones, 1994, there's a big section on this. To quote: Section 2.4 Encapsulaion "So far we have been able to combine state transformers to make larger state transformers, but how can we make a state transformer part of a larger program which does not manipulate state at all? What we need is a function, runST, with a type something like the following: runST :: ST s a -> a" "The idea is that runST takes a state transformer as its argument, conjures up an initial empty state, applies the state transformer to it, and returns the result while discarding the final state." ... Discussion of usage implications, and how this initial guess at type creates all kinds of potential usage problems ... "To put it another way, the argument of runST should no make any assumptions about what has already been allocated in the initial state, That is, runST should work regardless of what initial state it is given. So the type of runST should be: runST :: forall a . (forall s.ST s a) -> a This is not a Hindley-Milner type, because the quantifiers are not all at the top level; it is an example of rank-2 polymorphism (McCracken [1984]). Section 5.2 Types "Most of the type rules are the usual Hindley-Milner rules. The most interesting addition is the typing judgement for runST. Treating it as a language construct avoids the need to go beyond Hindley-Milner types. So rather than actually give runST the type runST :: forall a . (forall s.ST s a) -> a as suggested in the introduction, we ensure that its typing judgment has the same effect." - -- You're in a maze of twisty little statements, all alike. Public Key available from http://www.garbett.org/public-key -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.7 (GNU/Linux) iD8DBQE9WAthDtpPjAQxZ6ARAgsqAJ9i+oIdWHvQB80xmEhugQTklOtpvQCdFbM5 ol6XOKjp7FGdM3oetPUTw+E= =+exg -----END PGP SIGNATURE-----